How sea salt particles contribute to Metro Manila air pollution

The chemical interaction between air pollutants and sea salt particles can have effects on air quality, according to the study conducted by researchers from the Ateneo de Manila University-Manila Observatory and the University of Arizona. 

Researchers examined the chemical composition of air in Metro Manila—both daytime and nighttime—before, during, and after the implementation of the lockdown due to COVID-19. 

The study said that the heavy pollution in Metro Manila before the lockdown was highly effective at removing the chloride component from airborne sea salt, a process known as "chloride depletion."

“Further work has shown that chloride depletion varies with pollutant type and that water-soluble species, particularly sulfate and ammonium, are more sensitive to rainfall events lasting more than an hour. Variations in water-soluble species have also been associated with changes in aerosol composition linked to long-range transport and local emissions,” the researchers said. 

“Overall, during the pre-lockdown period, Cl depletion was more pronounced during the day, while absolute mass loss was greater at night,” they added. 

Grace Betito, the lead researcher from the University of Arizona, explained that chloride depletion can lead to a chain reaction that creates ground-level ozone that is harmful. 

“Yes, higher chloride depletion can have a negative effect on air quality. When chloride is depleted from aerosol particles, it can lead to releases of hydrochloric acid gas, which can later form reactive chlorine radicals. The chlorine radicals can enhance the formation of ground-level ozone by reacting with VOCs (volatile organic compounds) in the atmosphere,” she told GMA News Online. 

Further, because the lockdown significantly cut emissions from vehicles and industries, the air became less acidic, the study noted. This reduction in acidity meant the air was less capable of destroying airborne chloride, allowing more natural sea salt components to persist. 

“Although the percentage of chloride depletion decreased post-lockdown due to reduced acid levels, the absolute Cl mass loss increased, driven by higher sodium levels. Nitrate, calcium, and magnesium showed minimal changes, reflecting their stronger association with natural sources less affected by shifts in human activity,” the study said. 

The researchers recommended further studies to measure gas emissions and weather conditions together, and in greater detail, towards more effective, evidence-based government interventions.

“The results we presented in the paper are just preliminary and would benefit from more measurements with higher time resolution, as well as more sampling locations,” said Betito. — VBL, GMA Integrated News