Synoptic-Remote Sensing Analysis of the Persistence of Air Pollution in the Southern Slopes of Alborz (Tehran, Alborz and Qazvin Metropolitan)

 
Regional recognition and analysis of air pollution is one of the important components in the action plan for environmental sustainability. The present study was conducted to analyze the spatial distribution, the reasons for the persistence of air pollution, and the factors that cause it in the dense population-industrial area of the three metropolitan cities of Tehran, Karaj, and Qazvin. In this regard, the combination of station-satellite and atmospheric data between 2018 and 2023 was used. By using the air quality index (AQI), the days that had an average daily pollution index above 150 and were in an unhealthy condition and continued for at least 6 days or more in the five-year statistical period were extracted. Then, two periods from January 17 to 29, 2023, and December 19 to 28, 2022, when air pollution was the most persistent, were selected for analysis. The results of the synoptic analysis showed that during the duration of the pollution, the blocking pattern has caused the movement of the system to stop or slow down, and with the establishment of a strong ridge, the atmosphere of Iran and the region has been kept in a stable state. With its permanent establishment, the mentioned ridge has caused the stillness of the air. As a result of these conditions, pollution has yet to be discharged daily and accumulated. The average distribution and focus of pollutants with the help of satellite images shows that the highest amount of CO and NO2 is concentrated in the city of Tehran. In contrast, the highest amount of SO2 is concentrated in Qazvin province. It seems that the Shahid Rajaei power plant is responsible for a large share of SO2 pollution. The study of the atmospheric patterns of these 7 periods, when air pollution has been high, indicates the dominance of blocking patterns.
 
Extended Abstract
1-Introduction
Regional recognition and analysis of air pollution is one of the important components in the action plan for environmental sustainability. A large part of Iran's population is concentrated in the southern slopes of Central Alborz, including the three big cities of Tehran, Karaj, and Qazvin. According to the latest population and housing census, these three cities have 15,917,116 people living in urban areas, which have experienced high levels of air pollution. The purpose of writing this article is to analyze the spatial distribution and reasons for the persistence of air pollution and the factors that cause it in the dense population-industrial area of the three metropolitan cities of Tehran, Karaj, and Qazvin by combining station, satellite, and atmospheric data between 2018 and 2023. In this regard, the continuous trend of daily air pollution and the accumulation of pollutants according to the type of pollutant and in connection with the atmospheric circulation and synoptic patterns leading to the dominant atmospheric conditions in the region have been studied.
 
2-Materials and Methods
Air pollution data was collected from 32 pollution measurement stations of Tehran Air Quality Control Company and Environmental Organization between 2018 and 2023 and Air Quality Index (AQI) was used to identify polluted days. Air quality index data was analyzed in Excel software after receiving, and polluted days with high persistence were identified. Next, to identify the synoptic patterns effective in the persistence of air pollution, hourly geopotential height data were obtained from the database (ERA5) of the European Center for European Centre for Medium-Range Weather Forecasts (ECMWF) for levels of 850 and 500 hPa and converted to daily averages in the GrADS software environment. To investigate the relationship between atmospheric circulation patterns and ground data, horizontal visibility and temperature data from 3 meteorological weather stations in the study area were also obtained. Next, to check the atmospheric conditions on the selected days, the Skew-T data of the synoptic Mehrabad weather station, from the Wyoming University atmospheric database, was used. To determine the way of distribution and investigation of the focus of pollutants, and to complete the ground data, the products of the Sentinel 5 satellite and its sensor, namely Tropomi, have been used. The spatial map of the average concentration of pollutants was classified in ArcMap software and produced for analysis.
 
3- Results and Discussion
In the study period, a total of seven cases, infected days with a duration of 6 days and more with a daily average index of 150 and above were identified. Then, two dates from January 17 to January 29, 2023, and from December 8 to 19, 2022, when the air pollution was more persistent, were selected for analysis. The results of the synoptic pattern prevailing at the level of 850 and 500 geopotential meters in this 13-day interval, when the air pollution continues, show that a low center with a value of 5300 geopotential meters is closed over Central Europe. The tongues of this low altitude have spread to North Africa in Libya. In addition, a high-altitude center with a value of 5660 geopotential meters has been closed in the northeast of the Caspian Sea, which has remained in place for several days without moving. The mentioned high altitude acts as a barrier system and prevents western cyclones from entering Iran. The synoptic pattern of the 850 hPa level is also very similar to the 500 hPa level pattern. In addition to the sharp drop in temperature in the studied area, stable weather has dominated this area and the southern slopes of Alborz. This Blocking pattern with the stillness and stability of the air has caused polluting elements to accumulate in the stable atmosphere and increase their error. By referring to the Skew-T thermodynamic diagram, the inversion phenomenon can be observed between the levels of 900 and 800 hPa. The continuation of the Blocking pattern and the stability of the atmosphere caused the accumulation of suspended particles in the studied area. The sharp downward trend of the field of view is obvious in three stations. Sentinel 5 satellite images show that the pollutant behavior of CO and NO2 is similar to each other. The highest amount of SO2 is concentrated in Qazvin province. In this period, with the stable atmosphere and, the Shahid Rajaei power plant in Qazvin has accumulated a large amount of this pollutant. The Blocking pattern caused a similar incident on 8-19 December 2022.
 
4- Conclusion
It seems that the source of air pollution in all three study areas is relatively constant, which means that the production and release of air pollution are constant and continuous every day. In some periods, due to the constant production and emission of pollution, the air quality index increases significantly and continues, which is the reason for the climate. A point that has been less mentioned in previous studies is related to the persistence and accumulation of air pollution, which is considered one of the main components of environmental sustainability. The results of the current research are in line with the research (Cai et al., 2020) on the impact of the structure of blocking in the troposphere on severe winter air pollution in North China. Examining the atmospheric patterns of 7 identified cases where the air pollution has continued showed that the blocking patterns have caused the stability of the air and as a result the continuation and accumulation of air pollution. The blocking of atmospheric systems causes the movement of meteorological patterns to stop, during which the patterns governing the atmosphere remain in place for days and even weeks. These blocking patterns with the stillness and stability of the air have caused the continuation of air pollution.