Multiphase Buffering by Ammonia Sustains Sulfate Production in Atmospheric Aerosols

Multiphase oxidation of sulfur dioxide (SO₂) is an important source of sulfate in the atmosphere. There are, however, concerns that protons produced during SO₂ oxidation may cause rapid acidification of aerosol water and thereby quickly shut down the fast reactions favored at high pH. Here, we show that the sustainability of sulfate production is controlled by the competing effects of multiphase buffering and acidification, which can be well described by a characteristic buffering time, τ_buff. Both GEOS-Chem simulations and observations show that globally, τ_buff is long enough (days) to sustain sulfate production over most populated regions, where the acidification of aerosol water is counteracted by the strong buffering effect of NH₄⁺/NH₃. Our results highlight the importance of anthropogenic ammonia emissions and pervasive human influences in shaping the chemical environment of the atmosphere.