High levels of ammonia do not raise fine particle pH sufficiently to yield nitrogen oxide-dominated sulfate production

Abstract

High levels of ammonia (NH3) have been suggested to elevate ambient particle pH levels to near neutral acidity (pH = 7), a condition that promotes rapid SO2 oxidation by NO2 to form aerosol sulfate concentration consistent with “London fog” levels. This postulation is tested using aerosol data from representative sites around the world to conduct a thorough thermodynamic analysis of aerosol pH and its sensitivity to NH3 levels. We find that particle pH, regardless of ammonia levels, is always acidic even for the unusually high NH3 levels found in Beijing (pH = 4.5) and Xi’an (pH = 5), locations where sulfate production from NOx is proposed. Therefore, major sulfate oxidation through a NO2-mediated pathway is not likely in China, or any other region of the world (e.g., US, Mediterranean) where the aerosol is consistently more acidic. The limited alkalinity from the carbonate buffer in dust and seasalt can provide the only likely set of conditions where NO2-mediated oxidation of SO2 outcompetes with other well-established pathways. The mildly acidic levels associated with excessive amounts of ammonia can promote high rates of SO2 oxidation through transition metal chemistry, this may be an alternative important aerosol chemical contributor to the extreme pollution events.