Influencing Factors of Gas-Particle Distribution of Oxygenated Organic Molecules in Urban Atmosphere and its Deviation from Equilibrium Partitioning

Abstract. Gas-to-particle partitioning governs the fate of Oxygenated Organic Molecules (OOMs) and the formation of organic aerosols. We employed a FIGAERO-CIMS to measure gas-particle distribution of OOMs in a winter campaign in urban atmosphere. The observed gas to particle (G/P) ratios show a narrower range than the equilibrium G/P ratios predicted from saturation mass concentration C^* and organic aerosol content. The difference between observed and equilibrium G/P ratios could be up to 10 orders of magnitude, depending on C^* parameterization selection. Our random forest models identified relative humidity (RH), aerosol liquid water content (LWC), temperature and ozone as four influential factors driving the deviations of partitioning from equilibrium state. Random forest models with satisfactory performance were developed to predict the observed G/P ratios. Intrinsic molecule features far outweigh meteorological and chemical composition features in the model's predictions. For a given OOM species, particle chemical composition features including pH, RH, LWC, organic carbon, potassium and sulfate dominate over meteorological and gaseous chemical composition features in predicting the G/P ratios. We identified positive or negative effects, as well as the sensitive ranges, of these influential features using SHapley Additive exPlanations (SHAP) analysis and curve fitting with a generalized additive model (GAM). Our models found that temperature does not emerge as a significant factor influencing the observed G/P ratios, suggesting that other factors, most likely associated with particle composition, inhibit the gas/particle partitioning of OOMs in response to temperature change.