Impact of Urban Characteristics on Nitrogen Dioxide Concentration and Urban Heat Island Intensity: A case study of Tehran

ABSTRACT
Air pollution and the Urban Heat Island (UHI) effect have become critical challenges in megacities such as Tehran, particularly as a result of rapid population growth and ongoing urban expansion. This study investigates the relationship between key urban characteristics, including built-up areas, building height, and vegetation cover, and UHI intensity as well as nitrogen dioxide (NO₂) concentration, which represents a major urban air pollutant. Data on built-up areas and building height were obtained from the Global Human Settlement Layer (GHSL), while vegetation cover was mapped using the Normalized Difference Vegetation Index (NDVI) derived from Sentinel-2 imagery. UHI intensity was assessed using nighttime MOD11A2 land surface temperature products acquired at 22:30 with a spatial resolution of 1000 m for the period 2018 to 2022, while NO₂ concentrations were retrieved from the TROPOMI sensor onboard Sentinel-5 for the period 2019 to 2024. All data processing and analysis were conducted using the Google Earth Engine platform, with Pearson correlation analysis applied to examine the relationships among variables. The findings reveal significant positive correlations between both built-up areas and building height and NO₂ concentration (R² = 0.45 and 0.18, respectively), indicating that urban growth substantially contributes to increased NO₂ levels. A weaker positive correlation was observed between vegetation cover and NO₂ concentration (R² = 0.097), which is attributed to Tehran’s topographic conditions and the accumulation of pollutants in certain densely vegetated areas. The results also indicate strong positive associations between built-up areas and building height and UHI intensity (R² = 0.45 and 0.18), highlighting their significant role in intensifying urban heat island effects. Although the relationship was not statistically significant, an inverse association was observed between vegetation density and UHI intensity, suggesting that vegetation may play a mitigating role in reducing urban heat intensity. Overall, this study underscores the strong influence of urban morphology on both air quality and thermal dynamics, while highlighting vegetation as a potential strategy for moderating UHI intensity in rapidly growing cities such as Tehran.
Extended Abstract
Introduction
Urbanization exerts increasing pressure on environmental resources and has adverse effects on air and water quality, land availability, and local climatic conditions. Urban areas play a substantial role in the emission of greenhouse gases and, at the same time, are highly vulnerable to the impacts of climate change, including global warming and sea-level rise. Urban development alters surface radiation balance and moisture regimes, leading to changes in land use patterns and biogeochemical cycles. The present study examines the relationship between urban structural characteristics, including built-up areas, building height, and vegetation cover, and their effects on the Urban Heat Island (UHI) phenomenon and nitrogen dioxide (NO₂) emissions in Tehran. By employing multiple remote sensing datasets, this research seeks to provide an integrated assessment of urban structure, air quality, and urban heat island intensity.
 
Methodology
This study employs datasets from the Global Human Settlement Layer (GHSL) to assess the physical characteristics of the urban environment, with particular emphasis on the Global Built-up Surface dataset for quantifying built-up areas and the building height dataset for estimating building heights. These datasets are readily accessible through the Google Earth Engine (GEE) platform. Vegetation cover characteristics were extracted using the Normalized Difference Vegetation Index (NDVI) derived from Sentinel-2 satellite imagery. To address differences in spatial resolution and sample size, the study area, namely the city of Tehran, was divided into a uniform grid with a spatial resolution of 1000 × 1000 m. Within each grid cell, key variables were calculated, including Urban Heat Island (UHI) intensity, nitrogen dioxide (NO₂) concentration, mean building height, total built-up area, and vegetation cover. Subsequently, the relationships between urban physical characteristics and both UHI intensity and NO₂ concentration were analyzed using Pearson correlation coefficients. This methodological framework enables a systematic assessment of the interactions between urban structure, urban heat island characteristics, and air pollution at a consistent spatial scale.
 
Result and Discussion
Urban Heat Island in Tehran
During the study period from 2018 to 2022, the highest mean annual temperature in Tehran was recorded in 2022 at 17.7 °C, while the lowest mean annual temperature was observed in 2020 at 15.3 °C. In the suburban areas of Tehran, the highest average temperature reached 12.8 °C in 2022, whereas the lowest value of 10.7 °C was recorded in 2019. Seasonal analysis indicates that temperatures within the urban core of Tehran consistently exceed those of the surrounding suburban areas throughout the year, confirming the persistent presence of the Urban Heat Island (UHI) phenomenon.
UHI intensity in Tehran exhibits clear seasonal variability, reaching its maximum value of 5.2 °C during the winter months and decreasing to approximately 4.3 °C during the summer. Notably, the UHI effect intensifies during colder seasons, a pattern that can largely be attributed to increased anthropogenic activities, particularly the extensive use of heating systems in urban areas. In 2021, when the mean urban temperature declined to 14.9 °C, central districts, especially Districts 10 and 11, exhibited elevated temperature levels, with an average UHI intensity of 4.4 °C. This spatial concentration of higher temperatures highlights the influence of dense urban morphology and intensified human activities on local climatic conditions in Tehran.
Nitrogen Dioxide (NO₂) Concentration in Tehran
Analysis of nitrogen dioxide (NO₂) concentrations in Tehran from 2018 to 2024 indicates that Districts 7 and 8 consistently experienced the highest average NO₂ levels, whereas Districts 19 and 20 recorded the lowest concentrations. The highest overall NO₂ concentration was observed in 2021, while the lowest levels occurred during 2018 and 2019. The peak in NO₂ concentration in 2021 may be associated with post–COVID-19 pandemic conditions. Although reductions in vehicular traffic and industrial activities during lockdown periods in 2019 and 2020 likely contributed to lower emission levels, the subsequent resumption of economic activities in 2021 appears to have led to a renewed increase in pollutant emissions.
Seasonal analysis further reveals that NO₂ concentrations peak during December and January, coinciding with winter temperature inversion events that trap pollutants near the ground surface. In contrast, NO₂ levels reach their minimum between April and September, a period characterized by enhanced atmospheric dispersion and reduced emission intensity. In particular, April shows a marked decrease in NO₂ concentrations compared to March, which may reflect reduced urban and industrial activities during the Nowruz holiday period.
 
Conclusion
The results of this study indicate that the Urban Heat Island (UHI) in Tehran exhibits a distinct seasonal pattern, intensifying during the winter months as a result of increased fossil fuel consumption and weakening during the summer. In addition, UHI intensity decreases markedly during weekends and official holidays, such as Thursdays and Fridays, coinciding with reduced industrial activity, a pattern that is consistent with findings reported in previous studies. Further analysis reveals a statistically significant relationship between UHI intensity and urban physical characteristics, particularly built-up areas and building height. These urban forms contribute to heat accumulation, whereas increased vegetation cover is associated with a moderating effect on urban temperatures.
Similarly, nitrogen dioxide (NO₂) emissions follow a seasonal pattern comparable to that of the UHI, reaching peak levels in winter due to intensified urban and industrial activities and declining during periods of reduced activity. Correlation analysis indicates strong associations between NO₂ concentration and built-up density, building height, and vegetation cover. Specifically, areas characterized by higher building density and taller structures exhibit elevated NO₂ levels, while areas with greater vegetation cover are associated with lower concentrations. These findings underscore the critical role of urban morphology in shaping thermal and atmospheric conditions in urban environments and highlight the importance of vegetation in mitigating urban heat and air pollution.
 
Funding
There is no funding support.
 
Authors’ Contribution
All of the authors approved thecontent of the manuscript and agreed on all aspects of the work.
 
Conflict of Interest
Authors declared no conflict of interest.
 
Acknowledgments
We are grateful to all the scientific consultants of this paper.