Refined Sea Salt Markers for Coastal Cities Facilitating Quantification of Aerosol Aging and PM 2.5 Apportionment.

Sea salt (ss) aerosols in PM 2.5 are often quantified through source apportionment by applying sodium (Na + ) and chloride (Cl - ) as the markers, but both markers can be substantially emitted from anthropogenic sources. In this study, we differentiate ss from nonss (nss) portions of Na + and Cl - to better apportion PM 2.5 in a coastal tropical urban environment. Size-resolved ionic profiles accounting for Cl - depletion of aged ss were applied to 162-day measurements during 2012 and 2018-2019. Results show that the nss (likely anthropogenic) portions, on average, account for 50-80% of total Na + and Cl - in submicron aerosols (PM 1 ). This corresponds to up to 2.5 μg/m 3 of ss in submicron aerosols that can be ∼10 times overestimated if one attributes all Na + and Cl - in PM 1 to ss. Employing the newly speciated ss- and nss-portions of Na + and Cl - to source apportionment of urban PM 2.5 via positive matrix factorization uncovers a new source of transported anthropogenic emissions during the southwest monsoon, contributing to 12-15% of PM 2.5 . This increases anthropogenic PM 2.5 by ≥19% and reduces ss-related PM 2.5 by >30%. In addition to demonstrating Cl - depletion (aging) in submicron aerosols and quantifying ssNa + , nssNa + , ssCl - , as well as nssCl - therein, the refined PM 2.5 apportionment resolves new insights on PM 2.5 of anthropogenic origins in urban environments, useful to facilitate policy making.