Influence of south Asian outflow on secondary organic aerosol formation over the Indian Ocean: Inferences from water-soluble low molecular weight dicarboxylic acids and related organic compounds during ICARB 2018 experiment

Abstract

Low molecular weight dicarboxylic acids constituting a pivotal fraction of organic aerosols impact radiative forcing of Earth's atmosphere. However, a significant knowledge gap exists associated with their sources and formation pathways in the atmosphere. In this study, we report on water-soluble components such as dicarboxylic acids, oxoacids, and α-dicarbonyls as well as carbonaceous and ionic components in the continental outflow from the South Asian region to the Arabian Sea and the equatorial Indian Ocean during ICARB (Integrated Campaign for Aerosols, gases and Radiation Budget) 2018 experiment. Non-sea-salt sulfate (nss SO42−) was found to be the predominant component, followed by ammonium (NH4+) and organic carbon (OC) in fine-sized (<1.1 μm) aerosols. Total diacids carbon content accounted for 17% (on average) of total carbon (TC). Oxalic acid (C2) was the most abundant organic acid, contributing ~41% to total diacids, followed by terephthalic acid (tPh) and succinic acid (C4). Of these results, together with selected organic and inorganic mass ratios and linear relationships among the organic acids, we highlight the sources of organic acids over Indian Ocean. Over continental outflow regions in Indian Ocean, dicarboxylic acids are majorly derived from the emissions of biomass burning and combustion related sources, that underwent transformation in the atmosphere through photochemical reactions during the long range transport. However, over the deep oceanic environment of Indian Ocean, photo-oxidation of biogenic unsaturated fatty acids were found to be the important sources. The findings from this study will be helpful to improve further our understanding of the sources and formation processes of organic aerosols and their climate effects over the South Asian outflow regions.