Spatial and temporal variations of satellite-based aerosol optical depth over Iran in Southwest Asia: Identification of a regional aerosol hot spot

Knowledge of spatial and temporal variations of aerosols is essential for understanding the impacts of aerosols on air quality. Using aerosol products of the Collection 6 Terra MODIS Deep Blue, regional and temporal variations of aerosol optical depth (AOD) at 0.55 μm in sixteen locations spread over nine different regions of Iran are studied for the period 2001-2015. Monthly means of dust column mass density in three locations of Iran are also obtained from the MERRA-2 dataset. It is found that southwestern Iran experiences the highest annual mean AOD, while other regions experience significantly lower values. Indeed, southwestern Iran is identified as a regional hot spot of aerosols in Southwest Asia, significantly contributing to degrading the air quality in the nearby regions. Annual mean AOD values in most of the other studied locations are between 0.08 and 0.12. High AOD over southwestern Iran is strongly related to frequent dust outbreaks over the region all year long, although AOD values are higher from April to August, during which dust events are more frequent over Southwest Asia. In other, mostly urban populated areas, maximum AOD values occur from mid-winter to mid-spring due to significant aerosol emissions from combustion of fossil fuels, combined with shallow atmospheric boundary-layer depths, which lead to the development of a concentrated mass of aerosols near the surface. On the other hand, minimum values of AOD occur from August to November. Trend analysis indicated that none of the regions of Iran has experienced a noticeable increase or decrease in AOD during 2001-2015.