Vertical stratification of aerosols over South Asian cities.

This study examines vertically resolved aerosol optical properties retrieved from Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) onboard CALIPSO satellite over several cities across South Asia from March 2010 to February 2021. Atmospheric layer-specific stratification of aerosols and dominant aerosol sub-types was recognized over each city with their seasonal trends. A contrasting pattern in aerosol vertical distribution over cities across Indo-Gangetic Plain (IGP) was noted compared to non-IGP cities, with considerable dependency on geographic location of the city itself. In all the cases, total extinction decreased with increasing altitude however, with varying degree of slope. A clear intrusion of transported aerosols at higher altitude (>3 km) was also evident. Extinction coefficient of type-separated aerosols indicate robust contribution of smoke aerosols, urban aerosols/polluted dust, and mineral dust below 3 km height. At higher altitude (>3 km), dust and urban aerosols dominate over majority of the stations. Overall, 51% of total columnar aerosols remained within 0-1 km height over South Asian cities, slightly high over the IGP (57%) against non-IGP cities (39%). Such distribution also has a seasonal pattern with higher fraction of aerosols remaining below 1 km during post-monsoon (October-November, 62%) and winter (December-February, 72%) compared to summer months (March-May, 39%). When partitioned against planetary boundary layer (PBL), 41% (59%) of aerosols remained within the PBL (free troposphere) that too exhibiting strong diurnal variations irrespective of seasons. Dominating aerosol types and their contribution to total aerosol loading was explored by comparing type-based aerosol extinction against total aerosol extinction. Dust, smoke and urban aerosols emerged as three predominating aerosol types, while presence of marine aerosol was noted over the coastal cities. Major fraction of smoke and urban aerosols remained within 2 km height from surface. In contrast, efficient transport of dust aerosol above 2 km height was evident particularly over IGP during summer.

Three-dimensional nature of summertime aerosols over South Asia.

Three-dimensional (temporal-spatial-vertical) climatology of South Asian summertime (MAMJ, 2010-2019) aerosols and aerosol sub-types was explored using multiple high-resolution satellite-based observations and reanalysis dataset. Vertical stratification of aerosol layer and aerosol sub-types was identified using observation from space-borne lidar. Aerosol optical depth (AOD) was particularly high across the Indo-Gangetic Plain (IGP; AOD ± SD: 0.56 ± 0.12) and over eastern coast of India (AOD: 0.6-0.8), with prevalence of heterogeneous aerosol sub-types having strong spatial gradient. Clearly, aerosols over north-western arid part were highly absorbing (Ultra-violet Aerosol Index, UVAI > 0.80) and coarse (Ångström exponent, AE < 0.8), with an indication of desert/-mineral dust aerosols. In contrast, fine and moderate to non-absorbing aerosols (UVAI: 0.20-0.50) dominate from central to lower IGP, including in Bangladesh, with signature of anthropogenic emissions. Prevailing aerosols over twelve South Asian cities were classified into six aerosol sub-types constraining their particle size and UV-absorbing potential. Overall, mineral dust, smoke and urban aerosols were the three major aerosol sub-types that prevail across South Asia during summer. In particular, 58-70 % of retrieval days over Karachi and Multan were dust dominated; 57-64 % days were dust or urban aerosols dominated over Lahore, Delhi, Kanpur and Varanasi, and 56-77 % days were smoke or urban aerosols dominated over Dhaka, Kathmandu, Chennai, Mumbai, Colombo and Nagpur. Prevailing aerosols were vertically stratified as 50-70 % of total AOD was retrieved <2 km from the surface except in few cities where 70-80 % of AOD was retrieved <3 km height. Mineral dust and/or urban aerosols emerged as the most abundant aerosol types near the surface (<1 km) in all the cities except in Chennai, with their abundance remained as a function of emission sources and geographical location.