Aerosol climatology and determination of different types over the semi-arid urban area of Tehran, Iran: Application of multi-platform remote sensing satellite data

In this study, the combination of multiple satellite data was used to improve the assessments of aerosol properties including the aerosol optical depth, Angstrom exponent, UV aerosol index, and single scattering albedo over a semi-arid urban area during the period from 2008 to 2017. The data sets were derived from MODIS and OMI. The critical threshold values for each of the aerosol properties have been defined according to the previous studies to determine different aerosol types. Results of long term analysis of aerosol properties show that all parameters present a distinct monthly variation. A slightly increasing trend is detected for aerosol index values and this indicates the presence of more absorbing aerosols in recent years. There also is an obvious, consistent cooling effect of aerosols based on the variation of single scattering albedo. The aerosol optical depth relative frequency histogram shows a wide range of values with a maximum of about 40% around the range of 0.3 which is generally higher than some other polluted cities in the world. However, the Angstrom exponent peak around 0.8 likely means the dominance of aerosols in coarse mode. The results of the determination of different aerosol type show that, right after the mixed type aerosols, the urban/industrial aerosol type is the second-highest contributor to the aerosol composition and that it is followed by dust aerosols. On the other hand, maritime aerosols and smoke, present in less than 5% of the days studied, do not have a significant contribution. Seasonal analysis of different aerosol types indicates the dominance of large absorbing aerosols, i.e. dust during summertime, and small aerosols with non-absorbing properties, i.e. urban/industrial aerosols, mostly during cold seasons.