Complex gas-particle partitioning of nitro-phenolic compounds: field-based insights and determination of apparent activity coefficient

Gas-particle partitioning (GPP) of oxygenated semi-volatile organic compounds (SVOCs) is crucial for atmospheric organic aerosol formation, yet large uncertainties persist in its simulation due to challenges in obtaining accurate parameters. This study focuses on nitro-phenolic species (NPs), representative oxygenated SVOCs impacting solar radiative balance and atmospheric chemistry. Concurrent measurements of gaseous and particulate NPs at a subtropical coastal site showed particulate fractions ranging from 8.6% to 53%, which deviated from traditional theoretical estimates by factors of 0.26 to 104. To address these discrepancies, a field-based activity coefficient (zeta) was derived by integrating measured parameters and theoretical considerations. Incorporating zeta into a box model significantly improved simulations for mono-NPs and uncovered a more complex GPP process for di-NPs than previously recognized. The successful application of zeta in a regional model highlights its broader applicability and calls for more quantitative studies for various SVOCs.