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

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 (O<sub>3</sub>, NO<sub>2</sub>, OH, and NO<sub>3</sub>) 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 τ<sub>1/2</sub> on the order of seconds during the night, when the PAHs are rapidly oxidized by the gas-surface reaction with NO<sub>3</sub>. During the day, τ<sub>1/2</sub> is on the order of minutes and determined mostly by the surface layer reaction of PAHs with adsorbed O<sub>3</sub>. 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 O<sub>3</sub> (~2×10<sup>−7</sup> to ~2×10<sup>−6</sup>) and NO<sub>2</sub> (~5×10<sup>−6</sup> to ~10<sup>−5</sup>) at the different levels of NO<sub>x</sub> emissions and RH considered in this study. In contrast, those for OH and NO<sub>3</sub> 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

Chemical aging of single and multicomponent biomass burning aerosol surrogate particles by OH: implications for cloud condensation nucleus activity

“…At RH > 15 %, MNC is less reactive with OH, exhibiting γ < 0.07 due to competitive co-adsorption of water and OH (Slade and Knopf, 2014). Similar suppressions in gas uptake, due to competitive adsorption processes, have been observed in the case of OH and NO 3 uptake by BBA surrogate films (Slade and Knopf, 2013;Knopf et al, 2011), O 3 uptake by benzo [a]pyrene in the presence of water vapor (Pöschl et al, 2001), and O 3 and NO 2 uptake by benzo[a]pyrenecoated soot (Springmann et al, 2009;Kaiser et al, 2011 The presence of higher [O 3 ] may further decrease the OH reactivity of OA (Renbaum and Smith, 2011). Under the applied experimental conditions, the multiphase reaction kinetics involving highly viscous organic material are likely limited by surface-bulk exchange (Arangio et al, 2015;Slade and Knopf, 2014).…”

Quantifying aerosol mixing state with entropy and diversity measures

“…Note that from the information on per-particle composition, it is straightforward to calculate per-particle properties, such as hygroscopicity Zaveri et al, 2010;Ching et al, 2012;Tian et al, 2013), optical properties , or particle reactivity (Kaiser et al, 2011).…”

Kinetic studies of heterogeneous reactions of particulate phosmet and parathion with NO3 radicals

“…Compared to the results obtained in this study, the effective rate constants of phosmet and parathion with NO 3 radicals are 3 times slower than that of pirimiphos-methyl, while $5 orders of magnitude faster than that of pirimiphos-methyl with O 3 . Compared with NO 3 radicals, the effect of OH radicals on degradation of atmospheric particulate organic compounds may not be remarkable due to its lower concentration in the atmosphere (Kaiser et al, 2011). Therefore, under typical atmospheric condition at night, NO 3 radicals might be the most important oxidant for the degradation of atmospheric OPPs.…”

“…Under real atmospheric conditions, the phase and morphology of phosmet and parathion particles might be complex, and those distributed in the particle bulk may not be accessible for oxidation by NO 3 radicals. In addition, other factors such as atmospheric relative humidity (Kaiser et al, 2011), particle sizes (Huff Hartz et al, 2007), and the concentrations of particulate reactants and gas-phase oxidants (Ammann et al, 2003) can affect heterogeneous reaction. As a result, the lifetimes of phosmet and parathion particles may vary under different atmospheric conditions.…”