<p>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<sub>a,NH3</sub><sup>*</sup>). However, non-ideality in aerosol droplets is a major challenge limiting its broad applications. Here, we introduced a non-ideality correction factor (c<sub>ni</sub>) and investigated its governing factors. We found that besides relative humidity (RH) and temperature, c<sub>ni</sub> is mainly determined by the molar fraction of NO<sub>3</sub><sup>-</sup> in aqueous-phase anions, due to different NH<sub>4</sub><sup>+</sup> activity coefficients between (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>- and NH<sub>4</sub>NO<sub>3</sub>-dominated aerosols. A parameterization method is thus proposed to estimate c<sub>ni</sub> at given RH, temperature and NO<sub>3</sub><sup>-</sup> fraction, and is validated against long-term observations and global simulations. In the ammonia-buffered regime, with c<sub>ni</sub> correction the buffer theory can well reproduce the K<sub>a,NH3</sub><sup>*</sup> predicted by comprehensive thermodynamic models, with root-mean-square deviation ~0.1 and correlation coefficient ~1. Note that, while c<sub>ni</sub> is needed to predict K<sub>a,NH3</sub><sup>*</sup> levels, it is usually not the dominant contributor to its variations, as ~90% of the temporal or spatial variations in K<sub>a,NH3</sub><sup>*</sup> is due to variations in aerosol water and temperature.</p>