Long-term aerosol optical depth trend over Iran and identification of dominant aerosol types.

The paper focuses on analysis of long-term changes of aerosol optical depth (AOD) over Iran. It describes contributions of dominant aerosol in the aerosol load over Iran covering the period 1980-2018. For this purpose, a long-term AOD dataset from the reanalysis-based Modern Era Retrospective Analysis for Research and Applications (MERRA-2), the satellite-based Moderate Resolution Imaging Spectroradiometer (the new version of MODIS/Terra and Aqua) as well as a new AOD product (MERRA-2 MODIS merged) were used. The ground-based AOD measurements of the five Aerosol Robotic Network (AERONET) sites used for validation demonstrated better consistency of the MERRA-2 MODIS merged (MMM) product. Analysis of these datasets demonstrated high AOD in the southwest of Iran because of the proximity to the major source areas of natural mineral dust in spring and summer. In contrast, low AOD was mostly observed along the high elevation lands in the northern and western highlands. The trend analysis of AODs revealed differences between the AOD datasets, but agree on the positive trends over southwestern Iran and negative trend in northern Iran. Classification of major aerosol types indicated that the clean marine and mixed aerosols were the dominant aerosol types during the cold and hot seasons, respectively, and the increase of desert dust around 2010 was another obvious result in spring and summer. Our results indicate that the variation in dust aerosol has a key role in determining the AOD long-term changes in Iran which has contributed in regional climate change and environmental evolutions.