Temporal Changes and Spatial Distribution of Suspended Particulate Matter in the Southern Alborz Mountains Based on Satellite Data

Introduction

Air pollution became evident in human life when human settlements began to form as concentrated and stationary communities. Air pollution, caused by suspended particulate matter, refers to changes in the natural composition of the atmosphere due to the introduction of particles from both human and natural sources. Suspended particulate matter in the air consists of a mixture of solid and liquid particles that vary significantly in terms of shape, size, number, chemical composition, and origin. These differences cause some suspended particulate matter to have more severe health impacts than others. In megacities, the primary sources of suspended particulate matter are a combination of fossil fuel combustion from vehicles, construction equipment, furnaces, and power plants. Iran, like many other developing countries, is grappling with air pollution and is not exempt from this problem. Due to the geographical location of the study area in the southern slopes of the central Alborz Mountains and the comprehensive view of this region for spatiotemporal analysis of suspended particulate matter, the lack of air quality monitoring stations is a major challenge. This shortage is particularly noticeable in the cities of Karaj and Qazvin. Therefore, the primary objective of this study is to analyze the five-year (2018-2022) temporal and spatial patterns of suspended particulate matter in the study area. This analysis encompasses annual, seasonal, and monthly variations using the Aerosol Absorption Index (AAI) derived from Sentinel-5 satellite imagery, adopting a holistic approach to the region.



Methodology

To investigate the spatiotemporal variations of suspended particulate matter in the study area, the Aerosol Index (AAI) product from the Sentinel-5 satellite was utilized. Sentinel-5, as the first Copernicus mission satellite, is a powerful tool for atmospheric monitoring. The Tropospheric Monitoring Instrument (Tropomi) was launched aboard Sentinel-5 on October 13, 2017. These analyses were conducted using JavaScript programming language through the Google Earth Engine web platform. The outputs obtained from this index include several results for evaluating the temporal and spatial changes of these pollutants and understanding their distribution patterns in the environment. The products of this index include a five-year average of the suspended particulate matter index (from 2018 to 2022), an annual trend analysis over this period, and an analysis of seasonal and monthly variations. Subsequently, a spatial map of the average concentration of suspended particulate matter was classified and produced using ArcMap software. Finally, to more accurately discover the spatial distribution and intensity of particulate matter pollution, hot spots and cold spots and Getis-Ord Gi* statistics were used. This analysis can help better understand the spatial distribution of air pollution and identify critical areas, and ultimately show the difference in the intensity of air pollution between the metropolitan areas of Tehran, Karaj, and Qazvin.

Results and Discussion

A five-year average analysis of spatial distribution of suspended particulate matter reveals a high concentration of particulates, including dust and smoke, in the southern and eastern regions, particularly around Tehran and the Qom border. This concentration is linked to anthropogenic factors such as population density, industries, and traffic. Additionally, wind patterns and topography play a role in particulate distribution. Hot spot analysis using the Getis-Ord-Gi statistic showed that the southern regions are more polluted, while the northern regions have better air quality. Moreover, the suspended particulate matter index showed a decreasing trend from 2018 to 2020, reaching its lowest point in 2020. However, it exhibited an increasing trend from 2021 onwards, peaking in 2022. This increase can be attributed to various factors such as increased industrial activities and climatic changes. Pollution hotspots were concentrated in the southern regions of Tehran, Karaj, and Qazvin and remained relatively stable throughout this period. In other words, spatial variations in the concentration of suspended particulate matter have not been significant during this period, and the spatial distribution of suspended particulate matter has remained relatively stable. On the other hand, northern regions with lower population density and better air flow have cooler spots with less pollution. The results of the horizontal visibility field show that visibility at Mehrabad station has continuously decreased and has a significant correlation with the suspended particulate matter index. At Karaj station, visibility decreased from 2018 to 2020 and then increased until 2021, but decreased again in 2022. These changes indicate the significant impact of air pollution and suspended particulate matter on the reduction of horizontal visibility. Seasonal analysis of suspended particulate matter distribution and concentration, along with hotspot identification, reveals that the highest SPM concentrations occur during the summer months, particularly in the southern and central regions of Tehran, which are influenced by surrounding deserts and northerly winds. Similar patterns can be observed in the autumn season, while spring and winter exhibit different distributions. Hotspots linearly extend from southeast Tehran to west Qazvin, and the northern regions and Alborz Mountains are cold spots with lower pollution. The monthly distribution of suspended particulate matter reveals a consistent increase from May to September, reaching its peak in July. The highest concentrations were observed in the Tehran, Karaj, and Qazvin regions, particularly along the Tehran-Qazvin freeway. This increase is attributed to a combination of stable atmospheric conditions, rising temperatures, reduced wind speeds, traffic congestion, and industrial activities, leading to the accumulation of pollutants and deteriorating air quality.



Conclusion

A precise understanding of the impacts of particulate matter on human health and the environment requires continuous monitoring of particle concentration and size distribution on a global scale. The high variability of these particles in the atmosphere poses significant challenges for ground-based monitoring networks. Therefore, remote sensing technologies, especially Sentinel-5 satellite data, provide an efficient tool for examining the spatial and temporal distribution of suspended particulate matter. The results indicate that the highest concentrations of suspended particulate matter are concentrated in the central and southeastern regions of Iran, particularly around Tehran. Between 2018 and 2020, due to restrictions imposed by the COVID-19 pandemic, the concentration of suspended particulate matter decreased continuously and reached its lowest level in 2020. However, with the return to normal activities, pollutants increased again and reached their highest level in 2021. Summer, with its unique climatic conditions, experiences the highest concentrations of suspended particulate matter, particularly in Tehran. This region is influenced by dust storm sources in the surrounding deserts. Moreover, monthly variations reveal an increase in particulate concentrations from May to July. In-depth analyses can help evaluate the impact of pollution control policies and measures, and contribute to the development of predictive models for the future. Therefore, increasing the number of ground-based monitoring stations and utilizing remote sensing technologies are essential steps for optimal air pollution management and the implementation of sustainable environmental policy.