Aerosol Acidity Modulated by Temperature Through Semi-Volatile Species

Temperature modulates aerosol pH through several ways. While the increased solubility (Henry’s law constant) of semi-volatile gases at lower temperatures is well recognized, the temperature dependence of acid dissociation constants has received less attention. More importantly, the simultaneous increase in both Henry’s law constant and acid dissociation constant at lower temperatures amplifies volatility shifts for species like NH₃ and HNO₃. In our recent work (Campbell et al., 2024), we show that reduced NH₃ volatility at extremely cold conditions (< -30 ℃) enhances aerosol neutralization in ammonium-sulfate aerosols. Building on this, we investigate pH behavior across a broad temperature range in ammonium-sulfate-nitrate aerosols. Our results highlight that aerosol pH is governed by the interplay between NH₃ and HNO₃ volatility, modulated by ambient temperature and the relative abundance of total NH₃, total HNO₃ and total H₂SO₄. These findings provide key insights into aerosol acidity and secondary aerosol chemistry on a global scale.