On the formation pathways of secondary organic aerosols over a tropical coastal atmosphere

Organic aerosols (OA) account for substantial mass fraction of submicron aerosols (20-90%) and have potential impact on aerosol-cloud-radiation interaction in the earth's atmosphere. OA have multitude of sources and varying lifecycle processes and major uncertainty exists in understanding the formation mechanism of secondary OA (SOA), its evolution and impact on the climate relevant aerosol physical-chemical properties. Emerging studies show the dominance of aqueous phase reactions over photochemistry in the formation and processing of SOA at environments with high relative humidity. In this context, to delineate the formation pathways of SOA, long-term observations of real-time sub-micron aerosol composition and trace-gas concentrations and meteorological parameters were conducted from Thumba in southern peninsular India. This location, being a tropical coastal site with abundant solar radiation and moisture content, and prevalence of contrasting air masses in meso- and synoptic-scales, provides an opportunity to examine the OA formation pathways (photo- and/or aqueous-phase chemistry) in varying atmospheric conditions. The results revealed significant correlations between odd oxygen (a tracer for photochemistry) and SOA (with Pearson R>0.5) with a range of photochemical formation efficiencies over different seasons. The O:C ratio, which is an indicator of oxidation-degree of SOA, showed an increasing trend (∆O:C ~ 0.10-0.40) with odd oxygen (∆odd oxygen ~ 60 µg m-3) suggesting the processing of SOA via gas-phase photochemical oxidation. Further, to delineate the aqueous-phase formation pathways of SOA, the Aerosol Inorganic Liquid Water Content (AILWC) was estimated using Extended-Aerosol Inorganic Model (E-AIM). Though, significant correlations (R>0.60) were seen between SOA and AILWC at RH>90% during wet seasons, the change in O:C ratio (∆O:C <0.1) with AILWC (∆AILWC ~ 120 µg m-3) was not significant, which indicated that the SOA processing is unaffected by aqueous-phase reactions. Overall, our study suggested that the SOA-processing over this coastal atmosphere is governed mainly by photochemical reactions under different meteorological conditions.