Comparison of Vapor Pressure Estimation Methods Used to Model Secondary Organic Aerosol Formation from Reactions of Linear and Branched Alkenes with OH/NOx

Abstract

Modeling atmospheric reactions that lead to the formation of secondary organic aerosol (SOA) is an important tool for understanding the current and future impacts of human activity on the environment. Vapor pressure is a key parameter in modeling these reactions, as it largely determines the gas-particle partitioning of atmospheric oxidation products. However, the vapor pressures of many atmospherically relevant molecules are still poorly constrained. To aid modeling efforts, several structure–activity relationships (SARs) based on group contribution methods have been developed for estimating compound vapor pressures. The current study evaluates how four of these SARs: SIMPOL, EVAPORATION, SPARC, and Nannoolal impact the modeled predictions of SOA yields for reactions of C₈–C₁₄ 1-alkenes and C₉–C₁₅ 2-methyl-1-alkenes with OH radicals in the presence of NO_x. The models include well-constrained, quantitative reaction mechanisms developed by our research group from several previous environmental chamber studies of product yields, gas-particle and gas-wall partitioning, and secondary reactions with OH radicals. Based on our previous product studies and the results of this study, there was no need to account for particle-phase oligomer formation. Comparison of modeled and measured SOA yields provide insight into the major products responsible for SOA formation over the large range of carbon numbers, and the sources of discrepancies between model predictions and measurements. The generally moderate to poor model-measurement agreement exemplifies the need for further development of vapor pressure estimation methods, which have a major impact on atmospheric SOA modeling. The systems studied here could be useful to others interested in developing and evaluating models for simulating SOA formation.