Day-Night Shifts in Water-Soluble Ions of Size-Resolved Aerosols before and after the COVID-19 Lockdown in a Coastal Megacity: Metro Manila, Philippines

The COVID-19 pandemic-driven lockdowns offer a unique opportunity to examine how reductions in anthropogenic emissions impacted atmospheric aerosol composition in urban environments. This study investigates the day-night variability of size-resolved water-soluble ions in ambient particulate matter (PM) collected in Metro Manila before (November 2019–February 2020) and after (November 2020–February 2021) lockdown implementation. Using tandem Micro-Orifice Uniform Deposit Impactors (MOUDIs), aerosol samples were collected during daytime (06:00–18:00) and nighttime (18:00–06:00) periods and analyzed for key ionic species (sulfate, ammonium, nitrate, oxalate, sodium, chloride, calcium, and magnesium) via ion chromatography. Submicrometer water-soluble mass declined post-lockdown, particularly during daytime, reflecting suppressed secondary formation under reduced anthropogenic activity, with substantial reductions in sulfate and ammonium. In contrast, concentrations in the supermicrometer range increased due to naturally higher sea salt levels. Chemical ratios reveal notable features post-lockdown and during daytime due to especially reduced sulfate levels: reduced chloride depletion (on percent basis), higher ammonium-to-sulfate ratios pointing to more excess ammonia available for reactions beyond neutralizing sulfate, and support for aqueous-phase processing preferentially forming more oxalate relative to sulfate. These findings underscore how both photochemistry and changes in anthropogenic activity influence aerosol composition, with implications for air quality and atmospheric processing in coastal urban cities.