Impact of Urban Characteristics on Nitrogen Dioxide Concentration and Urban Heat Island Intensity: A Case Study of Tehran

Introduction

Urbanization places increasing pressure on environmental resources, adversely impacting air and water quality, land availability, and local climatic conditions (Das et al., 2020). Urban areas contribute significantly to greenhouse gas emissions while also being susceptible to the effects of climate change, such as sea level rise and global warming (IPCC, 2022). Urban development alters surface radiation and moisture balances, leading to shifts in land use and biogeochemical cycles (Oke, 1982; Lowry & Lowry, 2001). This study explores the relationship between urban structural characteristics—namely built-up areas, building heights, and vegetation cover—and their influence on the urban heat island (UHI) effect and nitrogen dioxide (NO₂) emissions in Tehran. Leveraging a variety of remote sensing datasets, the research offers an integrated assessment of urban form, air quality, and thermal dynamics. Advanced geospatial analysis facilitates a more nuanced understanding of the interconnections between urban morphology and environmental conditions.



Methodology

To assess the physical characteristics of the urban environment, this study utilizes remote sensing datasets from the Global Human Settlement Layer (GHSL), specifically the Global Built-up Surface dataset for quantifying built-up areas and the Global Building Height dataset for determining building heights. These datasets are readily available through the Google Earth Engine (GEE) platform. Vegetation cover is evaluated using the Normalized Difference Vegetation Index (NDVI), derived from Sentinel-2 satellite imagery. Given variations in data resolution and sample size, the study area—Tehran—is segmented into a uniform grid of 1000 × 1000 meters. Within each grid cell, key variables are computed, including the extent of the urban heat island (UHI) effect, nitrogen dioxide (NO₂) concentration, average building height, total built-up surface, and vegetation cover. Subsequently, the relationships between urban physical characteristics and both the UHI effect and NO₂ concentrations are analyzed using the Pearson correlation coefficient. This approach enables a systematic evaluation of the interactions between urban morphology, thermal dynamics, and air pollution at a spatially consistent scale.



Results and discussion

Urban Heat Island in Tehran

During the study period from 2018 to 2020, the highest average annual temperature in Tehran was recorded in 2022 at 17.7°C, while the lowest was observed in 2020 at 15.3°C. In contrast, the suburban areas of Tehran experienced a peak average temperature of 12.8°C in 2022 and a minimum of 10.7°C in 2019. Seasonal analysis reveals that temperatures within Tehran consistently exceed those in surrounding rural areas throughout the year, indicating the persistent presence of an urban heat island (UHI). The intensity of the UHI in Tehran varies seasonally, reaching an average maximum of 2.5°C in winter and increasing to 3.4°C during the summer months. Notably, the UHI effect intensifies during the colder seasons, a trend largely attributed to increased anthropogenic activities, particularly the widespread use of indoor heating systems in urban areas. In 2021, when the city’s average temperature dropped to 4.9°C, central urban districts—specifically 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 structures and human activity on localized climate conditions within the Tehran metropolitan area.

Nitrogen Dioxide (NO₂) Concentration in Tehran

Analysis of nitrogen dioxide (NO₂) concentrations in Tehran from 2018 to 2024 reveals that Regions 7 and 8 consistently exhibited the highest average levels of NO₂, while Regions 20 and 19 recorded the lowest concentrations. The highest concentration was recorded in 2021, whereas the lowest levels were observed in 2018 and 2019. The peak in NO₂ concentration during 2021 may be attributed to the aftermath of the COVID-19 pandemic. While reduced vehicular traffic and industrial activities during lockdowns in 2019 and 2020 likely contributed to lower emission levels, a resurgence in economic activity in 2021 may have led to an increase in pollutant emissions. Seasonal analysis further indicates that NO₂ concentrations are highest in December and January, coinciding with the occurrence of winter temperature inversions that trap pollutants near the surface. Conversely, NO₂ levels are lowest between April and September, a period associated with higher atmospheric dispersion and reduced emissions. Specifically, April exhibits a marked decrease in NO₂ compared to March, likely reflecting the decline in urban and industrial activities during the Nowruz holidays.

Conclusion

The results of this study demonstrate that Tehran’s urban heat island (UHI) exhibits a clear seasonal pattern, intensifying during the winter months due to increased fossil fuel consumption and subsiding in the summer. Additionally, the UHI effect is notably reduced on weekends and public holidays (e.g., Thursdays and Fridays), which coincide with decreased industrial activity—a pattern consistent with previous findings. The analysis further reveals a statistically significant relationship between UHI intensity and urban physical characteristics, particularly built-up areas and building heights. These urban forms contribute to heat accumulation, while increased vegetation coverage is associated with a mitigating effect on urban temperatures. Similarly, nitrogen dioxide (NO₂) emissions follow a seasonal trend analogous to that of the UHI, peaking in winter due to heightened urban and industrial activities and declining during periods of reduced activity. Correlation analyses indicate strong associations between NO₂ concentrations and built-up density, building height, and vegetation cover. Specifically, areas with high building density and taller structures tend to exhibit elevated NO₂ levels, whereas areas with greater vegetation cover are associated with lower concentrations. These findings underscore the critical role of urban morphology in shaping both thermal and atmospheric conditions in metropolitan environments and highlight the importance of vegetation in mitigating urban heat and air pollution.