Reply on RC2

Abstract. Semi-volatile NH4NO3 is a major component of atmospheric aerosols, and its environmental and climate effects are largely regulated by the gas-particle partitioning. The activity coefficient of NH4NO3, γAN, is one key parameter controlling the gas-particle partitioning, yet its dependence on meteorological condition and chemical profile remains uncertain. Here we investigated into this issue with comprehensive simulations and ambient observations, based on results of three widely-used thermodynamic models, i.e. ISORROPIA, E-AIM, and AIOMFAC. Across all models, γ2AN ranges between 10-2 and 10-1, with AIOMFAC results ~ 33 % lower than E-AIM and ISORROPIA. Correspondingly, AIOMFAC estimate higher particle phase nitrate values. The correlates positively with relative humidity (RH) and temperature, while RH generally contributes larger variations under typical scenarios. The effect of chemical composition on is more complex and is strongly modulated by RH. Furthermore, γ2AN responds more strongly to changes of particle chemical profile in E-AIM, whereas in ISORROPIA and AIOMFAC γ2AN is more sensitive to meteorological variations. As E-AIM is typically considered as the benchmark thermodynamic model, these results suggest the potential under-representation of chemical profiles in predicting γ2AN for ISORROPIA and AIOMFAC. The corresponding influence on 3-D chemical-transport model predictions of NH4NO3 are encouraged in future studies.