Abstract
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.