Spatiotemporal characteristics of Dust Aerosol Episodes over Asia and Caspian Sea based on contemporary climatological satellite data

The wide region of Asia is one of the most densely populated places of the Earth, hosting a large percentage of the Εarth's population. Thus, changes in climate and weather conditions affect the lives of many people. In Asia, there are many desert areas, from which large amounts of Dust Aerosols (DA) are emitted into the atmosphere, where they remain suspended from a few hours up to several days. DA are able to travel thousands of miles away from their source areas, among which the largest ones are the Taklamakan and Gobi Deserts in Central & East Asia and the Tar Desert in the Indian subcontinent. Apart from them, there are also other smaller deserts in Asia, i.e. Badain Jaran, Tengger, which also contribute significant amounts of DA. Furthermore, the Aralkum, Kyzylkum and Karakum areas East of the Caspian Sea contribute high dust loadings, too.

DA is a major contributor of aerosol burden in the Earth’s atmosphere, significantly affecting weather and climate conditions, through various interactions with radiation and clouds, while also deteriorating air quality and causing a series of health problems. DA alter the energy balance of the Earth-Atmosphere system, as they absorb and scatter primarily the solar, but also the thermal infrared radiation, thus influencing climate from the local to regional and global scales. Besides, DA act as effective Cloud Condensation Nuclei (CCN) or Ice Nuclei (IN), modifying cloud albedo and coverage, as well as the produced precipitation. All these dust effects are intensified under Dust Aerosol Episodes (DAEs), i.e. conditions of unusually high dust loadings, which occur every year with varying frequency and intensity, but with distinct seasonal and spatial characteristics. DAEs are originally determined on, and refer to, a pixel level, whilst days with an extended spatial coverage of DAEs are named Dust Aerosol Episode Days (DAEDs). Finally, series of consequent DAEDs constitute Dust Aerosol Episode Cases (DAECs), which are spatiotemporally extended and intense dust episodes that deserve to be identified and studied in areas like Asia.

In the present study, a satellite algorithm is used to identify DAEDs over Asia and the Caspian Sea, aiming to determine their spatial and temporal distribution emphasizing their frequency of occurrence and the associated dust loadings. The algorithm uses as input daily spectral Aerosol Optical Depth (AOD) and Aerosol Index (AI) data from MODIS C6.1 and OMI OMAERUV databases, respectively, spanning the 16-year period from 2005 to 2020. It operates on a daily basis and 1deg x 1deg pixel level and detects the presence of DA by applying appropriate thresholds on Ångström Exponent (AE) (calculated using spectral AOD from MODIS) and AI. Subsequently, the algorithm determines the occurrence of DAEDs and DAECs, yielding their frequency of occurrence, as well as the associated dust optical depth (DOD) on monthly and annual timescales. Thus, the algorithm outputs enable to build a climatology of spatiotemporally extended Asian dust episodes, as well as to derive their year to year variability and tendencies over the 16-year study period.