Impact assessment of continental and marine air-mass on size-resolved aerosol chemical composition over coastal atmosphere: Significant organic contribution in coarse mode fraction

Abstract

Size-segregated aerosols (five stages from <1.1 to >7 μm in diameter) collected from a coastal location in south-west peninsular India were analysed for water-soluble ionic species, carbonaceous aerosols and metallic species on a seasonal basis for two years (2017–19). The annual mean concentration of the components Na+, Ca2+, Mg2+, Cl−, NO3−, Al, Fe exhibited coarse mode (> 2 μm) enhancement; while NH4+, K+, SO42−, MSA−, oxalate, Organic carbon (OC), Elemental carbon (EC) peaked in fine mode (< 2 μm). The contribution of fine mode aerosols dominated under the influence of continental air-mass and a relative shift towards coarse mode size regime with respect to marine air-mass was observed. The relative contribution of analysed species showed the dominance of sea-salt fraction in coarse mode during monsoon, anthropogenic ions in fine mode during post-monsoon, crustal species in coarse mode during pre-monsoon and carbonaceous species in fine mode during winter. While nss-SO42− was found to be a major component of fine mode aerosol fraction, OC contributed significantly for both coarse and fine mode. Distinct seasonal variation in OC size-distribution was observed. Fine mode OC dominated during post-monsoon and winter while coarse mode OC peaked during pre-monsoon and monsoon, indicating its multiple sources and formation pathways. The water-soluble fraction of OC predominantly existed in fine mode and was modulated by changing synoptic air-masses. The water insoluble OC fraction was found to be modified by sea-land breeze activities and exhibited coarse mode nature. The significant contribution of OC in coarse mode size regime over this coastal atmosphere has been attributed to; the condensation of gaseous precursors on pre-existing coarse particles leading to the formation of coarse mode WSOC and the primary emission from natural terrestrial and marine organic matter resulting in the emission of coarse mode WIOC.