Capturing the Relative‐Humidity‐Sensitive Gas–Particle Partitioning of Organic Aerosols in a 2D Volatility Basis Set

Aerosol water affects the physicochemical properties and mass concentration of organic aerosols (OA), but it is typically omitted by air quality, weather, and climate models. We compare two classes of simplified models to estimate the OA water uptake and gas–particle partitioning of organic compounds. One class uses a single‐hygroscopicity‐parameter (κ) approach while the other is based on the reduced‐complexity Binary Activity Thermodynamics (BAT) model. We show that a BAT‐based two‐dimensional volatility basis set (VBS) model always predicts a higher OA mass concentration at elevated relative humidity (RH), for example, ∼16% at 80% RH, than any variation of the κ‐based method considered—even when BAT‐VBS predicts a lower water uptake. The main reason being that the BAT‐VBS model captures variations in effective saturation mass concentration of organics (C*) with RH, a feature that other VBS methods lack. The BAT‐VBS framework offers an efficient, RH‐sensitive treatment for reduced‐complexity OA modeling. Plain Language Summary Organic compounds in the environment are present in both the gas phase and in condensed aerosol particles when of sufficiently low volatility. The latter contributing to what is known as organic aerosol, a major component of air pollution. Water vapor is one of the most important species in the air, including due to its effect on aerosol water content as a function of relative humidity. However, the implications of absorbed water on the equilibrium distribution of organics between aerosol particles and the gas phase are neglected by most air quality, weather, and climate models. We show that capturing the organic aerosol water uptake tends to increase the mass of organics present in aerosols. Additionally, we show that commonly used approaches, even when accounting for water uptake into the organic aerosol, tend to fail to predict the complete amplification of organic aerosol mass. Our work, based on a sound thermodynamic framework, shows the importance of a feedback effect between water uptake, changing particle properties, and the subsequent uptake of additional semivolatile organics from the gas phase. Our findings are particularly pertinent to the improvement of the reduced‐complexity representation of organic aerosols in large‐scale models used for air quality and climate simulations. Key Points 2D volatility basis set models of organic aerosol predict diverging effects of water uptake on organic aerosol mass concentration An