The impact of non-ideality on reconstructing spatial and temporal variations of aerosol acidity with multiphase buffer theory

Aerosol acidity is a key parameter in atmospheric aqueous chemistry and strongly influence the interactions of air pollutants and ecosystem. The recently proposed multiphase buffer theory provides a framework to reconstruct long-term trends and spatial variations of aerosol pH based on the effective acid dissociation constant of ammonia (K_a,NH3^*). However, non-ideality in aerosol droplets is a major challenge limiting its broad applications. Here, we introduced a non-ideality correction factor (c_ni) and investigated its governing factors. We found that besides relative humidity (RH) and temperature, c_ni is mainly determined by the molar fraction of NO₃^- in aqueous-phase anions, due to different NH₄⁺ activity coefficients between (NH₄)₂SO₄- and NH₄NO₃-dominated aerosols. A parameterization method is thus proposed to estimate c_ni at given RH, temperature and NO₃^- fraction, and is validated against long-term observations and global simulations. In the ammonia-buffered regime, with c_ni correction the buffer theory can well reproduce the K_a,NH3^* predicted by comprehensive thermodynamic models, with root-mean-square deviation ~0.1 and correlation coefficient ~1. Note that, while c_ni is needed to predict K_a,NH3^* levels, it is usually not the dominant contributor to its variations, as ~90% of the temporal or spatial variations in K_a,NH3^* is due to variations in aerosol water and temperature.