Naturally occurring sea salt components—sodium and chloride—are reacting with Metro Manila’s polluted air and adding to the city’s pollution burden, a new study co-authored by researchers from the Ateneo de Manila University–Manila Observatory and the University of Arizona revealed.

The new study identifies sea salt aerosols as a significant, chemically reactive factor in Metro Manila’s air quality, showing how their components transform when they interact with the city’s already polluted atmosphere.

According to researchers, sea salt particles and urban pollution do not simply mix when they meet in the atmosphere; pollutants chemically react with and transform the sea salt particles, with potentially negative effects on the air we breathe.

The researchers analyzed the chemical makeup of Metro Manila’s air during both day and night, comparing conditions before and after the lockdown.

Aerosol samples gathered in uninterrupted 48-hour intervals showed higher concentrations of chloride and sodium in the post-lockdown period. 

Researchers noted that Metro Manila’s pre-lockdown pollution was strong enough to strip chloride from sea salt aerosols through “chloride depletion.” 

When pollution levels dropped during the lockdown, the atmosphere became less acidic and less capable of depleting airborne chloride, enabling more natural sea salt particles to persist.

The findings carry broad implications for public health, climate, and urban policy, underscoring the complex ways pollution behaves in a coastal megacity. 

The researchers urged more detailed studies that track gas emissions alongside weather patterns to support stronger, evidence-based government interventions.

Published in October 2025 in ACS Earth and Space Chemistry, the study 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 was authored by Grace Betito, Paola Angela Bañaga, Rachel A. Braun, Maria Obiminda Cambaliza, Melliza Templonuevo Cruz, Alexander B. MacDonald, James Bernard Simpas, Connor Stahl, and Armin Sorooshian.