Accumulation of absorbing aerosols over the Indian Sector of Southern Ocean during the austral summer: Role of thermal inversions

Abstract

Optical properties of aerosols and their direct radiative effect were analyzed from columnar Aerosol Optical Depth (AOD) and ambient Black Carbon (BC) mass concentration (MB) estimated during Indian Southern Ocean Expeditions (SOEs), conducted in the austral summers of 2016-17 (SOE-IX) and 2017-18 (SOE-X). The aforementioned observations were supplemented with concurrent vertical soundings of the lower atmosphere. Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) data aided in investigating the vertical distribution of aerosols up to 10 km altitude. The relative contribution of BC aerosols emanating from fossil fuel combustion and biomass burning showed the former to be predominant, as the latter is larger and preferentially scavenged during long-range transport to remote marine regions. The highest values of AOD at 500 nm (AOD sub(500); 0.156 plus/minus 0.026) and MB (90.78 plus/minus 40.11 ng m sup(-3)) were encountered in the region between 50 and 60 degrees S. Consequently, the atmospheric clear-sky Aerosol Direct Radiative Effect (ADRE; plus 3.10 plus/minus 0.38 W m sup(-2)) and heating rate (HR; 0.087 plus/minus 0.011 K day sup(-1)) were also observed to be the highest in the region. Multiple atmospheric inversions were detected between 40 and 69 degrees S. Although a much lower AOD500 (0.070 plus/minus 0.017) prevailed between 60 and 69 degrees S, the average clear-sky ADRE (plus 1.56 plus/minus 0.49 W m sup(-2)) and HR (0.044 plus/minus 0.014 K day sup(-1)) were fairly high. The mean MB in the region was high (68.81 plus/minus 28.40 ng m sup(-3)) for the pristine ice-covered region. Low-lying inversions prevented the vertical ventilation and dispersal of BC therein. Simulations of BC-induced atmospheric heating rates at three locations in the coastal Antarctic waters indicated a approximately equal to 3-5-fold increase, corresponding to an increase in ambient BC from 50 to 300 ng m sup(-3).