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

Abstract

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.