Detailed heterogeneous oxidation of soot surfaces in a particle-resolved aerosol model

Abstract. Using the particle-resolved aerosol model PartMC-MOSAIC, we simulate the heterogeneous oxidation of a monolayer of polycyclic aromatic hydrocarbons (PAHs) on soot particles in an urban atmosphere. We focus on the interaction of the major atmospheric oxidants (O₃, NO₂, OH, and NO₃) with PAHs and include competitive co-adsorption of water vapour for a range of atmospheric conditions. For the first time detailed heterogeneous chemistry based on the Pöschl-Rudich-Ammann (PRA) framework is modelled on soot particles with a realistic size distribution and a continuous range of chemical ages. We find PAHs half-lives, τ_1/2, on the order of seconds during the night, when the PAHs are rapidly oxidised by the gas-surface reaction with NO₃. During the day, τ_1/2 is on the order of minutes and determined mostly by the surface layer reaction of PAHs with adsorbed O₃. Such short half-lives of surface-bound PAHs may lead to efficient conversion of hydrophobic soot into more hygroscopic particles, thus increasing the particles' aerosol-cloud interaction potential. Despite its high reactivity OH appears to have a negligible effect on PAH degradation which can be explained by its very low concentration in the atmosphere. An increase of relative humidity (RH) from 30 % to 80 % increases PAH half-lives by up to 50 % for daytime degradation and by up to 100 % or more for nighttime degradation. Uptake coefficients, averaged over the particle population, are found to be relatively constant over time for O₃ (∼2 × 10⁻⁷ to ∼2 × 10⁻⁶) and NO₂ (∼5 × 10⁻⁶ to ∼10⁻⁵) at the different levels of NO_x emissions and RH considered in this study. In contrast, those for OH and NO₃ depend strongly on the surface concentration of PAHs. We do not find a significant influence of heterogeneous reactions on soot particles on the gas phase composition. The derived half-lives of surface-bound PAHs and the time and particle population averaged uptake coefficients for O₃ and NO₂ presented in this paper can be used as parameterisations for the treatment of heterogeneous chemistry in large-scale atmospheric chemistry models.