تغییرات زمانی و پراکنش مکانی شاخص ذرات معلق در دامنه جنوبی البرز مرکزی بر پایه داده‌های ماهواره‌ای

ABSTRACT
Air pollution is one of the primary environmental challenges in the metropolitan areas of Tehran, Karaj, and Qazvin. Due to the concentration of industries and population in this region, high levels of particulate matter pose a significant threat to human health and the environment. This research aims to analyze the spatiotemporal distribution of particulate matter concentration and identify critical areas on a five-year average basis, with annual, seasonal, and monthly variations. For this purpose, Sentinel-5 satellite aerosol absorption index data were used to investigate the spatiotemporal variations of suspended particulate matter in the southern slopes of central Alborz. The results showed that the highest concentration of suspended particulate matter was observed in the south and eastern regions of the area, including Tehran, the political border of Qom, and Karaj. Analysis of hotspots at a 99% confidence level indicates a high concentration of pollution in these areas. Between 2018 and 2020, particulate matter concentrations exhibited a decreasing trend, reaching a minimum. However, in 2021, concentrations surged to their peak. Seasonal and monthly analyses revealed that the highest particulate matter levels occurred during summer. This pattern underscores the influence of geographic factors, population density, and industrial activities on the spatial distribution of pollutants, emphasizing the need for region-specific interventions to manage air quality.
Extended Abstract
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. Like many other developing countries, Iran is grappling with air pollution and is not exempt from this problem. Due to the geographical location of the study area on 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, 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 used JavaScript programming language through the Google Earth Engine web platform. The outputs obtained from this index include several results for evaluating these pollutants' temporal and spatial changes 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, ArcMap software classified and produced a spatial map of the average concentration of suspended particulate matter. Finally, to more accurately discover the spatial distribution and intensity of particulate matter pollution, hot and cold spots, and Getis-Ord Gi* statistics were used. This analysis can help better understand the spatial distribution of air pollution, 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 the 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 decreased 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 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 suspended particulate matter concentration 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 autumn, 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 stable atmospheric conditions, rising temperatures, reduced wind speeds, traffic congestion, industrial activities, accumulating 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 suspended particulate matter's spatial and temporal distribution. The results indicate that the highest concentrations of suspended particulate matter are concentrated in Iran's central and southeastern regions, 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. With its unique climatic conditions, summer experiences the highest concentrations of suspended particulate matter, particularly in Tehran. Dust storm sources influence this region in the surrounding deserts. Moreover, monthly variations reveal increased 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. Thus, increasing the number of ground-based monitoring stations and utilizing remote sensing technologies are essential steps for optimal air pollution management and implementing sustainable environmental policies.
 
Funding
There is no funding support.
 
Authors’ Contribution
All of the authors approved the content 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.