Evaluating the effects of different tree species on enhancing outdoor thermal comfort in a post-industrial landscape

A frequently emphasized strategy to reduce the burden of heat in cities across the world is the implementation of street trees. Here, we examine the effects of deciduous and coniferous tree deployment on meteorological variables and pedestrian thermal comfort through analysis of the new dynamic thermal comfort (dPET) index, using the latest version of the computational fluid dynamics model ENVI-met. We performed on site observational measurements of air temperature (Ta), relative humidity (Rh), wind speed (Ws), and mean radiant temperature (MRT) at five different locations on the hottest day of summer 2023, in a post-industrial urban landscape located in Tehran, Iran. Observations were used to evaluate ENVI-met simulation performance and served as a baseline against which sensitivity experiments—based on a minimum (35%) and maximum (75%) intervention scenario for deciduous and coniferous trees—were compared against. Our analysis indicates that 35% and 75% deployment reduced Ta by 1.2 °C and 4.2 °C, respectively, for deciduous tree species, compared to a 0.9 °C and 3.1 °C reduction for coniferous species, during the hottest day of summer 2023. The maximum deployment scenario decreased MRT by approximately 60 °C and 43 °C for deciduous and coniferous tree deployment, respectively. The maximum tree deployment scenario decreased dPET by nearly 16 °C and 14 °C for deciduous and coniferous trees, respectively, during the time of day that diurnal heating is maximized. Our findings highlight micrometeorological and personalized thermal comfort effects associated with variable tree species type and extent through examination of a pedestrian’s ambulatory experience across diverse urban microclimates in a region of the world that is particularly understudied.


Introduction
Escalating concerns regarding global warming and its dire consequences have brought sustainable urban design and management into sharp focus [1].With global temperatures on the rise due to the rapid buildup of greenhouse gases in the atmosphere, there is a pressing need to find effective solutions that reduce urban environmental pressures resulting from the continued build-up of greenhouse gases and the direct effect of the built environment [2][3][4][5].At the heart of this endeavor is the revitalization of postindustrial sites, once scarred by the adverse effects of human activities, into thriving centers of ecological vitality [6,7].
These post-industrial sites serve as stark reminders of a past marked by rampant industrialization, where progress occurred simultaneously with environmental degradation.Once bustling with factories and heavy industry, these neighborhoods have been abandoned, leaving behind contamination and ecological disruption [8].Yet, within these challenges, an often-overlooked opportunity for environmental revitalization emerges.Revitalizing postindustrial areas not only reclaims lost green spaces in urban landscapes but also serves as a less-discussed yet vital application of urban heat mitigation.Unlike the prevalent focus on tree-planting in residential and commercial neighborhoods, it is also important to recognize the need for post-industrial revitalization via thermal reduction strategies as a currently unmet challenge.Diversifying our approaches to urban environmental challenges highlights the importance of this lesser-explored avenue [9][10][11].
The role of vegetation, especially coniferous and deciduous trees, is an important pathway in the pursuit of environmental revitalization [12].Plants offer a range of benefits beyond just appearances and aesthetic values [13,14].For example, conifers, with their needle-like leaves and adaptability to various microclimates, and deciduous trees, known for their diverse shapes and sizes, can enhance thermal comfort [15][16][17][18].Prior research on tree cooling effects has demonstrated that trees with darker crowns reduce direct sunlight exposure [19].Cui et al [20] determined that in order to enhance outdoor thermal comfort in Harbin (China), the optimal implementation scenario involved tree deployment covering 50% of a northeast-southwest orientation with an aspect ratio of 0.5.Tree deployment leading to a nearly closed canopy [21,22] with a higher leaf area density (LAD) value [23] has been linked to decreased mean radiant temperature (MRT) [24,25].In Xi'an, China, recent work demonstrated a significant correlation between green space area and air temperature reduction, identifying optimal ranges to minimize air temperature [26].Zhang et al [27] explored the mitigating effects of urban trees in a subtropical area and found that a tree with lower trunk height which is closer to the surface leads to a greater cooling effect.
Strategic arrangement of tree species has been proposed to optimally combat the burden of heat on urban environments, through provision of shade and enhanced evapotransporation [28][29][30].In subtropical regions, Li et al [31] found that planting larger-crowned deciduous trees on the windward side enhances thermal comfort and decreases pollutant concentrations.In Singapore, Liu et al [32] concluded that increased planting density of smallcrowned trees improves cooling efficacy, leading to urban cool islands.Prior research has examined the impact of various tree species on enhancement of outdoor thermal comfort in different climates, including tropical [15], sub-tropical [33], and Mediterranean [9].However, there remains a notable gap in the understanding of vegetation types and their cooling effects in arid desert climates.In particular, studies that examine the structural characterization of tree deployment (i.e.extent and species type) on cooling efficiency would be invaluable [34].
Thermally relevant indicators, beyond merely temperature, are necessary to assess the impact of urban street trees on human well-being [35].For example, recent work in Xi'an, China, used physiological equivalent temperature (PET) to assess the impact of green spaces on human thermal comfort [36].Wang et al [37] examined how tree characteristics including crown radius and trunk height relate to PET reduction, and determined that taller trees led to lower PET values.Other studies have looked at indicators like predicted mean vote [38][39][40] and universal thermal climate index (UTCI) [19,41] to assess outdoor thermal conditions.As there is no universally accepted best metric, combinations of them have also been used (e.g.PET and UTCI; [42,43]).The majority of prior research has used various indices in order to evaluate standing people and associated thermal comfort, with few assessing effects on mobile individuals as they move through different urban microclimates.The new dynamic thermal comfort (dPET) index [44] facilitates impact assessment of personal mobility through varied micrometeorological conditions.Therefore, research is required to examine the impact of different tree species on mobile individuals through use of the new dPET index.
The central objective of this study is to evaluate the effects of variable tree species deployment (i.e.deciduous and coniferous) on reduction of outdoor thermal stress in a post-industrial location in Tehran, Iran.To comprehensively explore thermal mitigation effects on outdoor thermal comfort, there is a need to quantify implications for dynamic metrics that account for pedestrian mobility.Through this work, we address the following fundamental questions: • What are the effects of varied tree species on thermal conditions experienced by walking pedestrians?• Among deciduous, coniferous, or a combination of both, which scenario would result in a greater thermal comfort benefit?

Methodology
We first conduct in-situ measurements of meteorological variables at the site location on the hottest day of summer 2023.Next, we use the ENVI-met microscale model to examine its ability to represent meteorological conditions during this extreme heat event.
Finally, we use the ENVI-met model to characterize impacts associated with varying tree species deployment (supplementary figure 1).

Study area
Our study location, Tehran, Iran, is located in an arid desert environment with cold winters (BWk climate zone in the Köppen-Geiger classification [45]).Furthermore, the average building height is approximately 29 m and the building coverage encompasses 43% of the site's area.Tree coverage accounts for only 5% of the site, and includes 26 Fraxinus excelsior tree species with an average height of 15 m.The Ray Cement factory, established in 1932, was closed in 1984 due to pollution and rising heat, which adversely affected residents' health, causing heart disease, asthma, and tuberculosis [46,47].This factory was included in the list of national monuments of Iran and was named as Tehran's cement industrial heritage site.The Municipality and Tehran City Renewal Organization were responsible for re-creating and transforming Ray Cement Factory into a museum of industry.Proposal submissions were solicited for the reconstruction of Ray Cement Factory, and many designs were sent to the municipality from abroad and inside the country, after which it was determined that the factory site would be turned into a museum.However, no landscape modification has occurred as of yet, and incorporation of green infrastructure is a possible next step in the site's landscape design.Currently, the Ray Cement Factory site subjects visitors to uncomfortable meteorological conditions during most summer days [48].The nearby synoptic station at Mehrabad International Airport, about 15 km from the site, indicates that Tehran frequently encounters hot and dry summer days, with temperatures ranging from 33 • C to 40 • C and relative humidity peaking around 45%.This context highlights the need for efficient and sustainable design of urban greenery [49].
The choice of Ray Cement factory as the case study stems from the realization that many industrial heritage sites worldwide, once operational, contributed significantly to local and global heat increases [50].These sites, often constructed using radiationabsorbing materials like concrete and brick, offer the potential for cooling urban areas if repurposed appropriately.Moreover, due to years of cement production, greenery is lacking at the site, though its presence is vital for safeguarding the remains from wind and low moisture content [13].

Field measurement, simulation tool and ENVI-met evaluation 2.2.1. In-situ measurements
In-situ measurements were conducted on 11th July 2023, the hottest day of summer, at the Ray Cement factory, from 8:00 to 18:00 LST.The measurements involved five distinct points (figure 1(b)), lasting for a period of 2 h each.Points were selected based on unique characteristics that illustrate microscale heterogeneity.The first point was situated near grassy areas, the second near tall building walls that created a deep canyon effect, the third directly exposed to solar radiation without shading, the fourth along a wide pathway between buildings with varying degrees of shade, and the fifth at the site entrance, shaded by a Fraxinus excelsior tree.Meteorological parameters including air temperature (Ta), relative humidity (Rh), wind speed (Ws), and global temperature (Tg) at a height of 1.8 m were measured using a HD32.1-ThermalMicroclimate Data Logger.The measurement instrument's features and sensors are detailed in supplementary table 1.

Simulation tool
To comprehensively assess the site conditions and examine the efficacy of tree deployment, a threedimensional microclimate model, ENVI-met (version 5.5.1), was employed.This model is rooted in thermodynamics and heat transfer principles [53,54] and has been utilized in various studies [55][56][57] and geographies, including Osaka, Japan [58], southern Italy [59] and Seville, Spain [60], to name a few.ENVI-met offers a spatial resolution on the order of meters and a timestep interval on the order of seconds [61].The latest version includes enhanced dynamic and radiative process representation [62] and is a widely adopted computational fluid dynamics (CFDs) model.ENVI-met incorporates vegetation characteristics, enabling accurate representation of plant-related effects [63,64].ENVI-met was executed for a 28 h simulation starting on 10th July at 20:00 LST, the hottest summer day of 2023 in Tehran [65].After a 4 h spinup, the subsequent 24 h were used for analysis [43,66].Initial inputs for the ENVI-met model and spatial dimensions (e.g.grid cell dimensions in the x-and y-directions of 2 m) are presented in supplementary table 2.
The newly introduced dPET (dynamic PET) in the ENVI-met software enables personalized thermal comfort simulation based on pedestrians' responses to meteorology and adaptive actions, offering insights into mitigating diverse reactions to micro-scale changes across the urban landscape [67].An ambulatory individual is expected to encounter fluctuations in ambient wind speed, temperature, and varying exposure to radiative conditions.Such aspects of one's experience with the ambient environment is unaccounted for in stationary thermal indices like the UTCI or PET.Identification of pedestrian travel paths that avoid heightened heat stress through implementation of green infrastructure (i.e.trees in this case) promotes effective utility of urban heat mitigation techniques that enhance heat resilience [68].Indeed, to evaluate pedestrian walkability across the built environment, various meteorological parameters, including Ta, MRT, Ws, and Rh [69], are considered in the assessment of dPET.The dPET index provides an opportunity to investigate important physiological parameters, including the skin temperature (T skin) and sweat production (SP).Unlike the limitations of the PET for assessing thermal comfort in hot conditions due to assumptions about clothing, metabolism, and standardization, dPET incorporates dynamic considerations, individual differences, and better metabolic measurement for improved accuracy [70].Additionally, dPET characterizes a pedestrian's thermal experience and considers walking speed, stop points, insulation provided by clothing, height, weight, and ancillary factors [20,71,72].Here, dPET was simulated for a 32 min walk for each of the scenarios (see table 1 for biometeorology simulation details).

Model validation
ENVI-met's reliability has been verified through numerous studies comparing its simulated output with in-situ measurements [20,[73][74][75].Given the considerable impact of meteorological parameters on pedestrian thermal comfort, this study aimed to evaluate ENVI-met output against Ta, Rh, Ws, and MRT, through comparison against collected data from measurement points (see section 2.2.1).In alignment with prior research, we used the coefficient of determination (R 2 ) and root mean square error (RMSE) to quantify the level of agreement between simulated and measured values [76,77].We used Jamovi software [78] to perform curve linear regression assessment on both simulated and measured data as 5 min intervals, generating R 2 and RMSE values.For a dependable model, the R 2 value should approach 1, and the RMSE value should approach 0 [79].Previous research in the field of ENVI-met often employed 1 h intervals for model validation [12,14,75].This research introduces a novel approach (i.e. 5 min intervals) to delve more deeply into the details of simulated variables and to compare them more accurately with the actual measured data.

Scenario design
Alongside the base scenario (BS; i.e. the existing configuration), six additional scenarios were formulated based on the i-Tree species program.The i-Tree program is designed to help users select the most appropriate species based on selected desired environmental services and the geographic area of focus.Subsequently, the program determines the most suitable tree species by evaluating the usersupplied weighting of environmental benefits for tree species at maturity, specifically at the designated site location.The outcomes generated an extensive collection of distinct tree species.Six tree species were chosen: three coniferous and three deciduous, considering characteristics such as height, width, LAD.Our selections were focused on maximizing resilience and water efficiency, especially critical for hot and arid climates (supplementary table 3).The simulation phase comprised three planting strategies: two simulations featured a single tree species, either deciduous or coniferous, while the third combined both species.We examined two tree configuration levels: 110 trees (35% coverage) and 235 trees (75% coverage), representing what is considered as a reasonable minimum and maximum intervention scenario.The scenarios were: 35% and 75% deciduous scenario (35% DS and 75% DS), 35% and 75% coniferous scenario (35% CS and 75% CS), 35% and 75% combination scenario (35% CBS and 75% CBS).As mentioned previously, the simulations spanned 28 h (with the initial four hours discarded for spinup), enabling comprehensive evaluation of tree and plant configuration impacts during both day and night.supplementary figure 2 illustrates the plant configurations across the site for the base scenario and alternative scenarios.

ENVI-met evaluation
In the validation process, as discussed in section 2.2.1, we utilize 5-minute intervals for both simulated and measured data from the five measurement locations.According to supplementary figures 3-7, ENVI-met simulations tend to overestimate both Ta and MRT values from the early morning until sunset.Notably, the temporal evolution of all variables is well reproduced.Moreover, ENVI-met demonstrates the capability to simulate meteorological data based on natural or artificial elements surrounding each location.Supplementary figures 4 and 7 illustrate the effect of locations, highlighting that both building shading and vegetation cover have the potential to reduce MRT and Ta values.
ENVI-met simulations demonstrated the highest MRT skill between 10:00 and 16:00, with RMSE values less than ∼1 • C during this portion of the diurnal cycle (figure 2).Conversely, ENVI-met simulations demonstrated the worst skill for MRT during the early morning and early evening hours, with RMSE values between ∼3 • C and 3.5 • C. ENVI-met performance for wind speed was also deemed satisfactory, as indicated by R 2 and RMSE values, with the highest skill evident from the early morning through the midafternoon hours, with reduced model skill through the early evening (R 2 values < 0.8).Our simulations generally depict good agreement among the suite of observed meteorological variables with wind speed (between 08:00 and 14:00 LST) and relative humidity (between 08:00 and 18:00 LST) exhibiting R 2 values less than 0.91 and 0.92, respectively.Of the 20 measured sample points (i.e. 4 distinct variables observed at five distinct times) only 3 had RMSE values greater than 1; 19 of the 20 simulation sample points had R 2 values greater than 0.8 and 10 of the simulation sample points had R 2 greater than 0.9.Overall, the level of agreement between the suite of simulated and observed variables was considered of sufficient reliability to move forward with sensitivity assessment resulting from tree deployment.

Assessment of tree species deployment
We evaluate the micrometeorological effect of tree deployment scenarios through an assessment of impacts on Ta, Ws, and MRT, for the hottest day during the summer of 2023.We examine the effect of tree deployment through domain-averaged analysis and at specific snapshots in time: 10:00, 14:00, 17:00, and 21:00 LST, the latter of which was designated as nighttime for the purposes of our discussion.Furthermore, the dPET index was computed for each scenario at the aforementioned hours.This calculation enabled us to pinpoint the scenario with the most effective cooling performance among the vegetation scenarios on improving outdoor thermal comfort for a mobile individual traversing across a range of micrometeorological conditions.

Impact on Ta
The spatial variability of Ta varies across the diurnal cycle and is dependent on the tree deployment scenario undertaken (figure 3).The current configuration (i.e.base scenario) consistently leads to higher Ta values at all times, with peak daytime temperatures exceeding 43 • C at all locations at 14:00 LST and nighttime temperatures remaining above 28 • C at all locations at 21:00 LST.The deployment of both 35% scenarios leads to generally consistent results with minimum Ta differences between them: peak temperatures are reduced by 3 • C-6 • C, relative to base scenario, along the locations of tree deployment.Among the 75% scenarios, the deciduous configuration achieves the most significant reduction in domain-averaged Ta (4.2 • C), followed by 75% combination scenario (3.6 • C), and 75% coniferous scenario (3.1 • C), when averaged across the diurnal cycle (supplementary figure 8).These results underscore the greater Ta reduction arising from deciduous relative to coniferous trees.Differences in Ta reduction are more apparent during peak temperature hours (i.e.10:00-14:00 LST), but are moderated during late afternoon and evening hours (after 17:00 LST) with less contrast evident between 35% and 75% deployment scenarios.It is important to note that although the impact of tree deployment is greatest during the sunlit portion of the diurnal cycle, in agreement with previous research, our results indicate that cooling of a reduced magnitude persists during the evening and nighttime hours (supplementary figure 8), in contrast with previous research (e.g.[2,80]).
In order to delve further into the effects of different vegetation scenarios on Ta, the mean Ta beneath the vegetation, at a height of 1.8 m, was evaluated (supplementary figure 9).The results indicate that through all the simulation hours deciduous species with a mean LAD of 1.06 benefit from a lower Ta compared to all other tree species, which indicates, as expected, that the higher values of LAD results in lower Ta across the study area.Simultaneously, there are negligible differences (i.e.< 0.2 • C) between 75% coniferous scenario (mean LAD 0.7) and 35% deciduous deployment, which indicates that although the number of trees in the 75% scenarios is greater than the 35% scenarios, the LAD has a great impact on the Ta reduction.

Impact on Ws
A key question associated with tree deployment is the effect on local circulation.Our simulations indicate that tree deployment has a marginal effect on Ws and therefore only modestly impacts ventilation (supplementary figure 10).For example, local microclimatic differences in Ws across the site during the time of peak heating (i.e.14:00-17:00 LST) are greater than differences arising from any of the tree deployment scenarios.Nevertheless, at 14:00, base scenario exhibited the lowest mean Ws (approximately 0.6 m s −1 ), while the tree deployment scenarios resulted in a small Ws increase; we emphasize that these differences are generally less than 1.0 m s −1 .
Our domain averaged assessment confirms this result, and highlights a small, but consistent decrease in Ws throughout the diurnal cycle.Peak differences in Ws generally scale with the diurnal increase in Ta, although there are relatively short periods (e.g.14:00-15:00 LST) with negligible or nearly negligible differences in wind speed among all simulations conducted.Overall, our analysis indicates the 75% coniferous scenario resulted in the greatest reduction in Ws (0.65 m s −1 ), followed by 75% combination scenario (0.54 m s −1 ), and 75% deciduous scenario (0.5 m s −1 ), relative to base scenario.These relatively modest differences indicate minimal impact of tree deployment on ventilation throughout the course of the diurnal cycle (supplementary figure 11).

Impact on MRT
We next investigate the effects of tree deployment that consider the radiation balance, providing a more comprehensive emphasis on the impacts on outdoor thermal comfort.Tree deployment has a significant impact on MRT, leading to the greatest reduction during daytime and a comparatively lesser effect during nighttime hours (figure 4).For example, while MRT values are generally above 60 • C for the majority of base scenario locations at 14:00 LST, MRT values corresponding to the tree deployment scenarios are generally less than 40 • C. The spatial differences during the daytime (generally greater than 20 • C) are considerably lower during the nighttime, wherein differences between base scenario and the tree deployment scenarios are generally within a 10 • C-15 • C range.
At 10:00, MRT values reach ∼80 • C for the base scenario, largely driven by the limited availability of shade (figure 4).Conversely, during the same period, a lowest MRT of ∼21 • C was simulated in the 75% deciduous experiment.This can be attributed to the absorption properties of deciduous tree leaves and branches, resulting in reduced transmissivity to the surface.Notably, the denser tree planting in the 75% scenarios led to a more pronounced reduction in MRT, with values across the domain generally below 30 • C.
The domain-averaged effect (supplementary figure 12) illustrates distinct patterns across various deployment scenarios that are more evident than Ta.Notably, the highest MRT values are observed for base scenario, which is particularly pronounced during daylight hours.In contrast, the 35% deployment scenarios form a closely clustered group, featuring MRT values consistently lower by approximately 20 • C-25 • C compared to the base scenario during the daytime.However, as also mentioned previously, reduced differences emerge during nighttime hours.The 75% deployment scenarios exhibit the most substantial cooling effects, characterized by mean MRT values consistently below 30 • C. Interestingly, the 75% deciduous scenario stands out as the one with the lowest MRT values within this group, during both daytime and nighttime; nighttime differences, however, generally remain within the 5 • C range.
As part of the MRT evaluation, the urban morphology indicator employed was the Sky-View Factor (SVF), measured on a scale from 0 to 1.A higher SVF value suggests an unobstructed sky view, while a lower value indicates a pronounced street canyon [33].Analysis of base scenario morphological parameters (see supplementary table 4) revealed SVF values ranging from 0.64 to 0.83 at specific points (i.e.receptors 1 and 3), indicating open skies that allows for transmission of incoming solar radiation without interference from nearby artificial structures or tree shading.The lowest SVF value of 0.29 was recorded at point 5, situated beneath the shade of a Fraxinus excelsior tree, capable of obstructing incoming radiation.
Furthermore, the influence of morphological variables on comfort conditions is illustrated by correlations between SVF and MRT (see supplementary figure 13).Positive relations (R 2 ⩾0.91) between SVF and MRT were observed during daylight hours, indicating that in open spaces, incoming solar radiation raises the MRT.Conversely, the positive relationship noted during daytime hours was absent in the evening across all deployment scenarios, signifying that ground long-wave radiation escapes as streets widen or SVF values increase [48].Our results do not indicate that increasing tree coverage, which creates canyons with low values of SVF (i.e. a greater degree of obstruction), leads to an increase of MRT values at pedestrian level (supplementary figure 13), which may be anticipated due to enhanced downward longwave radiation.Nevertheless, this result is likely dependent on structural characteristics of trees such as foliage density, crown height and other aspects, warranting future research.

Contribution to dPET by tree species deployment in summertime
We note that assessment of the impacts of various tree deployment scenarios on meteorological variables (i.e.Ta, Ws, and MRT) is important and has been the subject of intense prior research for urban environments across the globe.However, how these characteristics impact outdoor thermal comfort is of particular relevance to human-environment outcomes, which we next quantify using the dPET index (see section 2.2.2).
Our results illustrate that the base scenario leads to the highest value of dPET (53.07 • C; this value represents the average across the 32 min virtual walk) compared to other tree deployment scenarios (figure 5).In contrast, the 35% tree deployment scenarios reduce dPET by approximately 10 • C-15 • C in comparison to base scenario, which, as mentioned previously, is characterized by the least amount of shading spots.However, our results also indicate there are negligible differences among the 35% configurations.Furthermore, it is evident that among the 75% deployment scenarios, the deciduous species is able to enhance thermal comfort (i.e.lower dPET) more than other scenarios.It is evident that a pedestrian walking along the assumed route benefits from a higher number of zones with a dPET below (32.70 • C) due to the significant shading provided by a variety of tree species.This shading enhances their ability to reduce thermal stress during the daytime, especially in the 75% deciduous scenario, in comparison to the other 75% scenarios.Furthermore, the base case (i.e.contemporary) site situation exhibits the highest dPET value, reaching a maximum of 53.96 • C, when compared to all other alternative scenarios.This could pose a potential threat to pedestrian thermal safety, particularly on the hottest days of summer.Among the 35% scenarios, both the maximum and minimum dPET values indicate similar impacts on dPET reduction, falling within the Max.range of 40 • C to 42 • C. In contrast, the 75% deciduous scenario stands out An important question is how such a virtual walker would experience the ambient environment should it occur at other times during the diurnal cycle.We therefore repeat ENVI-met simulations and conduct such virtual walks at three additional hours: 14:00, 17:00, and 21:00 LST.Our results indicate that the highest values of dPET are observed in the base scenario and the greatest reduction in dPET occurs during the sunlit portions of the diurnal cycle (supplementary figure 14).Among the 35% deployment scenarios, deciduous tree species resulted in a greater decrease in dPET values and therefore improved outdoor thermal comfort, except during the mid-morning period (i.e.10:00 LST), wherein differences between coniferous scenario, deciduous scenario, and combination scenario were negligible.The 75% deployment scenarios demonstrate the most significant dPET reduction, characterized by mean values consistently below 35˙•) during the daytime and below 29 • C during nighttime.The 75% deciduous scenario simulation exhibits superior effectiveness in reducing dPET at specific points along the path compared to its counterparts during both daytime and nighttime.Moreover, minimal differences emerge during nighttime hours among all the scenarios, as simulated outdoor thermal comfort remains in a range between 26 • C-29 • C along the entirety of the pathway.
The consequences of reduced thermal comfort have direct meaning on the body's natural ability to compensate for perturbations in energy load that may lead to enhanced skin temperature (T skin) and reduced ability to produce sweat (i.e. the body's natural cooling mechanism; figure 6).Notably, during daylight hours, an individual traversing micrometeorological conditions as in the base scenario would undergo an elevated T skin that peaks at 36.4 • C at 12:00 LST-this denotes an increase in T skin that is at least 0.5 • C greater than all tree deployment scenarios (figure 6(a)).All 35% deployment scenarios exert a similar influence on T skin reduction during day, with negligible impacts during nighttime.Conversely, the 75% deployment scenarios demonstrate superior ability to reduce T skin, with a daytime reduction of nearly 1 • C at both 10:00 and 14:00 LST.Among the 75% scenarios, the deciduous deployment exhibits the most favorable impact on T skin, for both day and nighttime.The effect of enhanced T skin leads to greater SP, as a human attempts to offset the increase in (solar) energy loading.Reduction in solar exposure therefore plays a critical role in the body's ability to reduce cooling through this evaporative mechanism (figure 6(b)).We note substantial differences between base scenario and the tree species deployment simulations in their ability to decrease sweat produced (SP) that is greater during the daytime than the nighttime, when all scenarios indicate SP values below 50 g h −1 .Additionally, the 35% deployment scenarios cluster closely, consistently featuring SP values approximately 100-150 g h −1 lower than base scenario during the daytime.In contrast, the 75% scenarios are the most effective deployment configurations, reducing SP values by nearly 150-200 g h −1 during the daytime.

Summary and conclusions
In this research, the effectiveness of six tree deployment scenarios, aimed to examine the potential to improve the thermal comfort of pedestrians, was assessed in a post-industrial landscape within the Ray neighborhood, situated in the mega city of Tehran.These scenarios included deciduous and coniferous trees, and a combination of them, with a strategy of minimum (35%) and maximum deployment (75%) each.The effects of each tree deployment scenario on meteorological parameters (i.e.Ta, Ws, and MRT) was evaluated through use of a CFD micrometeorological model.The efficacy of tree deployment strategies was investigated for the hottest day of summer in Tehran in July 2023, coinciding with the hottest summer in the planet's history [81].We additionally calculated dPET, a novel index that characterizes the thermal comfort of walking pedestrians, to evaluate the effect of tree deployment scenarios of individuals at four different times during the extreme heat day.
Our results demonstrate the greater benefits imparted for deciduous over coniferous tree deployment in mitigating thermal heat.When replacing the contemporary site conditions with a 35% deciduous deployment strategy, a noticeable benefit emerges, resulting in a 1.2 • C reduction in ambient temperature during peak heating time.Moreover, a substantial contribution is evident with a 75% deciduous deployment strategy, showcasing a significant decline in average ambient temperature up to 4.2 • C.
Interestingly, the type of tree species deployed seems to have negligible effects on wind speed and local circulation.However, the deployment of 75% deciduous trees exhibits a substantial decrease in MRT values, reaching up to 60 • C. Comparatively, both 75% deciduous tree deployment and 75% coniferous tree deployment demonstrate similar effectiveness in reducing MRT.Additionally, the 75% deciduous tree deployment significantly improves the thermal environment, as indicated by a ∼31 • C reduction in dPET.This is primarily due to greater leaf reflection of incoming solar radiation.A denser leaf cover (i.e.greater LAD values) increases the overall reflection of sunlight, reducing the amount of solar energy absorbed by the plant, and therefore received at the surface.Our results also indicate a 35% deciduous tree deployment scenario that yields an average reduction of ∼35 • C in dPET during the same simulation period.In conclusion, both 75% and 35% intervention scenarios emerge as robust strategies for mitigating thermal stress and alleviating the burden of heat induced by the built environment.Nevertheless, the 75% intervention scenarios exhibit superior performance in enhancing the thermal environment, regardless of variations in tree species.
While this study is the first to investigate dPET impacts through model simulation (i.e.ENVI-met), there are some limitations that should be considered.First, the principal aim of our research was to evaluate the effects of tree deployment on dPET enhancement by considering only one group of people with identical weight, height, and age.In particular, for other vulnerable groups, including young children and the elderly, corresponding simulations should be conducted to determine the thermal stress threshold(s) for those specific groups.Second, this study only considered thermal heat and dPET simulation for the summer and the hottest day of the year.Future research could delve into the comparison of dPET in winter and consider the impact of different vegetation cover on dPET values during other times of the year (e.g.certain physiological thresholds may be crossed at other times of the year, such as the first heat wave during the summer before any acclimatization can occur).Third, there is a growing body of research considering the effects of urban heat on pedestrians in an experimental way, benefiting from observations and monitoring of personalized reactions to the thermal environment.For example, Wang et al [68] conducted a study in Mianyang, China, investigating walkability impacts through a dynamic attenuation model of heat stress.Huang et al [69] developed a quantitative method to assess the thermal usability of outdoor spaces through field experiments in Changzhou, China.Personalized and dynamic responses of individuals to extreme ambient thermal and radiative conditions that better characterize human health and well-being using wearable sensors are receiving increasing research attention [82].Currently, we lack quantitative thresholds signifying physiological benefits when an individual's ambulatory experience shifts from one category (e.g. with serious health implications) to another (e.g. with reduced health implications) in the context of the dPET metric.Therefore, future work should account for combined field/survey experiments with ENVImet simulations to assess and validate simulation output of dPET with real conditions experienced by pedestrians across diverse urban landscapes.
We note that the model validation is generally superior during the midday hours compared to the morning and evening hours.We hypothesize that the less than stellar temperature performance metrics for the morning and evening hours are due to a time-of-day effect, or potentially also due, in part, to a location effect.We also note that the greatest simulated effect of our thermal mitigation scenarios occurred during the portion of the day that aligned with the highest model skill.Nevertheless, we underscore the need to conduct additional in-situ observational measurements for further, refinement of model performance during the entire diurnal cycle.Indeed, our model validation process faced certain limitations.Our study area, designated as a preserved post-industrial heritage site, welcomed visitors only from morning until 18:00, necessitating that our measurement campaign aligns with these hours.Consequently, our measurements concluded at 18:00, resulting in a dataset spanning 10 h.Future observational campaigns designed for in-situ measurement across 24 h, or periods of longer duration, and extending to other seasons, are required.
Our results indicate that within post-industrial areas the presence of green spaces and the types of tree species deployed can play a crucial role in enhancing outdoor thermal comfort and mitigating thermal heat.Deciduous trees, specifically, exhibit a superior performance in reducing daytime dPET during the hottest summer day through provision of shade from direct sunlight.Future research should examine potential impacts during winter, as the senescence of deciduous trees permit solar radiation to pass through their reduced foliage during a time of the year when solar exposure may be deemed beneficial.Simulateneously, the water costs associated with tree deployment strategies examined here also require accounting.Water resource availability is a particularly important concern for urban areas located in arid or semi-arid environments, and the exchange of water for cooling may not be desirable or sustainable.
A pivotal outcome of our work underscores that a combination of coniferous and deciduous tree coverage, serving multiple ecological functions, establishes a sheltered microclimate across the site.This microclimate enhancement significantly improves outdoor thermal comfort for humans, with provision of notable cooling effects.This research provides valuable insights about post-industrial landscapes design and their contribution to outdoor thermal comfort, for urban designers and landscape architects through quantifiable design outcomes at personalized scales.Subsequent studies could explore additional design scenarios aimed at optimizing the presence of green spaces and plant life, including the integration of vertical greening and various tree species.Critically, our work provides valuable insight into the nuanced thermal dynamics associated with different tree deployment scenarios and illustrates the significant effect of tree species on the thermal comfort of mobile pedestrians.In the event that green infrastructure is incorporated as a possible next step in the site's landscape design, our results demonstrate the direct benefits of tree deployment on a pedestrian's ambulatory experience through this industrial heritage site.

Figure 1 .
Figure 1.(a) Geographical location of study: Ray, Tehran, Iran.(b) Location of the cement factory measurement points on the simulation day.

Figure 2 .
Figure 2. The relationship between measured and simulated variables.Each point in the scatterplot indicates the measured versus observed variable.The blue shading expresses the standard error for the nonlinear best fit (black curve).R 2 and RMSE values correspond to the best linear fit equation, which is shown in the upper left hand side of each panel.There are five different measurement points and their locations are shown in figure 1(b).

Figure 3 .
Figure 3. ENVI-met simulated Ta (1.8 m) for all tree deployment scenarios at the designated time during the hottest day of summer 2023.

Figure 4 .
Figure 4. ENVI-met simulated MRT (1.8 m) for all tree deployment scenarios at the designated time during the hottest day of summer 2023.

Figure 5 .
Figure 5. ENVI-met simulated dPET for all tree deployment scenarios for virtual walks starting at 10:00 LST (green circle labeled start) and concluding at 10:32 LST (red circle labeled end) during the hottest day of summer 2023.The yellow points along the simulated pathway display the considered stop points.

Figure 6 .
Figure 6.ENVI-met simulated (a) mean skin temperature of the human body at four specific hours during the simulated virtual walk illustrated in figure 5; (b) mean sweat produced by the human body at four specific hours during a simulated virtual walk illustrated in figure 5.