عنوان مقاله [English]
In this study, the distribution of the tropospheric ozone as an air pollutant and an important greenhouse gas has been investigated in various layers of the troposphere in Southwest Asia. This research has been conducted for a 5-year period (2012-2016) using the Copernicus Atmosphere Monitoring Service Reanalysis (CAMSRA) data set, the result of the third European Centre for Medium-Range Weather Forecasts (ECMWF) project on atmospheric composition reanalysis. The average monthly concentration of tropospheric ozone in Southwest Asia and its time series (6-hourly data for the 5-year period) in three areas in Iran (northwestern Iran, Tehran and southeastern Iran) show that the concentration of ozone has an annual cycle, with the maximum during summer. The maximum ozone in different layers of the troposphere (surface, 700 and 500 hPa) occurs during summer and can be attributed to different sources. The maximum in the lower layers (up to 700 hPa) is mostly anthropogenic, while in the middle to upper troposphere it is the result of the injection of stratospheric air into the troposphere. The high concentration of NO2 in highly populated metropolitan areas, such as Tehran and industrial areas at the Persian Gulf and the Gulf of Oman, contributes to the photochemical production of ozone. In these areas, the concentration of ozone is higher during daytime and in summer compared to nighttime and in winter. This is due to the increase in photochemical production of ozone when the incoming solar radiation is high. Also, there are two hot spots of ozone concertation at 500 hPa over two regions: the eastern Mediterranean region and the east of the Caspian Sea toward Afghanistan. Large-scale subsidence and the occurrence of the tropopause folding and/or the stratosphere to troposphere transport (STT) in these two regions, linked to the Indian summer monsoon, are the main causes of high concentrations of ozone in the middle troposphere. The monsoon diabatic heating can induce Gill-type Rossby waves that propagate westward and cause descent via the interaction with the mid-latitude westerlies. The topography of the region, e.g., the Zagros Mountains, is also effective in increasing this descent. In general, every horizontal airflow that encounters steep isentropic slopes at the upper and middle troposphere is forced to descend. We were able to detect a wave-like pattern in ozone concentration at the 300 hPa level, which can be linked to a corresponding pattern of vertical velocities in the region.
Furthermore, statistical studies of ozone concentration anomalies showed that high ozone concentration events occur frequently in southeastern Iran. This could be due to transient variations in the monsoon circulation over India, the Tibetan anticyclone and mid-level anticyclone, which also affects the transport of the stratospheric ozone in the region.