How aerosol pH responds to nitrate to sulfate ratio of fine-mode particulate

Aerosol acidity (pH), one of key properties of fine-mode particulate (PM_2.5), depends largely on nitrate and sulfate in particle. The mass contribution of nitrate relative to sulfate in PM_2.5 has tended to increase in many regions globally, but how this change affects aerosol pH remains in debate. In this way, we measured PM_2.5 ionic species and oxygen isotopic composition of nitrate in the eastern China, and predicted aerosol pH using the ISORROPIA-II model. When nitrate to sulfate molar ratio increases and thus PM_2.5 is gradually enriched in ammonium nitrate (NH₄NO₃), aerosol pH tends to increase. The oxidation of nitrogen dioxide (NO₂) by hydroxyl radical is responsible for most of nitrate formation (generally above 60%). These indicate that nitrate formation through gas-to-particle conversion involving ammonia and nitric acid results in increasing aerosol pH with increasing molar ratio of nitrate to sulfate. Conversely, aerosol pH is expected to decrease with increasing relative abundance of nitrate as ammonia emissions are lowered. Our research concludes that it should be considered to reduce aerosol NH₄NO₃ by reducing the precursors of nitric oxide and ammonia emissions, to substantially improve the air quality (i.e., reduce PM_2.5 levels and potential nitrate deposition) in China.