Gas-particle partitioning of m-xylene and naphthalene oxidation products: temperature and NO_x influence

Abstract. Volatile organic compounds (VOCs) react with atmospheric oxidants resulting in oxygenated products of lower volatility known as semi and intermediate volatile organic compounds (S/IVOCs) forming secondary organic aerosols (SOA). Those compounds can partition between the gas and particle phases, a critical process influenced by several environmental parameters, yet poorly constrained. This study aims to evaluate the effect of temperature and VOC/NO_x ratio on SOA formation and 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/NO_x ratios reduce it. Gas-phase m-xylene major products are C₃, C₅ and C₈ 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 saturation concentration (C_i^*) values towards lower values, though no clear relationship between C_i^* and oxidation state is observed. A comparison between observed and estimated volatilities using SIMPOL.1 model reveals systematic deviations for both light molecules and heavy compounds, suggesting a need for improved predictive models.