Gas-particle partitioning of m-xylene and naphthalene oxidation products: Temperature and NOx influence

Abstract

Volatile organic compounds (VOCs) react with atmospheric oxidants, resulting in oxygenated products of lower volatility known as semi-volatile and intermediate-volatility organic compounds (S/IVOCs), which form secondary organic aerosols (SOAs). Those compounds can partition between the gas and particle phases, a critical process that is influenced by several environmental parameters yet is poorly constrained. This study aims to evaluate the effect of temperature and the VOC/NOx ratio on SOA formation and the partitioning of individual SOA products from m-xylene and naphthalene OH oxidation. Experiments are carried out in an oxidation flow reactor (OFR), and products are identified and quantified using a proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS) coupled to a CHemical Analysis of aeRosol ONline (CHARON) inlet. Results show that lower temperatures significantly enhance SOA formation, while lower VOC/NOx ratios reduce it. Gas-phase m-xylene major products are C3, C5, and C8 compounds, whereas particle-product distributions exhibit a progressive increase from C2 to C8. In contrast, naphthalene products partition more readily into the condensed phase, with C8-C10 products dominating. Most of the oxidation products from both precursors exhibit a volatility distribution in the SVOC regime, with fewer in the IVOC regime. The decrease in temperature shifts the effective saturation concentration (Ci*) values towards lower values, although no clear relationship between Ci* and the oxidation state is observed. A comparison between observed and estimated volatilities using a model based on the group contribution method (SIMPOL.1) reveals systematic deviations for both light molecules and heavy compounds, suggesting a need for improved predictive models.