Physical-chemical characteristics of composite aerosols in the Indian Ocean sector of the Southern Ocean and its associated effect on insolation: A climate perspective

Abstract

Aerosol optical depth (AOD), black carbon (BC) mass concentration, aerosol size, along with wind parameters, were derived during Southern Ocean expeditions (SOE) 7 and 8 carried out in austral summer of 2013 and 2015, with an aim to analyse the effect of aerosol on incoming solar radiation. The data were complemented with trace metals from aerosol samples of SOE-6 conducted in 2012. The AOD spectra north of 40 degrees S followed Angstrom turbidity formulae while those in the south deviated from it. A statistically significant correlation (R2) of 0.79 (P « 0.0001) between the differences of AOD440 estimated from an Optical Properties of Aerosol and Cloud (OPAC) model and measured in situ and chlorophyll-a concentration revealed phytoplankton as a significant source of fine mode aerosols. Analysis of ten years of MODIS derived fine mode particle concentration indicated an increase as season advanced from winter to summer and a subsequent decrease towards the following winter, clearly showing a contribution from phytoplankton. BC mass concentration was found to be around 80 ng m sup(-3). Prevalence of trace metals such as Cu, Cd and Zn and the anions SO sub(4)-sub(4)sup(-2) and NO sub(3)-sub(3)sup(-1) were observed in this part of the world ocean. An inverse relation was observed between Cu and phytoplankton derived SO sub(4)-sub(4)sup(-2), indicating the detrimental effect of Cu on fine mode sulphate aerosols which are as significant as cloud condensation nuclei (CCN). The aerosol radiative forcing was found to be between 25 and -28 W sup(-2) to the north of the ITCZ while it was around 2-6 W m sup(-2) in the south. The associated heating rate was from 0.05 to -0.09 and from 0.01 to -0.02 K day sup(-1), respectively. The study revealed an increase in black carbon due to ship emissions. An increase in BC over the Southern Ocean atmosphere may have a far-reaching effect on the cloud formation and regional albedo.