Rural versus urban gaseous inorganic reactive nitrogen in the Indo-Gangetic plains (IGP) of India

Okhla was selected as an urban site, which is located in the South Delhi district. The site is dominated by heavy vehicular traffic and industries. Okhla had a population density of 10 935 inhabitants per square kilometre (28 320/sq mi), with a population growth rate of 20.59% during the decade of 2001–2011. The South Delhi district had a population of 2 733 752 in 2011, which is roughly equal to the total population of Jamaica or the US state of Nevada. The Okhla area is congested in terms of high-rise buildings, roads and other infrastructure. In the vicinity of the sampling site, there is a cremation ground, which can also contribute emissions of pollutants. Such waste also can be potential source of Nr. Table 2 gives the number of meat shops and their meat production in the Okhla locality.

The Mai village, which is located in the Jaunpur district of Uttar Pradesh state, was selected as the rural site. The total area of the village is 617 hectares, out of which 385 hectares (approximately 62%) are used for agricultural purposes. The area around the site is dominated by agricultural activities. A few industrial establishments are located at a distance of 13 km from the village, including one vegetable oil production factory. The combustion vapours and odour of this mill can be smelled in Mai. This region of Uttar Pradesh has three major crop seasons, namely i) Rabi (winter crops), ii) Kharif (monsoon crops) and Zaid (summer crops). The Rabi crop is dominated by corn, black gram, gram and mustard. The Kharif crop is dominated by rice, while the Zaid crop season is not cultivated much due to lack of water. Most fields remain open during the summer period. The farmers of Mai mainly apply DAP and urea fertilizers as a N supplement. The monthly variations of temperature and wind speed at both sites have been given in table 3.

The average concentrations of NH₃ were recorded as 40.7 μg m⁻³ at the Okhla site and 51.6 μg m⁻³ at Mai.

Table 4 gives descriptive statistics of the NH₃ at the Okhla and Mai sites.

At the Okhla site, NH₃ concentrations varied from 20 to 126 μg m⁻³, with an average of 40.7 ± 16.8 μg m⁻³, although the daily average concentration of NH₃ did not show a large variation during each respective month. However, relatively higher average concentrations were recorded during the months of December (50.8 μg m⁻³), January (49 μg m⁻³) and May (50 μg m⁻³). At Okhla, the average NH₃ concentration did not show much difference during the different seasons. During winter (November-January), summer (May-June) and the monsoon (July-September) season, the average NH₃ concentrations were recorded as 44.3, 42.5 and 38.9 μg m⁻³, respectively. Such a seasonal pattern of NH₃ is very similar to the pattern reported by Singh and Kulshrestha (2012) at the JNU site of Delhi city.

At Mai, the NH₃ concentration varied from 13 to 127.8, with an average of 51.6 ± 22.8 μg m⁻³. At this site NH₃ concentration had more pronounced seasonality. The average concentrations of NH3 during winter, summer and the monsoon season were recorded as 72.0, 47.2 and 30.7 μg m⁻³, respectively. The difference in NH₃ levels at the urban and the rural site may be attributed to the difference in source types and their strength. At Okhla, the possible sources of NH₃ are solid municipal waste, tissue waste, traffic, etc, which remain constantly active throughout the year, while at Mai, the major sources of NH₃ are agricultural activities and cooking. During the winter season, fertilizer application and other agricultural activities are at full swing, which emit NH₃. Gupta et al (2003) also observed the highest NH₃ during the winter season due to maximum vegetation growth and maximum fertilizer use during this period at the rural area.

In general, N fertilizer application, increased livestock production, solid waste generation and high population density are the possible sources of NH₃ in India. Generally, 240 kg per ha per year DAP in basal dressing and 480 kg per ha per year urea in top dressing are applied in the field. However, the total fertilizer amount depends on the crop and season. According to the statistics, consumption of major fertilizers has increased in recent years. From 1999–2000 to 2011–2012, urea consumption increased by 44%, whereas DAP consumption increased by approximately 61%. According to data from the Fertilizer Association of India, out of the total national consumption, North India consumes around 38% of urea and 40% of DAP. However, less than 30% of the applied N fertilizer is absorbed by the crops. More than 20% is evaporated as ammonia emissions (Cai 1997). Around 20% of urea-N is lost into the atmosphere immediately after its application in the soil (Whitehead and Raistrick 1990) Factors such as soil moisture and temperature, soil texture and the form of nitrogen affect the intensity of NH₃ evaporation, controlling the efficiency of the fertilizer. Fertilizer N use efficiency can vary from one environment to another (Fennand Hossner 1985). In addition, the changing food habits of people also contribute to the increasing levels of atmospheric NH_3. However, NH₃ concentration is likely only representative of a small area as it is mainly influenced by low-level local sources, whereas for NO_x, more remote sources also can play a role.

According to estimates, around 1175 Gg NH₃ is contributed by different kinds of fertilizer in India (Parashar et al 1998), with urea as the highest contributor. As shown in figure 2, around 94% NH₃ is contributed by urea fertilizer.

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The other major source of NH₃ is the livestock population, which contributes around 1433 Gg of NH₃. The livestock population is dominated by cattle, which emit a major fraction (∼73%) of atmospheric NH₃ (Parashar et al 1998). It is important to mention that solid waste and human excreta, etc are also important sources of ambient NH₃ at Okhla. As mentioned earlier, the site is highly populated and has poor sanitation arrangements. In addition, according to recent reports non-agricultural sources, such as vehicles equipped with catalytic converters, also contribute NH₃ (Bari et al 2003). Such non-agricultural emissions influence NH₃ mixing ratios at urban locations and nearby roads (Sutton et al 2000, Heeb et al 2008, Ianniello et al 2010, Sharma et al 2014).

Meng et al ( 2011) also found traffic to be an important source of NH₃ in Beijing during the winters.

Figure 3 shows the day-night variations of NH₃ during different months at Okhla and Mai. At Okhla, daytime concentrations of NH₃ varied from 4.9 to 139.1 μg m⁻³, with an average of 38.7 μg m⁻³, while nighttime concentrations varied from 3.7 to 114.2, with an average of 42.6 μg m⁻³. The high concentrations during the nighttime can be attributed to atmospheric stability due to the trapping of gaseous NH₃ near ground level at both sites (Burkhardt et al 1998, Cadle et al 1982, Singh et al 2001, Singh and Kulshrestha 2012).

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Figure 3(b) shows the average concentrations of NH₃ during the day and night times at Mai. Day concentrations of NH₃ varied from 9.9 to 148.8 μg m⁻³, with an average of 49.8, while nighttime concentrations varied from 5.9 to 145.7, with an average of 53.4 μg m⁻³. Unlike Okhla, where NH₃ concentrations were always recorded higher during the nighttime, at Mai, daytime concentrations were higher during the December, March, May and June months. Daytime high concentrations of NH₃ are supported by Behera et al (2013) due to large evaporative emissions from various sources. It is possible that daytime higher NH₃ concentrations are observed due to evaporation from fertilized fields since these months are typical months during which fertilizer is applied to most of the crops. In addition, agriculture residue burning during the summer season may also give rise to daytime higher concentrations of NH₃ at Mai. To assess the difference between the daytime and nighttime levels of gaseous NH₃, a paired t-test (two-tailed) was performed. The test showed that there was no significant diurnal difference at a 95% confidence level (P > 0.05) for both sites.

Table 5 gives a comparison of NH₃, concentrations at various sites worldwide. In general, NH₃ levels at both sites are much higher than at temperate sites. This is probably due to the tropical climate. The high temperature of tropics helps in the higher evaporation of NH₃ from agriculture fields, municipal waste dump yards and other sources. In addition, NH₃, being an alkaline gas with an abundance of alkaline dust, also favours its gaseous existence (Singh and Kulshrestha 2012), while most temperate regions have an acidified atmosphere in which the dominance of sulphuric acid consumes gaseous NH₃, forming NH₄⁺ and SO₄²⁻ aerosols (Kulshrestha 2013).

Table 6 gives descriptive statistics of NO₂ at Okhla and Mai. The average concentration of NO₂ at Okhla was recorded as 24.4 μg m⁻³, ranging from 5 to 63.6 μg m⁻³. At Mai, NO₂ varied from 2.5 to 40.6, with an average of 18 μg m⁻³. Figure 4 shows the monthly variation of NO₂ at Okhla and Mai. The seasonal mean NO₂ concentrations at Okhla were recorded as 39.6, 24.5 and 10.4 μg m⁻³ during the winter, summer and monsoon season, respectively. At Mai, the mean NO₂ concentrations during the winter, summer and monsoon season were recorded as 27.5, 17.2 and 4 μg m⁻³, respectively. The major source of NO₂ in a rural area is biomass burning (wood, crop residue and dung cakes) during cooking and heating. (Singh et al 2014). Generally, around 621 tons of biomass are burnt by a typical village of around 200 families in North India, as reported by Singh et al (2014). Okhla, since it is an urban site, had higher NO₂ than Mai. This is probably because the Okhla area is dominated by vehicular and industrial activities, which contribute significant amounts of NO₂ in air. Gurjar et al (2004) observed that NO_x emissions have risen significantly from 94 Gg in 1990 to 161 Gg in 2000. These results are almost in line with vehicular growth in the city. The number of vehicles in Delhi has increased from 2 432 295 in 1994 to 6 932 706 in 2010 (DSHB 2011).

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The possible reason for high NO₂ concentrations during the winter season is the formation of inversions in which pollutants are trapped in the boundary layer, whereas during the summer period, a higher mixing height allows dispersion in free tropospheric air, causing the dilution of the pollutants (Reddy et al 2012). In addition, higher photochemical activity also helps in breaking down the pollutants into other simpler compounds. Higher concentrations of pollutants during the winter season is a typical feature in the India region. Gupta et al (2003) have also reported higher concentrations of NO₂ during winters. A similar observation was reported by Kelly et al (1989) and Barbiaux et al (1992). However, the average concentrations observed in the present study are much higher than those reported in other studies for rural areas. Higher concentrations of NO₂ at Mai are probably due to NO₂ emissions from biomass burning during cooking and heating. In rural India, domestic cooking and heating is based on biomass burning (Singh et al 2014). In addition, the burning of rice and wheat straw in the industrial units located at ∼13 km from the sampling site could also contribute NO₂ at Mai. Also, local vehicular traffic can also give rise to NO₂ in the atmosphere. NOx emissions into the atmosphere are dominated by the combustion of fossils fuels and biomass, which represent 75% of total emissions, with more than 50% of that from fossil fuels alone. NO_x emissions are therefore strongly influenced by anthropogenic activities. Ammonia oxidation is a potential tropospheric source of NOx. The oxidation of ammonia in the atmosphere is initiated by the reaction with OH

$$\begin{eqnarray}&&{\rm OH}+{\rm N}{{{\rm H}}_{{\rm 3}}}-{\rm N}{{{\rm H}}_{2}}+{{{\rm H}}_{2}}{\rm O}\end{eqnarray} \tag{ 1 }$$

This reaction may play a very important role in controlling NH₃ concentrations, especially in tropics where OH concentrations are higher in the atmosphere (McConnell 1973).

At lower NOx levels, NH₂ reacts with O₃, forming the NH₂O radical

$$\begin{eqnarray}&&{\rm N}{{{\rm H}}_{{\rm 2}}}+{{{\rm O}}_{{\rm 3}}}-{\rm N}{{{\rm H}}_{2}}{\rm O}-{\rm NO}\end{eqnarray} \tag{ 2 }$$

The NH₂O radical reacts with oxygen and finally forms NO. Therefore, such a reaction sequence results in a source or a sink of nitrogen oxides, depending on ambient NOx concentrations (Delmas et al 1997).

Figures 5(a) and (b) show the variation of NO₂ at the Okhla and Mai sites during different months. The daytime concentrations of NO₂ varied from 5.0 to 72.6, with an average of 25.9 μg m⁻³, whereas nighttime concentrations of NO₂ varied from 5.0 to 66.2, with an average of 22.8 μg m⁻³. At Mai, daytime concentrations varied from 2.3 to 56.9, with an average of 19.8 μg m⁻³, while nighttime concentrations varied from 2.6 to 36.9, with an average of 17.7 μg m⁻³. Daytime higher concentrations of NO₂ at both sites are quite obvious due to various sources, such as industries, traffic, etc, which are more active during the daytime. A paired t-test (two-tailed) result showed the significant diurnal variation at a 95% confidence level (P < 0.05). In a study reported by NEERI (2008) at Delhi, during the summer season, the daytime NO₂ concentration was recorded as 40 μg m⁻³, while at midnight, the NO₂ concentration was 30 μg m⁻³. During the winter season, the daytime concentration of NO₂ was recorded at about 60 μg m⁻³, which was higher than was recorded in summer and in the post-monsoon season. Contrary to the winter season, the NO₂ level increased gradually after sunset to about 80 μg m⁻³ by midnight. It is worth mentioning that unlike NH₃ levels, which are affected by local sources, NO₂ levels can be affected by more remote sources.

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