On the nature of sea salt aerosol at a coastal megacity: Insights from Manila, Philippines in Southeast Asia

This study utilizes multiple aerosol datasets collected in Metro Manila, Philippines to investigate sea salt aerosol characteristics. This coastal megacity allows for an examination of the impacts of precipitation and mixing of different air masses on sea salt properties, including overall concentration and size-resolved composition, hygroscopicity, and morphology. Intensive size-resolved measurements with a Micro-Orifice Uniform Deposit Impactor (MOUDI) between July–December 2018 revealed the following major results: (i) sea salt levels exhibit wide variability during the wet season, driven primarily by precipitation scavenging; (ii) ssNa⁺ and Cl⁻ peaked in concentration between 1.8 and 5.6 μm, with Cl⁻ depletion varying between 21.3 and 90.7%; (iii) mixing of marine and anthropogenic air masses yielded complex non-spherical shapes with species attached to the outer edges and Na⁺ uniformly distributed across particles unlike Cl⁻; (iv) there was significant contamination of sea salt aerosol by a variety of crustal and anthropogenic pollutants (Fe, Al, Ba, Mn, Pb, NO₃⁻, V, Zn, NH₄⁺); (v) categorization of samples in five different pollutant type categories (Background, Clean, Fire, Continental Pollution, Highest Rain) revealed significant differences in overall Cl⁻ depletion with enhanced depletion in the submicrometer range versus the supermicrometer range; (vi) κ values ranged from 0.02 to 0.31 with a bimodal profile across all stages, with the highest value coincident with the highest sea salt volume fraction in the 3.2–5.6 μm stage, which is far lower than pure sea salt due to the significant influence of organics and black carbon. Analysis of longer term PM_2.5 (particulate matter with aerodynamic diameter less than 2.5 μm) and PM_coarse (= PM₁₀ – PM_2.5) data between August 2005 and October 2007 confirmed findings from the MOUDI data that more Cl⁻ depletion occurred both in the wet season versus the dry season and on weekdays versus weekend days. This study demonstrates the importance of accounting for two factors in future studies on sea salt: (i) non-sea salt (nss) sources of Na⁺ impact calculations such as for Cl⁻ depletion that typically assume that total Na⁺ concentration is derived from salt; and (ii) considering precipitation data over a larger spatial domain rather than a point measurement at the study site to investigate wet scavenging.