Multiphase buffer theory: explanations of contrasts in atmospheric aerosol acidity and its applications

Acidity is one central parameter in atmospheric multiphase reactions, and strongly influences the climate, ecological and health effects of aerosols. Yet, the drivers of aerosol pH remain to be fully resolved. Here we investigated into this issue with thermodynamic models and observations. We find that aerosol pH levels in populated continental regions are widely buffered by the conjugate acid-base pair NH₄⁺/NH₃, and in aerosols an individual buffering agent can adopt different buffer pH values. To explain these large shifts of buffer pH, we propose a multiphase buffer theory, and show that aerosol water content and mass concentration play a more important role in determining aerosol pH in ammonia-buffered regions than variations in particle chemical composition. These results imply that aerosol pH and atmospheric multiphase chemistry are strongly affected by the pervasive human influence on ammonia emissions and the nitrogen cycle in the Anthropocene. We further investigated into the applications of the multiphase buffer theory. Exemplary applications include to help explain the formation of severe hazes in China, to quantify the contribution of different factors in driving the aerosol pH variations, and to provide the framework to reconstruct long-term trends and spatial variations of aerosol pH, etc. Further investigations on its applications in aerosol and cloud chemistry studies are needed.