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 (PM₁). This corresponds to up to 2.5 μg/m³ of ss in submicron aerosols that can be ∼10 times overestimated if one attributes all Na⁺ and Cl^- in PM₁ 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.