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 (ζ) was derived by integrating measured parameters and theoretical considerations. Incorporating ζ 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 ζ in a regional model highlights its broader applicability and calls for more quantitative studies for various SVOCs.