Distribution of tropospheric ozone over southwest Asia

In this study, the distribution of tropospheric ozone as an air pollutant and an important greenhouse gas has been investigated in various layers of the troposphere over Southwest Asia. This research has been conducted for a 5-year period (2012-2016) using the Copernicus Atmosphere Monitoring Service Reanalysis (CAMSRA) dataset, the result of the third European Centre for Medium-Range Weather Forecasts (ECMWF) project on atmospheric composition reanalysis. The analysis of the monthly mean concentration of tropospheric ozone over Southwest Asia and its time series (6-hourly data for the 5-year period) over three areas in northwestern and southeastern Iran, and Tehran show that the concentration of ozone has an annual cycle, with the maximum in summer. The maximum ozone in different layers of the troposphere (at the surface, and 700 and 500 hPa) occurs during summer. The maximum concentration in the lower layers (up to 700 hPa) is mostly caused by anthropogenic sources, 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 in 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 the daytime and summer compared to the nighttime and winter. This is due to the increase in the photochemical production of ozone when the incoming solar radiation is high. Moreover, there are two hot spots of ozone concentration 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 fold and/or the stratosphere to troposphere transport (STT) in these two regions, linked to the Indian summer monsoon, are the main causes of the occurrence 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 midlatitude 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, the statistical analysis indicates that high ozone concentration events frequently occur in southeastern Iran. This could be due to transient variations in the monsoon circulation over India, the Tibetan anticyclone, and the mid-level anticyclone, all of which also affect the transport of the stratospheric ozone in the region.