Abstract. Aerosol acidity is a key parameter in atmospheric aqueous
chemistry and strongly influences the interactions of air pollutants and the ecosystem. The recently proposed multiphase buffer theory provides a
framework to reconstruct long-term trends and spatial variations in aerosol
pH based on the effective acid dissociation constant of ammonia
(Ka,NH3∗). However, non-ideality in aerosol droplets is a major
challenge limiting its broad applications. Here, we introduced a
non-ideality correction factor (cni) and investigated its governing
factors. We found that besides relative humidity (RH) and temperature,
cni 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 cni at a given RH,
temperature and NO3- fraction, and it is validated against long-term
observations and global simulations. In the ammonia-buffered regime, with
cni correction, the buffer theory can reproduce well the
Ka,NH3∗ predicted by comprehensive thermodynamic models, with a root-mean-square deviation ∼ 0.1 and a correlation coefficient
∼ 1. Note that, while cni is needed to predict
Ka,NH3∗ levels, it is usually not the dominant contributor to
its variations, as ∼ 90 % of the temporal or spatial
variations in Ka,NH3∗ are due to variations in aerosol water and
temperature.