Air pollution from biomass burning in India

Air pollution is the most difficult challenge India is facing today, and anthropogenic biomass burning (ABB) is one of the major and least understood sources, leading to serious health and climate implications by affecting air quality, visibility, atmospheric chemistry, the Earth’s radiation budget and biogeochemical cycling. Scientific understanding of the composition, chemistry and regional dynamics of biomass burning (BB) emissions is still limited, thus constraining the implementation of appropriate research and policy interventions. The broad categories, associated complex emissions and spatio-temporal variations of ABB in India are discussed here. Major types of ABB that are prevalent year-round all over India include BB for cooking, heating purposes and open waste burning. Moreover, crop residue burning (CRB) in the Indo-Gangetic plain (IGP) during pre- (April–May) and post-monsoon (October–November) seasons, CRB in South India during January–February, forest fires in Central India and slash-and-burn agriculture in Northeast India during March–May are the other dominant sources that are active during specific months and in specific regions. Over the IGP, CRB along with other episodic ABB events and stagnant meteorology result in severe air quality and poor visibility (<∼300 m) with reported daily mean PM2.5 concentrations shooting up to 15 times higher than Central Pollution Control Board standards. When compared with wheat residue burning, higher fire counts and emissions are reported during paddy residue burning events. During winters, BB’s contribution to 37% of the aerosol oxidative potential in Delhi indicates its health implications. Meta-analysis of data obtained from representative source apportionment studies on PM10 shows >24% BB contribution in Assam, Ahmedabad, Kanpur and Visakhapatnam, 20%–24% in Patiala and Kolkata, and 10%–14% in Delhi. With regard to PM2.5, BB contributions of >24% in Ahmedabad and Agra, and 15%–19% in Delhi, Mumbai and Kolkata are reported, while with regard to PM1, BB contributed 19%–24% in Kanpur, Patiala and Assam and <10% in Delhi. Observed spatio-temporal variations in ABB sources and composition over India call for region-specific solutions through airshed-based management.


Introduction
Anthropogenic biomass burning (ABB) is a global problem with considerable spatio-temporal variations (Crutzen et al 1979, Jacobson 2014, Karanasiou et al 2021).The episodic wildfires in forests, grasslands, etc, are considered natural, while humandriven intentional or accidental burning of biomass, e.g.wood burning for heating and cooking, open burning of waste, forest fires (FFs), crop/agriculture residue burning, is termed ABB.Natural (wild) fires, as a regulatory ecosystem service, play an important role in Earth system dynamics through carbon cycling, regeneration of vegetation and sustenance of ecosystems; however, ABB results in an additional burden of particulates (carbonaceous, watersoluble ionic species (WSIS) and metals) and trace gases (Bond et al 2013, Keywood et al 2013).Satellite-based average annual biomass burning (BB) emissions noted over tropical continents during 2001-2017 include carbon dioxide (CO 2 ) > carbon monoxide (CO) > non-methane organic compounds (NMOC) > particulate matter (PM) with aerodynamic diameter ⩽2.5 µm (PM 2.5 ) > organic carbon (OC) > nitrogen oxides (NO x ) > methane (CH 4 ) > ammonia (NH 3 ) > sulphur dioxide (SO 2 ) > black carbon (BC) (Shi et al 2020).The air pollutants arising from BB emissions directly and indirectly impact atmospheric chemistry, the Earth's radiation budget, climate processes, the global carbon cycle and nutrient dynamics, thus resulting in poor air quality, visibility degradation, disturbed hydrological cycles, pyroconvection, climate change and overall ecological imbalance.These concerns of local to global scale may further exaggerate the challenges of poverty, food shortages and biodiversity loss, posing an overall threat to human existence, particularly in developing countries like India.Discernment of the contributions arising from natural and ABB in the atmosphere is an additional challenge.More than 90 million global active (natural and anthropogenic) fires were reported during 2001-2020, with prevalence in the eastern and southern hemispheres (Xiang et al 2023).The global active fire events retrieved from the visible infrared imaging radiometer suite (VIIRS) during 2012-2017 show bimodal latitudinal occurrence of 18.01% fires between 5 • -11 • N and 32.46% between 5 • -18 • S, with unimodal longitudinal dominance of 32.34% between 15 • E and 30 • E (Li et al 2020).Three broad categories of BB are typically observed in Asian countries: (1) FFs (natural/anthropogenic), (2) crop residue burning (CRB) and (3) grassland burning (Shaik et al 2019).In Asia, India contributes to ∼18% of the total BB activities after China (∼25%) (Streets et al 2003, Taylor 2010).The type of BB varies from one continent to another with forests, woody savanna/shrubland and savanna/grassland burning dominating in Asia, Africa and the Americas, respectively.The BB events vary over a spatio-temporal scale.Figure 1 summarizes the types of BB prevalent in India, their spatio-temporal spread and personal to global scale implications.Indoor burning, which includes solid fuel burning for cooking and residential heating purposes, is common in India.It remains active at hourly scales and thereby directly impacts human health through personal exposure, particularly making women highly prone to such harmful impacts.Also, older family members and children are very vulnerable to indoor BB emissions (Chen et al 2017, Balmes 2019, Chanchani and Oskarsson 2021).Outdoors, wood burning for heating and in tandoors, etc, for cooking is persistent along with open waste burning in India.These sources persist for several days and thus impact air quality and community health at the local scale in urban and rural areas of the country.It further leads to reduced visibility, resulting in haze-like conditions, usually noticed in urban atmospheres (Chen et al 2017, Ramadan et al 2023, Sharma et al 2024).Small-scale fires and CRB span over several days to months and thus result in poor regional air quality and ecosystem

Findings from previous reviews
A thorough understanding of the nature, composition and impacts of ABB emissions is necessary to formulate effective environmental regulations to help mitigate the persistent air pollution problem.In this section, some of the findings from India and other countries, along with suggested recommendations discussed in previous review works, are summarized.A review assessment by Dutta et al (2022) attempted to identify the root causes of the unlawful practice of excess PRB in India, and thereby highlighted its negative impacts on the environment.In-depth analyses of approaches, environmental regulations, socioeconomic policy designs and future directions would provide numerous opportunities for the sustainable management of crop residues as paddy straws are high in nutrients and can be processed and transformed into value-added commodities; however, farmers still prioritize burning of the residue due to a number of significant factors, including a limited time frame for sowing wheat, a dearth of farm automated machinery, paucity of labour and a reduced willingness to use paddy straw as feed.Their assessment called for a collaborative endeavour from distinct fields, including farmers, industrial groups and communities, agrarian scientists and professionals from extended sectors, state government agencies, and other associated stakeholders, to operationalize current technologies and develop novel solutions to the issue.Singh et al (2021a) focused on rice straw burning and highlighted the alternatives to straw burning, including soil incorporation and electricity generation.They also explored the potential for sustainable use of agricultural waste, with life cycle assessments as one of the environmentally sound alternatives.Bhuvaneshwari et al (2019) explored the policy and technological obstacles impeding India's efforts to address CRB and suggested solutions that were overlooked in current policies.They emphasized the importance of socioeconomic variables for successful implementation and recommended institutionalizing procedures for collection, segregation and recycling.Lohan et al (2018) addressed the problem of paddy waste disposal in NW India.They discussed budgetary analyses and policy tools for sustainable residue management.Singh (2018) discussed the effects of stubble burning in Punjab and Haryana and strategies to reduce it.They stated that one metric tonne of straw burned discharges of 3 kg of particulates, 60 kg of CO, 1 460 kg of CO 2 , 199 kg of ash and 2 kg of SO 2 , which contributes significantly to global warming and acid rain.Lung illnesses, chest obstruction, eye discomfort and dry eyes are the primary problems that the locals deal with due to such high levels of pollutant emissions.They illustrated how the National Capital Region (NCR) of Delhi, Haryana and Punjab was severely affected by smog in October 2016 and November 2017, which further led to serious health implications, e.g.chronic obstructive pulmonary disease (COPD), pneumoconiosis, bronchitis, cataracts, corneal opacity and blindness.They also revealed that the road accident count also rose during the stubble burning time due to poor visibility.They also highlighted the importance of crop residue management to control this anthropogenic disaster.
The review studies on observations from China and Thailand show that BB is a major problem in these countries also.A comprehensive review of BB studies from China presents details of field measurements, lab experiments and the critical implications of BB observed in China (Chen et al 2017).They deduced that both secondary organic, as well as inorganic aerosols arising from BB, largely drive extreme haze events over China.To analyze the spatio-temporal variations of the fire count (FC) over south, southeast and eastern Asia, they retrieved the FC data from the Terra moderate resolution imaging spectroradiometer (MODIS) satellite for 15 years (January 2001-December 2015).They found much lower FCs over the Indian and Indochinese regions due to high rainfall during the summer monsoon season, whereas higher FCs were observed over the Siberian region due to seasonal forest and peat fires.Zhao et al (2017) used emission inventories of combustion of agricultural straw to estimate their critical impacts on air quality and visibility over China.For China, the suggested solutions include reusing and recycling of agricultural straw in the form of animal feed utilization, electricity production and economical bioethanol production, along with prescribed open burning for bulk disposal of the straw.
Suriyawong et al (2023) presented an overview of the status of BB and the PM released due to BB over various parts of Thailand.It is a country with a strong agricultural sector and, consequently, a lot of biomass waste accumulates, which is routinely incinerated outdoors by agricultural industries to supply energy.The recommendations for Thailand mainly involved the management and monitoring of forest emissions, biomass processing, transformation, application and usage.Adam et al (2021) integrated several viewpoints on BB-derived PM to describe its physical and chemical properties while highlighting its major environmental effects.They examined the basis for the detection and apportionment of PM during BB events in addition to their health effects.A series of reviews (Koppmann et al 2005, Reid et al 2005a, 2005b) were conducted on BB emissions and their properties.Reid et al (2005a) examined the literature based on the estimation and modelling techniques of the optical characteristics of BB particulates.They observed high variability in the optical properties due to the transient characteristics of fires, the dynamics of the transformational process of smoke and their estimation strategies.Reid et al (2005b) focused on the estimation of smoke fragment size, chemistry, characteristics and emission parameters.They demonstrated that extremely significant variations in measured particle characteristicsparticularly with regard to particle carbon budgetsare reported in the literature.Their analysis of emission unpredictability revealed that the emission factors (EFs) of boreal, temperate and a few tropical vegetations still have very high levels of ambiguity.Depending upon a probabilistic evaluation of an extensive data set of BB measurements, they offered concise models for particle size and emission parameters.Hodshire et al (2019) provided an overview of the ageing of BB-induced PM levels and source tracers.They found differences in the ageing of BB aerosol from laboratory experiments and outdoor field campaigns mainly due to: (a) fluctuations in fluxes and chemistry; (b) variations in dilution pace, splitting up and overall load; (c) line and chamber wall losses; and (d) differences in preliminary estimation time.Furthermore, an evaluation of the database on the EFs of PM available in the literature by Simões Amaral et al (2016) revealed that BB in compacted form, i.e. pellets, emitted lower particulate EFs, whereas there were elevated PM 2.5 emissions from the combustion of forest biomass.Their study also provided an increasing trend of EFs as: burning in combustors < residences < field combustion < laboratory burning.Wan et al (2019) provided a discussion of various analytical techniques for determining the typical organic acids produced from BB in airborne aerosols and frozen cores.Their distributions in different natural matrices, such as urban, woodlands, countryside, coastal and polar zones, were also shown and contrasted.They also used aromatic acid levels and ratios for the determination of the origin of various kinds of biomass.Additionally, the use of aromatic acids in ice as well as snow samples to replicate past records of BB emissions was discussed in their review study.To the best of our knowledge, no extensive review on the types and sources of BB-led air pollution in India has been conducted, and thus this review provides the much required information on this contemporary topic.

Overview of air pollution in India and BB-air pollution link
India, at present, is struggling with an air pollution crisis.High levels of atmospheric PM, particularly finer ones (PM 2.5 ), are the major source of airpollution-related illness burden and a reliable indicator of associated death rates globally (Balakrishnan et al 2019).For a number of years, Indian cities have consistently been among the top 20 most polluted in the world, surpassing the pollutants' standard limits set by the World Health Organization and the central Pollution Control Board (CPCB) (Garaga et al 2018).The characteristics and severity of air pollution episodes are a function of location-specific natural and anthropogenic processes.India is known for its diverse climatic conditions, land-use patterns and region-specific anthropogenic activities.Given its diversity, location-specific tailor-made strategies are required to mitigate air pollution.In this direction, the National Clean Air Programme (NCAP) was launched by the Ministry of Environment, Forest and Climate Change in 2019.The identification of 131 non-attainment cities, setting the targets for source apportionment (SA) studies and PM reduction, are some of the major landmark objectives of NCAP (Yadav et al 2022).Gradually, it was realized that as air pollutants undergo horizontal and vertical transport and transformation in the atmosphere, it will be more appropriate to follow the airshed approach (Sharma and Yadav 2023).Airshed-based management calls for integrated air quality management strategies across city/state borders to find solutions to the problem of air pollution (Guttikunda et al 2023).The severe haze episodes of Delhi are a good example of regional cross-border transport of pollutants in IGP (Singh et al 2023).The long-range transport of CRB emissions from NW India is noticed even in the eastern Himalayas (Mukherjee et al 2022a).Various sources of air pollution in India include secondary aerosols, vehicular emissions, biomass/refuse burning, industrial emission, dust, fossil fuel combustion and sea salt (Yadav et al 2022).Amongst various natural and anthropogenic sources, ABB (for field clearing (CRB), human-driven FFs, BB for cooking/heating/recreational purposes and open waste burning]) is one of the major sources of air pollution in India (Gunthe et al 2021, Mishra et al 2023).While poor air quality is a concern across the nation, identified seasonal hotspots of pollution in IGP, particularly in the NCR, are the result of multiple factors, namely, CRB, firecracker emissions during the Diwali festival and prevailing meteorology.In October-November months, PRB results in a very high PM 2.5 load in New Delhi (Bray et al 2019).The gradual increase in PRB from 30 million tonnes in 1960 to 50 million tonnes in 2016 is reported (Kumar et al 2019).In the Punjab-Haryana region, over the years, agricultural practices have changed with the transition from less water-intensive crops to high-yielding varieties of rice.As this region has an additional challenge of groundwater depletion, the enactment of groundwater conservation policies resulted in a delay of rice sowing in June (monsoon season) and harvesting in late October-November.Thus, the shift in crop pattern, low socioeconomic status and implementation of a recent groundwater conservation policy have led to large-scale stubble burning in November every year in this region, where the farmers are forced to clear the stubble in a short time window (Mukherjee et al 2023).The MODIS FC data corroborates continuous enhancement in CRB events from 2000 to 2010 and also a shift in the CRB peak after 2010 over NW India (Sawlani et al 2019).

Composition of particulates and gases in BB emissions
The BB emissions include primary particulates (aerosols) and their precursor gases (Hodshire et al 2019).The chemical composition of BB emissions depends upon the types, characteristics, amount and moisture of biomass fuels, fire behaviour, combustion stages (flaming/glowing/ smouldering) and several other parameters (Andreae 2019).Figure 2 shows species arising from BB emissions.The morphology, composition and characteristics of BB emissions alter with age, leading to alterations in optical properties and changes in their cloud-forming and ice-nucleating potential.The high time resolution study of the physical and chemical properties of fresh and aged BB emissions is an important topic of smoke particle chemistry research.Once airborne, the BB emissions age, transform and get transported; thus it is difficult to identify the exact contributions from specific distinct sources.Several factors, namely, the type of fuel used and its moisture content, the combustion conditions (flaming or smouldering) and other meteorological factors, affect the composition of BB emissions.Sub-micron and fine particulates dominate over coarse particles in BB emissions in India (Badarinath et al 2004, 2009, Nirmalkar et al 2019), while compositionally, the carbonaceous fraction predominates in BB emissions (Reid et al 2005b, Garbaras et al 2015).Due to smaller size fractions of particulates emitted from BB, evidence has shown that these particulates witness long-range transport (Sahu and Saxena 2015, Sawlani et al 2019, Lalchandani et al 2022, Mukherjee et al 2022b), especially from NW India due to CRB (Sawlani et al 2019).BB particulates in India are reported to make a significant contribution to the oxidative potential (OP) (∼37%) (Puthussery et al 2022), thereby, having substantial health implications.Moreover, during BB events, the BC input to total aerosol mass is ∼14% in India, which further has significant implications for the determination of radiative forcing due to such particulates (Badarinath et al 2009).Nonetheless, a dearth of spatio-temporal measurement data regarding combustion sources, their chemical properties and size distribution results in significant uncertainty in the effects of BB-derived aerosols on regional and global climates (Singh et al 2021b).The Fire Laboratory at Missoula Experiments (FLAME 1 and 2) were conducted in Missoula, Montana during 2006-2007, where 33 different plant materials were burnt.In each experiment, 50-250 g of fuel was ignited in a controlled laboratory environment and the reconstructed fine mass concentration was calculated by adding the concentration of emitted inorganic salts, elemental crustal dust, organic matter and light-absorbing carbon.The burning experiments of Asian rice straw during FLAME showed unique properties, such as high mass fractions (∼0.5) of salt, low light-absorbing carbon, bimodal volume size distributions and the lowest mass scattering efficiency (1.5 m 2 g −1 ) (Levin et al 2010).The EFs of CO were within the ranges 8-9, 11-12 and 14-29 gkg −1 from the burning of briquette, wood and cow dung cake, respectively, whereas those of PM were within the ranges 0.8-1.8,2.1-2.2 and 2.4-3.7 gkg −1 due to wood, briquette and dung cake burning, respectively (Venkataraman and Rao 2001).The EFs of PM are similar to that mentioned by Simões Amaral et al (2016).

Carbonaceous particulates
The carbonaceous fraction of particulates includes OC and elemental carbon (EC), where OC may contribute as high as 90% of the total carbonaceous particulates (Wang et al 2007, Hodshire et al 2019, Singh et al 2019).The BB events form the major source of primary carbonaceous particulates in the atmosphere and are responsible for 85% primary organic aerosols and ∼59% BC emissions globally (Bond et al 2004, 2013, Hallquist et al 2009, Andreae 2019, Wan et al 2019).The organic fraction consists of a myriad of species, namely, n-alkanes, polycyclic aromatic hydrocarbons (PAHs), aromatic resin acids (e.g.hydroxybenzoic, vanillic, syringic, dehydroabietic acid), anhydrosaccharides (levoglucosan, mannosan and galactosan from thermal degradation of cellulose and hemicelluloses), steroids, terpenoids and methoxyphenols from lignin and some minor components, namely, furan-derivatives such as 2acetylfuran and 2-furan methanol (Simoneit 2002, Yadav et al 2013a, 2013b).The byproducts of pyrolysis of major biomass fuel components (cellulose, hemicellulose, lignin, sporopollenin and suberin) are widely employed to trace the BB sources (Shi et al 2019b).Typically, lignin burning produces phenols, aldehydes, ketones, acids and alcohols, which preserve the hydroxyl and methoxy groups on the original benzene ring.Source tracers originating from lignin burning can be from softwood lignin (gymnosperms), hardwood lignin (angiosperms) or grass lignin.Pyrolysis of softwood lignin mainly releases methoxyphenol.Combustion of pine wood mainly emits vanillic acid and vanillin (along with small amounts of syringic acid, syringaldehyde and panisic acid) making it an excellent source tracer specific to the burning of coniferous woods (Li et al 2021).Likewise, hardwood lignin burning also releases methoxyphenol along with syringyl acetone, acetosyringone, syringic acid and disyringyl dimers.The smoke from oak wood burning has significant levels of syringic acid and syringaldehyde, and the syringyl molecular tracers can also be used as key identifiers of angiosperm smoke.Burning of hardwood, specifically, releases syringic acid, whereas dehydroabietic acid is a partially transformed byproduct of conifer combustion (Fu et al 2009, Wan et al 2019).Combustion of the lignin of grasses also emits a variety of organic source tracers, namely, n-alkanoic acids, levoglucosan and phenolic compounds, including syringyl acetone, catechol, guaiacyl acetone, dimethoxyphenol, syringic acid, vanillyl acetic acid, vanillic acid, p-anisic acid, p-anisaldehyde and minor amounts of other p-coumaryl, vanillyl and syringyl-type species and syringic acid.When subjected to combustion, cellulose decays in two different ways.At 300 • C, it initially decomposes, after which it starts to bind, cleave, undergo fission and finally, it starts to transglycosylate to generate tar anhydrous carbohydrates and volatile compounds.From a particular source, the second pathway generates molecular tracers such as levoglucosan, the furanose isomer and dianhydride (Simoneit 2002).The respective amount and different kinds of biomasses (such as hardwood, softwood and herbaceous plants) along with the photochemical oxidation mechanisms can be determined using the diagnostic ratios among aromatic acids (Wan et al 2019).Additionally, for the equivalent quantity of biomass burned, the ratio of the mass concentration of two aromatic acids is indicative of the biomass and thus more specific than concentration used independently for source determination.One of the most widely used diagnostic ratios is that of vanillic acid/syringic acid (Va/Sy) for the determination of BB sources of particulates (Fine et al 2004, Fujii et al 2015, Myers-Pigg et al 2016, Wan et al 2017).A further description of these ratios can be found in section 5.1.Levoglucosan is a commonly used source marker for BB emissions (Nirmalkar et al 2019, Sullivan et al 2019a, Jiang et al 2020, Pio et al 2022, Yadav et al 2022, Devaprasad et al 2023) and is said to be present in fine particulates arising from domestic wood burning.By pyrolyzing cellulose and hemicellulose, two isomers of levoglucosan known as mannosan and galactosan are formed, which are also used as BB source markers (Li et al 2021).Furthermore, PAHs, as one of the most harmful classes of airborne pollutants, are also emitted in the atmosphere during partial combustion of different biomasses (wood, etc) (Chen et al 2017).Terpenoid natural metabolites and their thermally transformed equivalents are also used as BB markers.Alkyl nitriles, alkanones, alkanedioic acids and other analogous lipid molecules that are simultaneously released, such as the wax derived from flora, are typically not specific to their source but nevertheless can be utilized to confirm BB contributions.Compared to levoglucosan, aromatic acids and their ratios are distinct BB tracers (Wan et al 2019).A laboratory experiment conducted on 56 laboratory burns confirmed that furan, benzofuran, 2-furaldehyde, 2-methylfuran and benzonitrile serve as potential BB tracers (Gilman et al 2015).

Inorganics
The fractional contribution of inorganic species remains generally low in BB emissions when compared to carbonaceous fractions.The inorganic components make up about 12%-15% of the composition of BB particulates, dominated by alkali earth species and halides that further include insoluble dust, ash, water-soluble ions (potassium and ammonium sulphate) (Bamotra et al 2022) and some inorganic acids (Reid et al 2005b).However, trace inorganic composition in fine and fresh smoke particulates is nearly 10%, which mostly includes potassium, chloride and calcium.The fresh BB emissions have high chloride content and low sulphate, nitrate and ammonium concentrations.However, with ageing, the chloride content decreases and an overall increase in sulphate, nitrate and ammonium takes place.One confirmed pathway is conversion of potassium chloride (KCl) into potassium sulphate (K 2 SO 4 ) and potassium nitrate (KNO 3 ) (Yokelson et al 2009).It is also noted that in the presence of excess ammonia, emission of gas-phase hydrochloric acid from the burning of waste containing plastic may result in reduced visibility.The emission of highly water-absorbing chloride from these sources results in increased water uptake and thus particle growth, leading to poor visibility and haze episodes (Gunthe et al 2021).The non-sea-salt (nss) water-soluble potassium is a commonly used marker for BB.It is generally emitted in the flaming phase; however, other BB tracers like levoglucosan are emitted at all stages.This is one of the reasons that leads to poor correlation between nss K + and other tracers (Sullivan et al 2019a).The inorganic content, particularly the salts present in particles, arising from BB emissions influences their hygroscopic behaviour and thus the fractional contribution of the inorganic content is important to understand the implications of BB emissions.It has been observed that as the ratio between the total carbon to inorganic ions increases, the hygroscopicity decreases (Carrico et al 2010).The higher inorganic content (ammonium chloride (NH 4 Cl), KCl, sodium chloride (NaCl) and aluminium oxide (Al 2 O 3 )) of Asian rice straw burning during FLAME indicates that these particles may readily act as cloud condensation nuclei (CCN).The noted higher hygroscopicity, i.e. water uptake capability, of Asian rice straw is responsible for a greater influence on visibility reduction and haze formation (Levin et al 2010).

Gaseous emissions
Numerous hazardous gaseous pollutants are released during BB (Chen et al 2017).Globally, BB is the second-largest source of trace gases in the air (Bond et al 2004) with substantial contributions of carbon dioxide (CO 2 ), methane (CH 4 ), molecular hydrogen (H 2 ), nitrogen oxide (NO x ), methyl chloride (CH 3 Cl), carbonyl sulphide (COS) and numerous volatile organic compounds (VOCs) from worldwide BB emissions (Crutzen et al 1979).Gaseous species arising from BB play an important role in tropospheric and stratospheric chemistry.However, the accurate estimation of gaseous emissions from BB is again difficult due to the complex mixing, cracking and oxidation reactions (Koppmann et al 2005).Nearly 90% of the BB carbon gets converted to either CO 2 or CO by oxidation reactions (Reid et al 2005a).Due to mixing with the surrounding air and photochemical processing, especially for higher or reactive VOCs with narrow atmospheric lifespans, VOC levels and dynamics change as a plume ages (Koppmann et al 2005).For instance, the most prevalent VOCs produced during the burning of sugarcane stalks are ethane, ethene, propane, ethyne, n-butane, propene, i-pentane, n-pentane, toluene, and i-butane and C 2 -C 4 hydrocarbons.Benzene and toluene are major VOCs released from barbecues (Chen et al 2017).Moreover, higher VOCs significantly contribute to the total VOCs emitted during BB, thereby having impacts on the overall photochemistry in the atmosphere.Aromatic species, i.e. benzene, toluene, ethyl benzene, phenol, etc, and methane, aldehydes, methanol and furans are released into the atmosphere during the burning phase at higher temperatures (250 • C-500 • C) when the polymer matrix of wood gets disintegrated.Furthermore, CO released from partial BB can be dispersed over a large area and utilized as a marker of BB plumes in the upper troposphere due to the relatively prolonged life span of CO in the troposphere (Koppmann et al 2005, Mafusire et al 2016).The long-term (2001-2017) mercury emissions inventory shows that a high average annual amount (497 Mg) of mercury is emitted from BB in tropical continents with 41%, 31% and 28% contribution arising from Africa, Asia and the Americas, respectively (Shi et al 2019a).
The chemical fingerprints provided by directly released and thermally transformed molecular markers from BB are specific to their respective sources and are thus helpful for distinguishing contributions from the burning of single or several plant species in particulates (Simoneit 2002).To track BB emissions and aid SA, various chemical tracers, such as potassium (K + ), levoglucosan, mannosan and galactosan, are used in numerous studies carried out in Southeast Asia (Simoneit 2002, Adam et al 2021).Levoglucosan and K + are both typical tracers for BB that are widely employed to understand the source origin of particulates (Kaushal et al 2018, Shi et al 2019a, Devaprasad et al 2023, Li et al 2023).Moreover, compared to the typical inorganic tracer K + , the organic tracers in BB emissions are more source-specific (Simoneit 2002).

Methods used in India and globally to study BB emissions
The approaches/methods used to investigate BB-led air pollution depend upon the overall objective of the study.The field measurements followed by laboratory analysis and statistical tools allow quantification of BB tracers to understand the contribution, composition and sources of BB emissions.While the multiple satellite tools and model simulations help in understanding the extent and pattern of BB events and their implications on optical properties (Zhang and Smith 2007), the combination of EFs along with field data on the type of crop burnt, production statistics, etc, also help in determining the BB-led air pollution events (Streets et al 2003).

Ground-based field and laboratory measurements
As a result of convoluted atmospheric mechanisms, such as photochemical ageing, phase separation/partitioning and deposition, as well as varying chemical life spans, typical tracers as well as particular ratios are limited in their ability to target transported pollution sources and define the inputs from respective biomasses.Isotopic values, e.g.δ 13 C and δ 15 N, can be used to identify different BB categories (Agnihotri et al 2015).For instance, mean δ 13 C values of ∼ −26‰ and −12‰ typically indicate burning of C3-and C4-type biomasses, respectively (Martinelli et al 2002), whereas the average δ 15 N values of ∼11.5‰, ∼10.5‰ and ∼14.5‰ substantiate C3, C4 plants and cow dung cake combustion, respectively (Agnihotri et al 2015).Isotopic ratios help in accurate source identification.However, very few studies from India have used isotopic concentration and ratios to decipher types of BB events (Pavuluri et al 2010, Agnihotri et al 2015, Sawlani et al 2019, Devaprasad et al 2023).The reported δ 13 C value for PRB emissions over Patiala was −28.9 ± 1.1‰ (Devaprasad et al 2023), while Sawlani et al (2019) observed lower average δ 13 C values (−26.4 ± 0.5‰) during Delhi smog 2016 in comparison to normal conditions (−25.8 ± 0.3‰).Determination of ratios between different organic tracers is the most prevalent and fundamental method for determining the type of BB in the atmosphere.Five important ratios used include the ratios of: (1) levoglucosan to mannosan (Lg/Mn) (Schmidl et al 2008); (2) levoglucosan to OC (Lg/OC) (Pio et al 2008); (3) levoglucosan to potassium (Lg/K + ) (Devaprasad et al 2023); (4) vanillic acid to syringic acid (Va/Sy) (Fine et al 2004, Fujii et al 2015, Myers-Pigg et al 2016); and (5) vanillic acid to p-hydroxybenzoic acid (Va/Hyd) (Thepnuan et al 2019).However, these mutually exclusive ratios pose a challenge in determining the sources at times, particularly if there are multiple emission sources (Wan et al 2019).Moreover, higher Va/Hyd ratios pointing towards higher contributions from vanillic acid than p-hydroxybenzoic acid suggest coniferous burning (Otto et al 2006, Iinuma et al 2007).Iinuma et al (2007) reported a Va/Hyd ratio of 8.75 for pine wood burning in Europe.The Va/Hyd ratio was also estimated in a Eurasian Arctic ice core (Grieman et al 2017), which correlated very well with fires in Siberian forests and tundra lands, especially the three peaks of significant preindustrial burning, which were fairly detected in the ice core data.The ratios of BB tracers to OC and EC concentrations are also widely used to trace emissions from various kinds of BB (Bari et al 2009).Rastogi et al (2014) found a good linear correlation between OC and K + and a distinct OC/K + ratio of∼16 in Patiala, Punjab, India.Other carbon-based diagnostic ratios for identifying BB sources include K + /OC, K + /EC and char-EC/soot-EC.To begin, K + /OC and K + /EC can be used to detect BB.For instance, in a study by Kaushal et al (2018), the nssK + /OC and nssK + /EC ratios ranged from 0.04 to 0.09 and 0.06-0.01,with an average of 0.06 and 0.09, respectively, at Pohara; the nssK + /OC ratio is comparable to that found for tropical forest wood burning, whereas nssK + /EC is somewhat comparable to or higher than that for wood and coal combustion.However, at Dharamshala, nssK + /OC implied a similar source to that for Pohara, whereas the nssK + /EC ratio spanned the range of 0.04 to 0.20, with an average of 0.09, suggesting coniferous wood and coal burning for heating.Furthermore, a high char-EC/soot-EC ratio in Pohara confirmed that BB is dominant as char-EC is considered to be emitted mostly by BB and is employed as an important BB tracer.For domestic coal burning in Xi'an, China, char-EC/soot-EC ratios ranging from 1.50 to 3.00 were reported (Cao et al 2005).A strong and positive correlation was seen among K + , OC, EC and water-soluble organic carbon (WSOC) during the BB events, pointing towards a notable contribution of CRB to carbonaceous aerosols during the BB episode.In Cao et al (2016), the OC/EC, K + /OC, K + /EC and WSOC/OC ratios for agricultural-residue combustion emissions were found to be 10.4 ± 0.5, 0.054 ± 0.006, 0.57 ± 0.07 and 0.48 ± 0.01, respectively.The OC/EC ratio of 10.4 was more than that for the WRB (3.0), whereas it was similar to that for PRB (10.6) (Cao et al 2016).Figure 3 shows a schematic of important chemical constituent/source tracerspecies-based diagnostic ratios to confirm BB sources and to further decipher the type of biomass burnt.The most common method to identify the BB source uses the OC/EC ratio (<4-5 indicates fossil fuel burning, whereas above 5 generally signifies BB emissions) (Cao et al 2005).Rajput et al (2014) reported an OC/EC ratio of ∼10.6 and 3 for paddy and WRB, respectively.A Lg/K + ratio of 0.1-0.2 is reported for CRB in Asia (Cheng et al 2013, Mkoma et al 2013) and ratios of 0.38-22 and 4.35-58.8are reported for FF smoke of the Amazon and U.S., respectively (Fine et al 2004).Pashynska et al (2002) reported a Lg/K + ratio above 40 for CRB, whereas this ratio between 15-25 and 3-10 indicated hardwood and softwood burning, respectively (Schmidl et al 2008).The ratio of Va/Sy is also used to trace the type of burning, and if it is within 0.40-5.03, the burning of herbaceous plants is reported.However, Va/Sy ratios within the ranges of 0.12-4 and 8.57-11.9indicated hardwood and softwood combustion, respectively (Wan et al 2019, Li et al 2021).Furthermore, with variation in the ratios of nssK + /OC and nssK + /EC, the type of BB can be determined.Ratios of nssK + /OC and nssK + /EC varying from 0.03-0.06and 0.04-0.20,respectively, are reported for tropical wood burning and for coniferous wood and coal burning, respectively (Mkoma et al 2013, Kaushal et al 2018), whereas pine wood burning showed an nssK + /EC ratio of 0.20 (Schauer et al 2001).However, only limited number of studies targeting the quantification of organic tracers have been carried out in India (Kaushal et al 2021, Huma et al 2016, Yadav et al 2020 and references therein).

Studies using satellite products/reanalysis datasets
Remote sensing is widely used in almost every field of science, and it is the most feasible method for detecting and estimating the energy released and pollutants emitted from large-scale BB.Some of the available products are capable enough of detection of both FCs as well as emissions from BB.Some of the routinely used satellite products/reanalysis databases for detecting BB events through the detection of fire, emissions, and both fire and emissions in India and other countries are given in figure 4 along with their resolution and timelines for data availability.However, it is pertinent to mention here that the satellites mentioned in figure 4 are not the only satellites used to measure BB emissions globally; also, all BB-emitted species cannot be measured using a satellite-based approach.To detect emissions, the products include: (i) cloudaerosol lidar and infrared pathfinder satellite observations (CALIPSO); (ii) the ozone monitoring instrument (OMI); (iii) measurement of pollution in the troposphere (MOPITT); (iv) modern-era retrospective analysis for research and applications (MERRA); and (v) the infrared atmospheric sounding interferometer (IASI).To detect fire and emission, VIIRS, MODIS and fire INventory from NCAR (FINN) are commonly used,.Meanwhile, to detect only fire, the available satellite products include: (i) the medium resolution imaging spectrometer (MERIS;) (ii) the global fire emissions database (GFED); (iii) the along track scanning radiometer (ATSR); and (iv) global burnt area satellite products (GBASP).Reanalysis products are comprehensive datasets that assimilate observations from various sources into numerical models of the Earth's atmosphere.These products provide a coherent and consistent record of the Earth's observation over time, incorporating data from multiple instruments, satellites and groundbased observations.In figure 4, MERRA-2 is the only reanalysis product, while all the other products are satellite-based.Some of the products, such as FINN and GFED, are slightly different from pure satellite products and can be called merged products.For instance, FINN, a model-based fire emissions inventory, provides estimates of BB emissions, including wildfires and prescribed burns, on a global scale.It utilizes modelling techniques to estimate the amount of PM and trace gases released into the atmosphere due to BB events.Meanwhile, GFED is a database that provides information on global fire emissions.The GFED is a collaborative effort that compiles and disseminates data on BB emissions, including wildfires and prescribed burns, on a global scale.It incorporates satellite data, ground-based observations and models to estimate the amount of trace gases and particulate matter released into the atmosphere due to fire events.
These products aid in understanding BB emissions through variables such as the aerosol optical depth (AOD), absorbing AOD (AAOD), aerosol index (AI), Angström exponent (AE), single scattering albedo (SSA), datasets for precursors and trace gases, vertical profiles, FCs, fire radiative power (FRP), smoke-plume properties, long-range transit and mapping of burnt regions (Kharol and Badarinath 2006, Kaskaoutis et al 2011, 2014, Singh et al 2017, 2022, 2023, Shaik et al 2019, Dutta and  Chatterjee 2021, Kant et al 2022, Montes et al 2022).The MODIS onboard the Terra (EOS AM-1) and Aqua (EOS PM-1) satellites are widely used to detect BB activity around the world (Chen et al 2017).The daily global AOD, fine mode fraction and AE across land (0.47, 0.55 and 0.66 µm) and ocean (0.47, 0.55, 0.65, 0.86, 1.20, 1.60 and 2.10 µm) can be retrieved by MODIS (Terra + Aqua).Particulate types, such as urban/industrial, BB, dust, urban mixed, can be examined on the basis of the link between AOD vs the fine mode fraction and AOD vs AE (Kaskaoutis et al 2012).Furthermore, the fluctuation in brightness temperatures acquired from MODIS data of the 4 and 11 µm channels is used by the fire-detection system to identify the fire spots over the globe (Giglio et al 2003).Throughout the summer monsoon, FCs are much lower in India and Indo-China due to heavy rainfall, but much higher in Siberia due to seasonal forest and peat fires.The number of fires in the eastern area of China is extremely high, particularly during the winter and spring.Arid regions, such as the Tibetan Plateau, central and western China, have far lower FCs (Huang et al 2012).The seasonal mean spatial profile of AOD550 demonstrates that FCs have no effect on aerosol load and seasonal scale distributions.Infrequent fires and their resulting plumes are proven to have a major effect on regional AODs, but not on seasonal particulates.As a result, the seasonal mean AOD spatial pattern over South and East Asia is largely influenced by local anthropogenic pollution and the huge impact of sand and dust storms during the spring and summer.However, at the local level, particularly in India and China, greater BB during the winter and spring appears to alter AOD distribution (the lower AODs are linked with significantly lower FCs during the summer).The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) is a CALIPSO satellite tool that helps in understanding the vertical profiles of particulates and clouds at 532 and 1064 nm, as well as the linear depolarization profile at 532 nm (Winker et al 2009).The CALIPSO level 1 records at 532 nm (version 3.30) are extensively used to gauge the long-term movement of BB particulates, yielding data primarily on the levels and types of the smoke plumes (via attenuated backscatter coefficients and volume depolarization ratios), vertical profiles and the infusion height (Kaskaoutis et al 2014).Additionally, the CALIOP products may distinguish between aerosol types by considering smoke in their retrievals, allowing quicker detection of BB aerosols (Chen et al 2017).

Implications of BB emissions
The globally prevalent phenomenon of BB emits huge amounts of gaseous as well as particulate pollutants, posing a serious threat to the overall ecosystem by influencing the climate, visibility, hydrological cycles, the Earth's radiation budget and human health.Three broad categories of implications are as follows.
6.1.Implications on climate and Earth's radiation budget BB emissions influences climate processes at the global to regional scale.Plant and agricultural straw fires alter the chemistry of the atmosphere, which affects climate change and the water cycle by modulating surface energy fluxes and Earth's radiation budget (Randerson et al 2006).Even a minor increase in global temperatures may lead to ecosystem shifts and biome reorganization, affecting the natural fire regime (Temoltzin-Loranca et al 2023).Globally, BB emissions are responsible for 1/5 of CO 2 and other greenhouse gas (GHG) emissions, including FFs, slash-and-burn farming and combustion of wood debris (Jacobson 2014).In contrast to GHGs, which are consistently dispersed and stay in the atmosphere for long periods of time, carbonaceous aerosols produced by BB activities are dispersed unevenly.These aerosols can have a localized climate impact, affecting certain places rather than the entire globe.Furthermore, their impact on the climate is more complicated, with multiple mechanisms and interactions at play (Chen et al 2017).Depending upon the composition, the particulates arising from BB lead to surface cooling and heating, resulting in decreased convection and development of convective clouds.However, BB-induced PM escalates the number of CCN and thus leads to a decrease in the size of cloud droplets.This results in the beginning of precipitation much higher above the cloud base compared to clean atmospheric conditions (Keywood et al 2013).BB particles are externally mixed and have complex hygroscopic features.Measurements also show that plants growing in saline soil may emit more hygroscopic particles upon burning, at least at sufficiently high fire temperatures (Petters et al 2009a).BB particles enhance ice nucleation (Petters et al 2009b) and are important for cold cloud formation.BB emissions contain ice active minerals and, as the smoke ages, the organic carbon coating around the mineral active sites gets removed and thus the ice nucleating ability of BB particulates enhances upon ageing (Jahl et al 2021).One of the major direct forcing factors for global warming in BB aerosols is the BC fraction, which largely controls the absorption of visible solar rays (Jacobson 2001).The light-absorbing smoke plumes typically trap energy from the sun, heating the middle and lower atmosphere while decreasing net radiation at the surface of the Earth (Keywood et al 2013).Additionally, by disrupting the Sun's rays and serving as the CCN, smoke particles cool the Earth's surface, stabilize the atmosphere as a whole, lower evaporation and alter the ever-evolving and microphysical behaviour of the cloud, potentially inducing feedbacks on cloudiness, rainfall and the water cycle, as well as global circulations (Lee et al 2014a(Lee et al , 2014b)).Evidence from studies conducted in India also shows an inverse relationship between AOD and ultraviolet erythemal (UV ery ) radiations reaching the Earth's surface during BB events (Madhavi Latha et al 2004).The Southeast Asian carbonaceous aerosol layer, also referred to as an atmospheric brown cloud, is observed to be partially accountable for surface cooling, decreased evaporation, a latitudinal imbalance in sea surface temperature, prolongation of the onset of the Asian monsoon and local drought, with BB playing a significant role in this process (Ramanathan et al 2005).Because BB emissions contain several chemically reactive compounds, they are capable of having a considerable impact on atmospheric chemistry.Tropospheric ozone (one of the powerful GHGs) formation is reported to be induced by the ageing of BB plumes, which emit numerous pollutants, including the ozone precursors (NO x , CO and VOCs) produced in overabundance by fire emissions, thereby allowing more atmospheric routes for the generation of ozone (Real et al 2007).The efficiency with which BB emissions generate ozone is determined by dilution and physicochemical conversion during transportation by air, as well as due to the combination of various other emission sources.BB is reported to account for ∼10%-25% of the net worldwide photochemical generation of tropospheric ozone (Granier et al 2000).Throughout the drier months, elevated ozone plumes cover vast areas of the tropical and subtropical continents, with levels approximately three times greater than the ambient level.However, the impact is exacerbated over the Atlantic by concurrent upsurges in human-induced NO x emissions in comparatively pristine locations (Reeves et al 2010).The production of tropospheric ozone is seriously concerning as it is reported to impose an additive radiative forcing of 0.35 Wm −2 (Monks et al 2009).The ageing of a BB plume allows the accumulation of secondary pollutants, ozone and highly oxygenated substances, such as secondary aerosols (sulphates, nitrates and organics) (Yan et al 2008).Because of the coverage with water-soluble substances, aged smoke particles have increased cloud condensation activity; condensation of VOCs with numerous functional groups generated during combustion tends to be a primary component of secondary organic aerosol.BB emissions impact tropospheric oxidants, particularly the hydroxyl radical, which is crucial to the lifespan of CH 4 , tropospheric ozone and halogens, hence influencing ozone levels in the stratosphere.The methyl bromide (CH 3 Br) released by BB also contributes to the degradation of stratospheric ozone, which is highly crucial for survival on Earth's surface.

Haze development and visibility impairment
A haze event is characterized by visibility of less than 10 kilometres and relative humidity below 90% for more than four hours (Du et al 2011).The reduced visibility in the atmosphere is primarily attributed to a high aerosol load (Chen et al 2017).The transport of a smoke plume in the urban environment leads to increased secondary aerosol concentration, aggravating haze pollution and/or enhancing the probability of substantial haze pollution due to the interplay of chemical and physical reactions in static weather (Ding et al 2016).Furthermore, using a decade-long (2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014) dataset by the Weather Research and Forecasting model along with a BB aerosol module, operated by high-resolution BB inventories, Lee et al (2017) employed surface visibility and PM levels estimations over the Southeast Asian region.They discovered that almost all instances with extremely poor visibility (<7 km) during the last ten years were caused by fire particulates.In Southeast Asia's biggest cities, fire aerosols solely accounted for a significant portion of low-visibility occurrences (visibility <10 km): as much as 39% in Bangkok, 36% in Kuala Lumpur and 34% in Singapore.BB in Sumatra is a significant source of fire-produced PM 2.5 in Kuala Lumpur (50%) and Singapore (41%), while BB in mainland Southeast Asia contributed the most to the total fire-produced PM 2.5 in Bangkok (99%).Fire episodes were observed to be accountable for about 50% of the overall haze exposure events.During dry seasons, land clearing is carried out by burning the lowland forests as well as plantations and, further, the presence of peat leads to a large amount of smoke as it sustains the smouldering fires for a longer duration.Severe haze episodes were reported in Indonesia in 1997 and 2013.The analysis of molecular markers and the diagnostic ratios of specific organic substances reported during a Singapore haze episode confirmed that the BB emissions arising from Sumatra and Kalimantan were transported to Singapore, causing the Singapore haze episode and a reduction in visibility in 2006.An El Nino-induced drought further aggravated BB emissions, resulting in a severe haze episode in Singapore in 2015 (Wiggins et al 2018).It was reported that peat burning in Sumatra and Borneo significantly (∼85%) contributed to high aerosol load in Singapore.Puthussery et al (2022) reported prominent haze with low visibility (<10 km) during a post-monsoon interval over Delhi, India.The primary BB emissions in Delhi have distinct effects on the development of nanoparticles at night (Mishra et al 2023).The primary organic vapours are driven out of equilibrium by increased BB emissions along with the abrupt drop in temperatures at night.This causes the vapours to condense and develop into haze-causing nanoparticles.Wherein low-volatility vapours generated during the day through oxidation lead to hazy conditions by stimulating the proliferation of particles.In October-November, episodic increases in PRB and emissions from firecrackers along with stable meteorology lead to severe haze episodes in NW India, particularly in Delhi.However, BB-led haze episodes are reported from other parts of India as well.A dramatic decline in visibility (3.8 km) was noticed during a BB episode in Dochhumahmum, Mizoram (Badarinath et al 2004).

Implications on human health
Health implications (respiratory and cardiovascular diseases) and the associated high mortality risk from exposure to BB emissions are well known (Karanasiou et al 2021).Various methods of transformation, such as thermal, chemical, electrochemical and biochemical, are frequently employed for conversion of biomass into alternative energy sources during burning.The gases, i.e.CO 2 , CO, VOCs and NO x , and particulates are released by biomass during conversion (Zhang and Smith 2007).BB in the open atmosphere releases a variety of PAHs, which are typically released during the partial combustion of biomass, thereby adding to the particulate load (Singh et al 2013).Particle-bound PAHs are considered to be a serious health risk to humans as they are primarily attached to dust particles in the air (Kim et al 2013) and can reach the human respiratory system through inhalation.The particulates settle in various parts of the respiratory tract during inhalation and exhalation, contingent upon their size, form and their velocity.For instance, these fine particulates can descend to the alveoli of the lungs and can become further absorbed into the bloodstream.The tiny particles penetrate the alveolar epithelial wall and enter the alveolar sacs.Following this, the particles pass through the interstitial space and into the lung capillary's blood line.Particulates are typically removed from the bronchial wall by mucociliary elimination after deposition.The degree of exposure (e.g.duration of exposure), the level of toxins while exposed, the level of toxicity, the mode of exposure (e.g.ingesting, inhaling or dermal contact), age and previous medical history will all primarily determine the health impacts.Particulate inhalation can cause lung cancer and a number of respiratory illnesses, including COPD, long-term bronchitis, cystic fibrosis, asthma and respiratory allergies (Chen et al 2017).Air pollution is believed to be the major catalyst of the rise in lung cancer, COPD and asthma cases over the last few decades, particularly throughout developing nations.More than 2.4 billion households worldwide rely on biomass for cooking or heating, and over 90% of rural residents in low-and middle-income nations get their energy from solid fuel sources such as wood, agricultural waste, burning charcoal and coal (Smith et al 2004).Furthermore, solid fuel is reported to be responsible for nearly 3.55 million deaths every year.Elderly people, women and children are the most vulnerable to the emissions of indoor BB pollutants, while women are more highly impacted due to more exposure to kitchen smoke and more fragile airways, rendering them more susceptible to COPD (Ali et al 2021).However, men are more likely than women to develop lung cancer and COPD as they are more likely to become hooked on tobacco (Chen et al 2017).The levels of the pollutants released from the burning of solid fuels are two to three times higher indoors.Through the induction of several toxicity pathways, including oxidative strain, DNA methylation and gene stimulation, these pollutants can result in a multitude of health hazards.Lower birthweights, acute illnesses of the respiratory tract, anaemia and early fatalities are more common in exposed children (Ali et al 2021).Johnston et al (2012) demonstrated that landscape fire smoke attributed to nearly 339 000 deaths per year.The chemical reactions in BB plumes lead to the addition of several chemicals (direct products as well as byproducts) in the atmosphere.The high concentrations of NH 3 and NO 2 in the BB plume promote sulphate synthesis through aqueousphase processes, where nitrous acid (HONO) is produced as one of the byproducts (Nie et al 2015); HONO is carcinogenic in nature, and its chemistry results in the production of carcinogenic nitrosamines (Monks et al 2009).A few studies from India also revealed the critical impacts of CRB on the lung functions of healthy populations, especially in children.Elevated concentrations of PM 2.5 and PM 10 pertaining to agricultural CRB have negative and unrecoverable effects on the pulmonary function tests of small children, rendering them at much higher health risks (Awasthi et al 2010, Saggu et al 2018).

Meta-analysis of BB (%) contribution in SA studies from India
The offline SA studies involve three steps: field sampling, laboratory-based chemical analysis and receptor modelling.Here, to understand the BB (%) contribution in PM mass at different locations, the meta-analysis of source data obtained from SA studies discussed in Yadav et al (2022) is presented.Only those studies where complete SA is performed and the BB (%) contribution to the total PM load is clearly given are discussed here.This meta-analysis provides insights into the fractional contribution of BB to the total PM load over India; however, the observations are restricted to a location and the given time period of the study.Figure 5 shows the BB (%) contribution in five categories: <10 (%), 10-14 (%), 15-19 (%), 20-24 (%) and >24 (%).To assess the BB contribution to PM 10 , SA data from 15 studies was analysed: three from IGP (Mehta et al 2009, Murari et al 2020, Saggu and Mittal 2020), five from Delhi (Sharma et al 2014, 2016, Gupta et al 2018, Jain et al 2018, 2020), four from Peninsular India (Gummeneni et 5(c)).A study on the effect of BB emissions on non-refractory PM 2.5 composition in Delhi found clear evidence of long-range transport of CRB in adjoining states as a major cause of haze episodes in Delhi (Lalchandani et al 2022).The BB contribution of ∼45% to organic aerosols in Delhi, India (Lalchandani et al 2022) is considerably higher than that reported in three European cities (Athens (15.3% BB), Paris (15.1% BB) and Zurich (10.6% BB))n a similar study conducted by Chen et al (2022).Metaanalysis of the BB (%) contribution in SA studies from India shows substantial fractional contribution from BB emissions in India.Up to 33% BB contribution in PM 2.5 was noted in SA studies from India (Yadav et al 2022), while a comprehensive SA study from 11 European and Asian cities found 16% BB contribution to the PM 2.5 load.(Almeida et al 2020).Also, due to the unique BB emissions and their complex implications on human health, climate and the overall ecosystem, it is important to focus on BB-led air pollution in India.

Studies on BB emissions from different parts of India
The ground-based studies pertaining to BB emissions from India have mostly focused over the IGP with few studies from other parts of India.However, the satellite-based studies provide a good understanding of BB emissions over India.A total of 48 studies, as listed in table 1, are reviewed here to understand the spatio-temporal variability of BB events over different locations in India.Overall, the majority (>50%) of these investigations were ground based, where offline/online measurements of PM and its constituents, along with monitoring of BC and gases, were conducted to understand the role and contribution of BB to total particulate and gaseous concentrations A detailed discussion on their findings follows.

Multiple site studies from IGP
To examine the effects of WRB, 16 air pollutants (PM 10 , PM 2.5 and PM 1 , ozone (O 3 ), CO 2 , CO, oxides of nitrogen (NO, NO 2 and NO x ), SO 2 , NH 3 and VOCs) were measured in eight cities of North India in 2019 (Ravindra et al 2023).The System of Air Quality and Weather Forecasting and Research mobile laboratory was used to continuously monitor the ambient air pollutants and other weather-related parameters.The average concentration of PM and VOCs was high in semi-urban areas, NO x was high at an urban location and NH 3 peaked at a rural site.The data acquired from NASA/NOAA Suomi-National Polar-orbiting satellite (https://firms.modaps.eosdis.nasa.gov/) with a VIIRS sensor aboard showed 21 509 and 7006 fires in Punjab and Haryana, respectively.Under the Aakash project, a comprehensive campaign was led by RIHN along with various institutions in IGP in September-November 2022 (Singh et al 2023).To capture the influence of CRB emissions, a dense network of 32 compact and useful PM 2.5 instruments with gas sensors and 7 P-sensors was deployed in Punjab, Haryana, Delhi NCR and western Uttar Pradesh.They observed that PM levels were the highest in the source region (Punjab), followed by the intermediate region (Haryana) and Delhi NCR.They revealed that CRB pollutants not only result in poor air quality at the source, but also have a direct effect on the regional air quality of the whole IGP.This study confirmed that the alarming PM 2.5 concentrations with hourly means of 1000-1400 µg m −3 and even the daily mean up to 900 µg m −3 in IGP is a result of CRB plume transport and stagnant meteorological conditions.Montes et al (2022) focused on the contributions from WRB and PRB in the IGP (the North-Indian states of Punjab, Haryana and Uttar Pradesh).They used data for FCs from a multi-spectral sensor VIIRS and, to retain only cropland FCs, a MODIS MCD12Q1 V06 land cover product with 500 m × 500 m spatial resolution was used.The MERRA-2 atmospheric reanalysis data were used to understand the seasonal trends in PM 2.5 surface levels and their link with meteorological parameters.In addition, biomass burned and crop production statistics and hourly PM 2.5 data from CPCB were used.The analysis showed the occurrence of two peaks of FCs and FRP coinciding with the burning of rice and wheat residues in October-November and April-May, respectively.Wheat and rice residue burning generally takes place in preand post-monsoon season, respectively.The postmonsoon rice residue burning period showed higher FC (79 385 fires) in comparison to the pre-monsoon WRB period (33 096 fires).Similarly, emissions were higher in rice (406 Gg yr −1 ) when compared to wheat (245 Gg yr −1 ).This study revealed a high correlation between PM 2.5 concentrations and the frequency of fires, suggesting that the farming of wheat and rice may be a reliable indicator of seasonal discrepancies in particulate concentrations.The episodes of WRB show a very strong correlation with positive irregularities in particulate level, emissions and fire incidences (Montes et al 2022).Singh et al (2022) investigated aerosol optical properties from 2004 to 2018 using MODIS (both on Terra and Aqua satellites) data and Level 3 OMI data (OMAER-UVd_v003) of SSA and AI during months with predominance of BB events (October-November) and the dust period (April-June) at three locations over IGP: Ballia, Kanpur and Jaipur.During the BB period, increased AOD was detected over all three locations with the highest AOD at Kanpur.Throughout the study period, Kanpur had the maximum AOD levels during the BB phase.In the BB phase, anthropogenic aerosols were identified to be dominant.Furthermore, prospective source areas and transport routes of these aerosols were analysed using a potential source contribution function (PSCF) and concentration-weighted trajectory (CWT) analysis.
The predominance of PSCF (>0.7) was noted from the NW direction during the BB period, and the CWT analysis confirmed that the elevated AOD during the BB period was getting contributions from IGP in addition to local areas.Saxena et al (2021) analyzed the effects of air pollutants released from CRB events in Haryana on the air quality of Delhi.The levels of both coarse and fine PM were two to three times higher than the threshold given by National Ambient Air Quality Standards (NAAQS), whereas NO 2 and SO 2 levels were well within the limits.As observed by MODIS fire counts, they showed that the rabi burning period witnessed approximately three times more highly intense fires than that during kharif burning in Haryana.Backward trajectories' analysis showed that the pollutants released from fires in Haryana reached Delhi through air masses, thereby affecting its air quality.Chandra and Sinha (2016)  levels.These findings implied that smoke aerosols from PRB predominate.They also found a change in the aerosol size distribution towards larger volume and smaller particle size, indicating the presence of fresh soot particles.The BB particulates were found to alter the atmospheric gas composition, optical and radiative aerosol properties, aerosol radiative forcing and climatic repercussions over northern India dramatically.

Single-site studies from Delhi, Lucknow, Kanpur and Agra
The real-time monitoring of non-methane volatile organic compounds (NMVOCs) from December 2020-May 2021 using the proton-transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) at an urban background site in Lucknow, India showed solid fuel combustion as one of the prominent sources (Jain et al 2023).The SA conducted using a multi-linear engine-positive matrix factorization (PMF)-model confirmed BB as the principal source of NMVOCs and secondary organic aerosol (SOA).The two major identified sources of organic compounds in Lucknow were traffic and solid fuel combustion.(Jain et al 2023).An interesting study (Mishra et al 2023) found that the low-volatility vapours arising from BB emissions in Delhi led to night-time new particle formation and hazy conditions.It is important to note that this haze build up is due to the BB taking place for residential cooking and heating purposes in the IGP.To determine the origins of condensing organic vapours, they applied PMF on gas-phase data collected from the filter inlet for a gases and aerosols chemical ionization mass spectrometer.
They also observed that small growing particles were almost entirely formed of organics, while the presence of NH 4 Cl in accumulation mode suggested condensation of NH 3 and hydrochloric acid.Non-refractory -PM 2.5 and BC measurements were conducted by Lalchandani et al (2022) to investigate the effects of BB emissions on the chemical makeup of PM 2.5 and the generation of secondary aerosols in Delhi during post-monsoon and winter haze events.They reported four unique haze periods in Delhi, each with a distinct chemical profile of PM 2.5 .However, the BB organic aerosol (BBOA) was the main component in all hazy periods, and it was predominantly present in an aged state with high O/C (0.71 and 0.46 at two sites).The authors also examined how BBOA and BC contributed to the total carbonaceous PM mass.They observed that the influence of BBOA and BC rose as PM levels increased.To understand the implications of BB emissions, fog events and Diwali fireworks on the OP of PM 2.5 aerosols, measurements were performed from October 2019 to January 2020 in Delhi (Puthussery et al 2022).This study showed that throughout the winter fog periods, BB particulates accounted for 37% of the OP; however, during the post-monsoon time span, traffic-related pollutants, secondary sulphates and oxidized organic particulates were the key determinants of the OP.Around the Diwali festival, fireworks-related emissions were a dominant contributor to the OP (up to 72% on Diwali eve).The study highlighted the need to comprehend the impact of periodical and episodic origins on the health-relevant aspects of PM 2.5 , including BB and Diwali crackers.In Agra, ground-based and MOPITT-retrieved CO observations indicated the influence of PRB in October and November during 2015-2019.The enhancement of CO by 20%-79% during the PRB period confirmed the long-range transport of PRB from NW-IGP to Agra (Kumari et al 2021).During the winter of 2018, the high time and mass resolved measurements of VOCs using two PTR-TOF-MS were conducted simultaneously in Delhi (an urban site) and at a suburban site in Faridabad suburb, India (Tripathi et al 2021).Out of the three episodic events discussed in this study, the second episode was found to have a strong (r 2 = 0.90) acetonitrile-CO correlation and a higher molar emission ratio of ∆acetonitrile/∆CO, confirming contributions from BB emissions.The enhancement in furfural, acetonitrile, CO, methanol and formaldehyde during Holi bonfire festival on 2 March 2018 indicated wood burning in the region.Sawlani et al (2019) revealed three major factors responsible for Delhismog-2016 episode: transport of CRB emissions from NW India, nearly stagnant meteorology and firecracker emissions following Diwali night.They analysed the total carbon, nitrogen and sulphur concentrations and their respective stable isotopic ratios, i.e. δ 13 C, δ 15 N and δ 34 S, using an elemental analyzer coupled with an Isoprime isotope ratio mass spectrometer.For the normal and smog periods, the average δ13C findings were −25.8 ± 0.3‰ and −26.4 ± 0.5‰, respectively.Slightly reduced δ 13 C values during smog could be interpreted due to supplementary carbonaceous matter contributions from CRB in Punjab and Haryana along with notable ageing processes pertaining to stagnant meteorological conditions.The average δ 15 N values were found to be 13.5 ± 7.1‰ and 16.9 ± 7.2‰ during normal and smog conditions, respectively.Secondary processes are reported to be the main cause of the rise in δ 15 N of PM 2.5 during the smog period.The δ 34 S showed average values of 3.8 ± 1.5‰ and 1.2 + 1.9‰ during normal and smog periods, respectively.The lower average values of δ 34 S during smog show similarities with sulphur emissions from firecrackers.Furthermore, the noted FCs here substantiate that after 2010, a delay (early October to late October or early Nov) in CRB is noticed.This may possibly be due to enactment of ground water policy rules in 2009.Shamjad et al (2015) investigated the chemical, optical and microphysical properties of aerosols in Kanpur to estimate the contribution of brown carbon (BrC) to direct radiative forcing (DRF) during February-March of 2013.The PMF analysis was carried out on high-resolution time-of-flight mass spectrometer (HR-TOF-AMS) data, and the value of the BB indicator (f60, the ratio of m/z 60 to total organic mass) was used to confirm BB episodes.
The BB episodes resulted in a spike in negative surface DRF and an increase (∼35%) in the absorption of solar radiation by particulates in the atmosphere.The average values of shortwave cloud-free open-sky DRF induced by overall particulates at the top of the atmosphere, surface and within the atmospheric column were −6.1 ± 3.2, −31.6 ± 11 and 25.5 ± 10.2 Wm −2 , respectively.The BrC was reported to account for 18% of the total DRF at the top of the atmosphere, 13% at the surface and 12% in the atmosphere.

Single-site studies from Punjab
In October-November 2014, diurnal variations in carbonaceous aerosols arising from PRB were studied in Patiala, Punjab (Devaprasad et al 2023).
Comprehensive measurements (dual carbon isotopes, chemical species (OC, EC, total OC, levoglucosan) and water-soluble organic fraction) showed the dominance of biogenic over fossil fuel sources.The higher f bio (0.85 ± 0.07) at night when compared to day f bio (0.80 ± 0.03) indicated more night-time PRB in the region.The abundances of BrC species increased with increasing f bio , whereas mass absorption efficiency did not suggest emission of those BrC which are not extra absorbing.Also, the mass normalized OP was found to increase with atmospheric processing/ageing of BB emissions.Singh et al (2021b) conducted measurements in Beas, Punjab, during October 2016, May 2018 and August 2018.
In addition, they burnt five biofuels (buffalo dung, shahtoot (Morus indica), guava (Psidium guajava), red saal (Shorea robusta), and kail (Pinus wallichiana)) and collected the emitted particles on a quartz filter.They observed that the carbonaceous particulate percentage in PM 2.5 was approximately twice for the post-monsoon season when compared to the pre-monsoon period.A greater input from secondary organic carbon (SOC) as an outcome of increased photochemical activity in dry environments was noticed in the pre-monsoon season.Carbon fractions of sampled aerosols revealed substantial contribution from biofuels.The C3 plantderived emissions were noted and also prevalence of night-time CRB was observed.Overall, the study emphasized BB's considerable role in enhanced carbonaceous particulates and levels of PM 2.5 in North India, with seasonal changes and the dominance of fresh aerosols from regional sources.Another study sought to identify and assess stubble burn areas in Punjab's principal rice-producing regions, Patiala and Ludhiana, during 2014-2018 using Landsat 8 OLI data (Chawala and Sandhu 2020).For identification, statistical thresholding and the normalized burn ratio index are used.Their findings showed a reduction of 32% and 40% in stubble burn areas in Patiala and Ludhiana, respectively.Considerable increases were noted in various pollutant levels (respirable suspended particulate matter, NO x and SO 2 ) during stubble burning times.During stubble burning events, NO x and respirable suspended particulate matter emissions are higher than that of SO 2 .A study examined the effects of PM fractions emitted during wheat and rice farming seasons on school childrens' (10-16 years) health in rural areas of Patiala, Fatehgarh and Sangrur from September 2014 to December 2015 (Saggu et al 2018).During these CRB periods, the monthly average PM levels were found to be three to four times higher than the limits set by NAAQS, as validated by MODIS data images.The pulmonary function tests of the school children, including forced vital capacity and forced expiratory volume, declined with increasing PM levels during the period.The reduction in lung function was slightly higher in males than in females.Their findings pointed towards the severity of such BB events and their serious impacts on human health, especially in children.Over Patiala, a semi-urban site in NW India, Sharma et al (2017) examined the impact of PRB on the radiative properties of aerosols to understand its implications on the regional climate.In October and November of 2008 and 2009, continuous measurements of AOD using a multi-wavelength radiometer and MicrotopII sunphotometer, PM via a highvolume sampler, BC via an aethalometer and aerosol radiative forcing calculation using Santa Barbara discrete ordinate radiative transfer (SBDART) code was performed.The observed increase in the AOD, AE, PM concentration and BC values and decrease in SSA indicated increased atmospheric forcing due to PRB emissions in Punjab.Another diurnal study (Rastogi et al 2016) was conducted in Patiala, where one year (October 2011-September 2012) PM 2.5 measurements followed by chemical characterization (carbonaceous and WSIS) were performed.This study confirmed substantial PRB in the autumn and WRB contributions in the summer as major sources of carbonaceous aerosols.The SOA formation was identified to be continuous, accounting for 10%-40% of PM 2.5 mass.Secondary inorganic compounds contributed 15%-40% of PM 2.5 , with an elevated contribution during the winter and the lowest contributions during the summer and rainy seasons.In general, the study illustrated that BB emissions have a considerable influence on the chemistry and temporal patterns of particulates over North India.Singh and Singh (2015) analyzed the potential of crop residue for power production by using a geographical information system.They stated that in total, crop residue in Punjab was 14.53 MT, of which the overall energy potential was found to be 227 300 TJ whereas the total power capacity of unused agricultural residue was 1000 MW.Their findings could help in designing the power plants in the future in all districts of Punjab.Rastogi et al (2014) found up to 300% higher night-time concentration of WSIS and carbonaceous species in Patiala (source region of PRB) during peak PRB (October-November 2011).More than 50% contribution from scattering species (OC, sulphate and nitrate) to total PM 2.5 load, a good linear correlation between OC and K + and a distinct OC/K + ratio of ∼16 was also noted during the study period (October 2011-March 2012).Kharol et al (2012) looked into variations in BC aerosol mass levels over Patiala, Punjab, during the CRB season (particularly stubble burning).Elevated BC mass concentrations (over 20 µgm −3 ) were detected by an aethalometer on several days, which were closely connected with the region's intense CRB practices.Ground-measured AOD values were found to be higher (more than 1.0) throughout the research period, along with a rise (>1.2) in AE values, showing the dominance of fine-mode aerosols over the region.Satellite data from MODIS revealed the presence of fire spots as well as an increased aerosol burden over the region under study, lending credence to the impact of BB activities.Awasthi et al (2011) analysed the effect of BB, primarily CRB (including both wheat and rice burning), on PM size and mass distribution over rural Punjab, India.During the study interval, they found that PM 2.5 formed a major share (55%-64%) of the total respirable suspended PM, implying that smaller particles were predominant during CRB, especially throughout the rice crop season.The study revealed that CRB significantly affects the concentration levels of different sized PM, although the general distribution of PM remained essentially consistent.
In another study from Patiala, PM  (Mittal et al 2009).Aerosols were collected from five distinct signature sites during the stubble burning and non-stubble burning periods in 2007.The region's air pollution is significantly influenced by the burning of WRB and PRB, as evidenced by the results, which indicated an alarming rise in aerosol, SO 2 and NO 2 concentrations during stubble burning times.

Studies from NE Indian region
Slash-and-burn agriculture practice, i.e.Jhum cultivation, is common from March to May in the NE Indian region.However, the long-range transport of CRB events from the NW states also leads to higher OC loadings in the eastern Himalayan states.
A year-long study from Darjeeling, an urban location in India's eastern Himalayas, showed 6% contribution in OC loadings from long-range transport of BB emissions from the NW States (Mukherjee et al 2022a).They conducted WSIS analysis and the monitoring of OC and EC, and used FC data and concentrated weighted trajectory analysis.They found that BB emissions contributed ∼39% to EC levels whereas they contributed ∼46% to OC levels.They observed that the long-distance BB plumes, including CRB in the NW India, also triggered carbonaceous particulates in the eastern Himalayan zone, and localized BB activities, such as residential heating and kitchen cooking, contributed significantly to OC emissions.They also observed that the SOC build up at night is due to the aqueous-phase oxidation of gases arising from BB emissions.Overall, the study highlights the local and transported BB emissions as a potential source of carbonaceous aerosols in India's eastern Himalayan region.et al 2004).In this study, BB emissions from shifting cultivation areas of moist tropical deciduous forests were studied.Radiative forcing of BB aerosols was calculated by shortwave aerosol radiative forcing.The burning period coincided with an increase in AOD and the total columnar water vapour.An increase in accumulation-mode particles along with bimodal size distribution was observed during CRB.The average (diurnally) radiative forcing by aerosols was estimated to be −65 Wm −2 , −87 Wm −2 and −59 Wm −2 over pre-burning, burning and postburning days, respectively.At the same location (Dochhumahmum) in Mizoram, Madhavi Latha et al (2004) explored the effect of BB particulates on UV ery .Throughout the BB interval, the study addressed the link between tropospheric particulates and groundreaching erythemal rays.Throughout various phases of burning, the AOD ranged from 0.5 to 4.2.The data revealed that while burning, direct sun irradiance decreased but diffused radiation increased.They found that with every 0.1 rise in AOD during the BB period, there was a 0.01 Wm −2 drop in the surface-reaching UV ery .They calculated the combustion efficiency (CO 2 fraction in total gaseous carbon emitted during fire) to identify flaming (⩾90%) and smouldering (⩽90%) stages and found dominance of the flaming stage during BB events in Mizoram.Badarinath et al (2009) studied optical properties of BB emissions and their effects on atmospheric radiative dynamics in Arunachal Pradesh's tropical moist evergreen forests in 2005.The ground-based measurements (mass-size distribution of aerosols, AOD, UV-B radiation, BC and SO 2 ) revealed a dominance of accumulation-mode particle load, an increase in AOD and a decrease in UV ery rays during BB emissions.However, throughout BB instances, diurnal variations in BC aerosol levels intensified throughout the morning and evening times compared to the afternoon hours.Compared to background values, higher accumulation-mode loading (14 times), BC concentrations (∼6 times) and SO 2 levels (∼3 times) were observed during BB periods.

Studies from other parts of India
Lan et al (2022) conducted a study to estimate the influence of variations in agricultural emissions on India's air quality and also to determine the input of district-level efforts using a GEOS-Chem adjoint modelling technique.They found that nearly 44 000-98 000 PM-associated premature mortalities every year were observed due to CRB.The states of Punjab, Haryana and Uttar Pradesh were responsible for 67%-90% of these deaths.Interestingly, they revealed that in Punjab alone, burning for two hours sooner could prevent up to 9600 premature deaths annually.In the interim, while more study is needed to determine the practicality and cost-effectiveness of this approach, their results encouraged the implementation of focused The fraction of crop residues burned in fields ranged from 18% to 30% nationally, with significant regional variation.Open burning was found to contribute substantially to emissions of BC, organic matter and CO (about 25%) and, to a lesser extent, to PM 2.5 and CO 2 emissions (9%-13%), while negligible contributions to SO 2 emissions (1%) were found.
To determine EFs for carbonaceous aerosols, controlled combustion experiments were conducted at the National Physical Laboratory, Delhi, where various fuels, i.e. fuelwood, dungcakes, agricultural residues, charcoal and soft coke, were burnt in a fabricated instrument setup.Sample collection was conducted on a filter by connecting a high-volume sampler to one end of a chimney.This study showed maximum contributions of OC (1.1-3.2Tg) and BC (0.3-1.0 Tg) from residential sectors, with the highest emissions from dungcakes (Parashar et al 2005).

Present challenge of BB in India: types and spatio-temporal variations
BB is one of the leading current causes of air pollution in India.An average increase of 1477 fire incidences per year are observed in the country (Shaik et al 2019).This section provides a summary of prevalent categories of BB in India and the associated challenges.The meta-analysis of available data on BB events in India and the Indian studies reviewed here reveal four major sources/types of BB: (i) BB for heating and cooking purposes, (ii) open waste burning, (iii) CRB and (iv) FF.The year-round prevalent BB sources in India include indoor and outdoor BB for heating and cooking purposes, and open waste burning.The incidences of BB for heating purposes increase in the winter months, especially in urban slums, rural areas and high-altitude locations, while biomass is burnt in tandoors (clay ovens) for cooking purposes all over India and the other two sources (CRB and FF) dominate over particular regions in particular months.In IGP, CRB is prevalent during pre-(April-May) and post-monsoon (October-November) season, and in South-India, it is the predominant type of ABB during January-February.During March-May, FFs in Central India and slash-and-burn agriculture in NE India lead to additional burdens of PM and gases.In eastern and NE regions, dominance of BB over fossil fuel is observed, whereas in Southern India, fossil fuel and BB are reported to contribute equally (Habib et al 2006).The present challenge of BB in terms of prevalent types is discussed as follows.

Wood burning for heating and cooking purposes
For heating and cooking purposes, BB is routinely conducted in India.In the IGP region, BB for residential heating and cooking is the prominent source of ultrafine particles, which leads to the night-time haze conditions.During winters, primary organic vapours from BB emissions condense under low-temperature conditions, resulting in new particle formation.The high contribution (∼70%) of this process to the total particle number concentration during haze events was observed by Mishra et al (2023).Wood burning for heating purposes in indoor as well as outdoor environments is particularly prevalent in the winter season.While several government initiatives/schemes are in place for clean fuel transitions in Indian households, outdoor wood burning remains unchecked.In hilly terrains, BB emissions lead to high carbonaceous aerosol load in the winter season.It is reported that coniferous wood is burnt for heating purposes, while angiospermic wood is burnt in tandoors for cooking.BB emissions aggravate as the tourist inflow increases in the hilly regions (Kaushal et al 2018).The BB practice is the cause of high PM 2.5 loads and severe human health implications.It is anticipated that ∼13% of premature mortality can be checked and India can meet the average PM 2.5 standards if Indian households completely shift to clean fuel practices (Balakrishnan et al 2019).In Lucknow, one of the major sources of NMVOCs was found to be solid fuel burning, confirmed by the presence of its tracers, including furans, furfurals and nitrogencontaining compounds (Jain et al 2023).At a rural site in Chhattisgarh, small-scale fires (hardwood and rice straw burning) were identified as major sources of BB aerosols (Nirmalkar et al 2019).These small-scale BB events for heating and cooking purposes are found throughout the country, and behavioural change at an individual level is essential for controlling this type of BB in India.

Open waste burning
Open burning of unsegregated waste results in mixed emissions.Heaps of unsegregated waste include both biodegradable and non-biodegradable material.Open waste burning is common in rural and urban areas, where heaps of waste are burnt for easy clearing.Incinerators use controlled conditions to ensure complete combustion, while open burning of mixed waste (paper, wood, plastic, rubber, etc)   waste is a common waste management practice, particularly in rural and urban areas of low-income countries.The unsustainable practice of open burning at dumping sites is the leading cause of air pollution, causing serious human health problems in India.Due to the mixed nature of burnt waste, the contributions from waste burning are difficult to trace back and thus are least understood.It is reported that open burning of waste is the neglected/unrecognized emission source and the contribution from this unofficial burning remains missing in current emission inventories.In the absence of appropriate steps to control these emissions, open waste burning may emerge as the largest source of poor air quality in India (Sharma et al 2022).

Crop residue burning
The practice of CRB for field clearing makes dominant contributions to the total BB emissions in India.In the NW-IGP, WRB in pre-monsoon (April-May) and PRB in post-monsoon (October-November) seasons are prevalent practices.The states of Punjab and Haryana jointly contribute to 26% of the overall annual fire events in India.In the rice season, an average of 15 456 (77.08%)FCs were recorded by MODIS, while the wheat season witnessed 3296 (16.44%)FCs (Shaik et al 2019).During pre-monsoon season, a higher contribution from SOC, due to escalated photochemical activity, is observed (Singh et al 2021b, Mukherjee et al 2022a), while PRB episodes witness higher PM levels, FCs, PM 2.5 , OC, EC and PAHs emissions compared to WRB (Rajput et al 2011, Shaik et al 2019) (figure 6).A strong correlation of PM 2.5 with fire frequency substantiates that paddy and wheat farming might be a valid indicator of seasonal variations in PM levels.Meta-analysis of current data reveals that the high PM 2.5 concentrations in IGP are an outcome of CRB plume transport and stationary meteorological conditions (Singh et al 2023).
Moreover, Singh et al (2022) confirmed that the increased AOD in the BB period had input from IGP and local regions using the CWT approach.Diurnal variations in PRB with higher night-time PRB was noted from the source region.During haze periods in Delhi, BBOA was found to be a major component present in aged form (Lalchandani et al 2022).These events have serious health implications and the OP, a robust indicator of PM toxicity, tends to increase with the ageing of BB emissions (Devaprasad et al 2023).Puthussery et al (2022) found that BB aerosols are responsible for 37% of the OP, substantiating the health implications posed by BB emissions.PRB emissions have shown enhanced atmospheric forcing in Punjab (Sharma et al 2017) and higher than normal night-time WSIS and carbonaceous fractions in Patiala (Rastogi et al 2014).To understand the optical properties of BB aerosols in Kanpur, Shamjad et al (2015) revealed that BB events lead to a rise in negative surface DRF and ∼35% increase in the absorption of solar radiation.As analysed by Kaskaoutis et al (2014), high AOD variability can be ascribed to the potential aerosol transport pathway eastwards through the Ganges valley from the BB source region.These BB plumes possibly have an impact on Central India, the Arabian Sea and the Bay of Bengal, thereby adding to the pollution levels of Asia.In Agra, during the PRB period, CO enhancement of 20%-79% was observed, ascertaining the long-range transmission from NW IGP (Kumari et al 2021).Nitrophenol, an excellent marker of an aged BB plume, is formed by the reaction of phenols and NO x .A study from Darjeeling by Mukherjee et al (2022a) showed longrange transport of BB plumes from NW India and revealed that BB accounted for 39% and 46% of EC and OC, respectively.The post-monsoon season had recently seen unparalleled levels of harmful air quality in north India, particularly in New Delhi and the NCR.It is reported that PRB in Punjab and Haryana contributes up to 70% of the fine particulate pollution in the NCR over the post-monsoon season.Stubble burning has been more common in recent years, with an estimated 4 000 farm fires per day in Punjab only throughout the post-monsoon months of 2020-2022.The shift of seasons and lessening of southwest monsoon prevailing winds at the beginning of November allow northern breezes to blow and carry the pollutants eastward towards the NCR region.The detrimental health effects in terms of chronic respiratory ailments, premature deaths and a projected six year loss in average lifespan due to severe air pollution episodes in NCR has led to an economic burden of 30 billion USD per year (Mukherjee et al 2023 and references therein).

Forest fires
Forests are the principal component of the terrestrial habitat and play a crucial role in preserving ecosystem health.India is one of the top ten countries in the world with respect to the total forest area covered.The total forest and tree cover of India is 809 537 sq.km, which is ∼24.62% of the total geographical area of the nation (https://frienvis.nic.in/Database/Status-Forest-Tree-2019-2021_3548.aspx).The enhancement in the frequency of the FF instances can be attributed to the rise in the dry periods in recent years, fluctuating rainfall trends, an increase in human-induced FF for their cattle and other needs, or due to the increase in the agricultural area (Vadrevu et al 2013a).CO emissions from BB events are well noted.According to data retrieved from MOPITT CO signals during peak BB events, evergreen forests emit higher mean CO (439.06 ppbv) in NE India when compared to mean CO (194.83 ppbv) emitted during CRB events in Punjab, India (Vadrevu et al 2013b).Moreover, it was found that FRP is a more reliable predictor than FCs for capturing CO signals during evergreen forest burning events.Natural FFs are essential for rebooting forest vegetation; however, a rise in these events because of climate change, along with the additional burden from anthropogenic FFs, results in the release of huge amounts of carbon into the atmosphere and a positive feedback mechanism, further intensifying the phenomenon of climate change.The frequencies of FFs have surged in India over the previous two decades, especially in the Himalayan areas (Vadrevu et al 2019), causing devastation to forests.On the basis of burned area and biomass density over a period (1995)(1996)(1997)(1998)(1999)(2000), the estimated contribution of forest and shrubland burning in India is 32 Tg yr −1 , with the dominant contribution arising from open and dense forest burning (27 Tg yr −1 ).Regional scale analysis shows the prevalence of forest burning in central and the east-NE India region, with maximum FF events occurring in February-May (Venkataraman et al 2006).The NW Indian states show a seasonal gradient for the start of FFs: February in Uttarakhand, March in Himachal Pradesh, April in Jammu and Kashmir and May in Ladakh (Kumar and Kumar 2022).The slash-and-burn agriculture practice is common in the Eastern Ghats and NE India.In February-April and March-May, tropical mixed deciduous forests are burnt in the Eastern Ghats and NE states of India, respectively (Vadrevu et al 2015).During BB events as part of slash-and-burn agriculture (Jhum cultivation) in Mizroam, the AOD and UV ery were found to be inversely related to each other.Also, based on the combustion efficiency, predominance of the flaming stage of BB over the smouldering stage was noted in Mizoram (Madhavi Latha et al 2004, Badarinath et al 2009).Overall, natural and anthropogenic FFs are a prevalent type of BB in India.

Conclusion and future perspectives
Natural BB helps to maintain the ecological balance; however, ABB leads to an additional burden of gases and particulates in the atmosphere, affecting the Earth's radiation budget and climate processes globally.While the regional phenomenon of severe haze episodes and visibility impairment deter dayto-day life, locally the complex emissions (particulate and gases) arising from BB affect human health.Due to the significant fractional contribution of ABB to the total aerosol load and the severity of implications from ABB emissions, ABB-led air pollution has emerged as a major challenge in India.Over time, a better understanding of the causes, improved planning and coordination amongst stakeholders across state borders and impactful and sustainable interventions/practices of clearing and further use of crop residue, farmer-friendly government schemes, limited use of firecrackers and CPCB-guided continuous monitoring of air pollutants before and after Diwali have resulted in better air quality over IGP.However, studies on quantitative assessment of this improvement are still lacking and should be conducted to understand the effects of interventions carried out to date.Although substantial steps are in place to understand BB emissions, continuous efforts at individual and community levels along with multi-sectoral initiatives are necessary to control BB emissions in India.Following broad interventions may further help in seeking sustainable solutions to BB-led air pollution in India.
10.1.Target 1: the first and foremost goal should be to reduce accumulated biomass residue CRB is a pertinent and noteworthy category of ABB in India and is one of the significant causes of the poor air quality in the IGPs.To tackle this problem in a sustainable manner, the goal should be to reduce accumulated biomass residue in agricultural fields.The introduction of alternative crops with crop diversification initiatives, particularly for the Kharif crop season, may help in achieving this goal.This requires focused research efforts to look for suitable alternatives, possible transitions in crop cycles and also awareness amongst stakeholders.Intense research on crop diversification followed by successful field trials and then acceptance and adaptation at mass level will help in achieving a permanent solution to the problem.

Target 2: application of innovative technologies to clear/use the accumulated biomass residue
Another major challenge is to sustainably clear/use the accumulated biomass.Adoption of new and innovative technologies, e.g.turbo seeder technology, conservation agriculture (zero-tillage, integrated management) and inter-cropping systems, and the development of compact portable and hand-held machinery for easy operation by farmers may help to minimize conventional burning of crop residues.Utilization of crop residue in multiple ways, including animal bedding, sunshade material, feed for cattle, soil mulching, composting, use of rice straw as cofiring with coal for power generation, biofuel, biogas, bio-compressed natural gas, can be considered.Use and promotion of CPCB-guided sustainable practices, such as biomass pellet systems (as adopted by Punjab Pollution Control Board) and other insitu and ex-situ crop residue management strategies, should be adopted more widely in India.Biochar production is one such sustainable practice, where biochar is obtained from various biomass forms, such as chipped wood, plant leftovers, manure or other crop residue products.Implementing such sustainable and innovative technologies will lead to a reduction in the additional carbon burden and thus will also contribute to mitigation of the climate change problem.Financial incentives and subsidies to encourage end users may motivate them to adapt green technologies, the use of renewable energy sources and innovative farming practices.accurate and correct data on sources, composition and emission factors of BB serves as a foundation for planning and execution of any intervention.Given the complexity of emitted particulates and gases, it is difficult to discern the contributions arising from natural and anthropogenic sources in the atmosphere.To trace back the type and source of BB is an additional challenge.Most molecular markers provide overlapping information on sources and thus the use of combinations of source tracers and diagnostic ratios is highly recommended for confirmation of sources of BB.Comprehensive SA studies using stable isotopic ratios and associated diagnostic parameters should be conducted.Better understanding of the problem of BB in rural and remote areas can help policymakers to formulate airshedbased guidelines.Improved measurements through diverse approaches, such as sensor networks, drone technology and aircraft measurements, should be widely adapted to gain a better understanding of BB sources.Furthermore, hybrid studies integrating ground-based and satellite-based studies are highly recommended.• Capacity building initiatives: most of the studies reviewed here are focused over the IGP; however, it will be interesting to explore the influence of BB emissions from other less studied parts of India, such as the Indian Himalayan region and peninsular region.The setting up of regional fire laboratories with sophisticated instrumentation is required for comprehensive laboratory measurements to improve the current knowledge on existing emission factors and source tracers, particularly for specific types of biomass burnt in India.
Collaborative studies by teams of agricultural scientists, atmospheric scientists, meteorologists, epidemiologists and doctors will help to understand the exposure of BB-led air pollution.

Figure 1 .
Figure 1.Spatio-temporal spread of different types of BB, namely, solid fuel burning for heating and cooking, waste burning, CRB/small-scale fires and large FFs, along with their respective implications.

Figure 2 .
Figure 2. A schematic showing major (particulate and gaseous) species emitted from Biomass Burning.

Figure 3 .
Figure 3.A schematic of important chemical constituent/source tracer-species-based diagnostic ratios to confirm BB sources and to further decipher the type of biomass burnt.

Figure 4 .
Figure 4. Details of satellite products/reanalysis databases along with their spatio-temporal resolution that are routinely used to confirm BB events through detection of fire, emissions and both fire and emissions.

Figure 5 .
Figure 5. Percentage contributions of BB in different size fractions (a) PM10, (b) PM2.5 and (c) PM1 over different locations in India, as reported by receptor -modelling-based SA studies.
(Badarinath et al 2004, 2009, Madhavi Latha et al 2004, Awasthi et al 2011, Rajput et al 2011, Rastogi et al 2014, 2016, Nirmalkar et al 2019, Puthussery et al 2020, 2022, Gunthe et al 2021, Tripathi et al 2021, Singh et al 2021b, 2023, Lalchandani et al 2022, Devaprasad et al 2023, Jain et al 2023, Mishra et al 2023).However, six studies (Habib et al 2006, Vadrevu et al 2013b, Sahu and Saxena 2015, Dutta and Chatterjee 2021, Singh et al 2022, Ravindra et al 2023) analyzed the variables obtained from satellite data/reanalysis products to decipher the occurrence of BB events over India.Some studies (Kharol et al 2012, Kaskaoutis et al 2014, Sahu and Saxena 2015, Shamjad et al 2015, Sharma et al 2017, Shaik et al 2019, Kumari et al 2021, Montes et al 2022, Mukherjee et al 2022a, Ravindra et al 2023) used a hybrid approach by employing both ground-based and satellite-based approaches to understand BB sources and composition.To investigate the influence of CRB in NW India, multi-site studies were conducted in the IGP.Moreover, from Delhi, one hybrid and six ground-based studies are discussed here.Seven ground-based and two hybrid investigations from Patiala and one ground-based study are available from a village (Daulo Nangal in Beas, Punjab).However, from Northeast (NE) India, two ground-based investigations from Mizoram, one from Arunachal Pradesh, one hybrid study from Darjeeling, Assam and one satellite-based study from the whole of NE India were found.
is commonly seen in dumping areas (Ch 5: Incineration and Open Burning of Waste, 2006 IPCC Guidelines for National Greenhouse Gas Inventories).The open burning of

Figure 6 .
Figure 6.Comparison of two prevalent types of CRB in Northwestern India.

Table 1 .
Details of studies on BB-led air pollution over different locations in India.
2.5 associated carbonaceous fractions and PAHs were investigated from October 2008 to May 2009 (Rajput et al 2011).They found 4-5 times lower PM 2.5 , OC, EC and PAHs and about 3-5 times lower five-and six-ring PAHs (normalized to EC) during WRB when compared to PRB.Awasthi et al (2010) investigated the health effects of agricultural CRB by studying the dynamics in pulmonary function tests in children (10-13 years) and young adults (20-35 years).The study was conducted in the agricultural region of Sidhuwal village in Patiala from August 2008 to July 2009.Their findings showed that increased PM levels from CRB resulted in decreased values in pulmonary function tests, especially in children.They revealed that CRB is a crucial health hazard, specifically for children.An investigation of the effects of open stubble burning on aerosol, SO 2 and NO 2 levels in the ambient air was carried out at ground level in the Patiala area of India May) and the dual-phase period of pre-monsoon and post-monsoon season for CRB are the prevalent fire periods.Long-term satellite retrievals revealed that the effects of BB go far beyond the source region, affecting North India and neighbouring oceanic regions.The study identified significant interannual and intra-seasonal variations in BB in the Indian subcontinent.Punjab and Haryana were identified as hotspot regions for BB in India, contributing to 26% of the overall yearly fires in India.They also noted higher FCs during PRB 15 456 (77.08%) than WRB events 3296 (16.44%).The Satellite FC data of ATSR sensors have shown large spatio-temporal variations in BB trends over India(Sahu et al 2015).

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Awareness and behavioural shifts at individual and community level: it is critical to raise awareness about the health and climate risks of BB-led air pollution.Measures such as public awareness campaigns in local languages, education about cleaner practices and behavioural modifications at the individual and community level are necessary (a) to adapt sustainable practices for use/clear accumulated crop residue, (b) to minimize dependency on BB for residential cooking and heating purposes and (c) to stop mixed waste burning.Training programmes for concerned stake holders to gain practical hands-on experience in necessary skills and technologies in remote locations should be organized.Farmer outreach activities should be undertaken to educate farmers about the harmful consequences of CRB on both the environment and human well-being.It is very important to involve stakeholders from under-privileged groups of society by educating them through awareness drives.the Department of Science and Technology-Promotion of University Research and Scientific Excellence (PURSE) Grant (SR/PURSE/2023/204).SNT acknowledges support under a J C Bose National Fellowship under the aegis of Science and Engineering Research Board.SY and SNT acknowledge the financial support received jointly from the Ministry of Earth Science and Swiss National Science Foundation in the form of the Indo-Swiss joint research Project (MoES/INDO-SWISS/2/2022-PC-I) 'ICE nuCleating paRticle and cloUd condensation NuClei properties in the Northwestern Himalayas (ICE-CRUNCH)' .KS acknowledges the University Grants Commission for providing financial assistance in the form of a Junior Research Fellowship.Awasthi A, Agarwal R, Mittal S K, Singh N, Singh K and Gupta P K 2011 Study of size and mass distribution of particulate matter due to crop residue burning with seasonal variation in rural area of Punjab, India J. 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