Revisiting the Key Driving Processes of the Decadal Trend of Aerosol Acidity in the U.S

Acidity is one essential parameter in determining the aqueous phase physical and chemical processes in the atmosphere and strongly influences the climate, ecological, and health effects of aerosols. Traditionally, aerosol acidity is thought to increase with emissions of atmospheric acidic substances (SO₂, NOx, etc.) and decrease with that of alkaline ones (NH₃, dust, etc.). However, decade-long observations in southeastern U.S. seem to disagree with this hypothesis: while the emissions of NH₃ versus SO₂ enhanced by over three times, the predicted aerosol acidity is stable, and the observed particle-phase ammonium-to-sulfate ratio is even decreasing. Here, we investigated into this issue with the recently proposed multiphase buffer theory. We show that historically, there is a transition in the dominant drivers of aerosol acidity in this region. Under the ammonia-poor conditions before ∼2008, the acidity is governed by HSO₄ ^-/SO₄ ^2- buffering and the water self-buffering effect. Under the ammonia-rich conditions after ∼2008, aerosol acidity is mainly buffered by NH₄ ⁺/NH₃. Buffering from the organic acids is negligible in the investigated period. In addition, the observed decrease in ammonium-to-sulfate ratio is due to the increased importance of non-volatile cations, especially after ∼2014. We predict that until ∼2050, the aerosols will remain in the ammonia-buffered regime, and the nitrate will remain largely (>98%) in the gas phase in southeastern U.S.