Detailed heterogeneous oxidation of soot surfaces in a particle-resolved aerosol model
1Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY, USA
2Department of Atmospheric Sciences, University of Illinois at Urbana-Champaign, Urbana, IL, USA
3Institute for Terrestrial and Planetary Atmospheres, School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY, USA
*currently at: Department of Physics and Astronomy, Universität Würzburg, Würzburg, Germany
Abstract. We simulate the heterogeneous oxidation of condensed phase polycyclic aromatic hydrocarbons (PAHs) on soot particles in an urban atmosphere using the particle-resolved aerosol model PartMC-MOSAIC. 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 PAH half-lives τ1/2 on the order of seconds during the night, when the PAHs are rapidly oxidized by the gas-surface reaction with NO3. During the day, τ1/2 is on the order of minutes and determined mostly by the surface layer reaction of PAHs with adsorbed O3. Such short PAH half-lives may lead to efficient conversion of hydrophobic soot into more hygroscopic particles, thus increasing the particles' aerosol-cloud interaction potential. Despite its high reactivity 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 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 NOx 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 PAH half-lives presented in this paper can be used as parameterisations for the treatment of heterogeneous chemistry in large-scale atmospheric chemistry models.