Detailed Heterogeneous Chemistry Implemented in a Particle-Resolved Aerosol Model
Kaiser, Jan Christopher
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In this study, heterogeneous chemical reactions between trace gases and atmospheric soot particles are investigated using a particle-resolved aerosol model. The model accounts for physical and chemical processes in the atmosphere that change both particulate and gas phase composition. Four reactive gases, namely the major atmospheric oxidants O3, NO2, OH, and NO3, are considered to compete with non-reactive water vapor for active surface sites on the soot particles coated with polycyclic aromatic hydrocarbons (PAHs). For this purpose, the state-of-the-art particle-resolved aerosol model PartMC-MOSAIC (Particle Monte Carlo model, coupled to the MOdel for Simulating Aerosol Interactions and Chemistry) has been extended to include heterogeneous chemicalkinetics based on the recently developed Poeschl-Rudich-Ammann (PRA) framework. PartMC-MOSAIC enables us to model continuous soot emissions with a realistic particle size distribution and to track each particle's composition individually over the course of a 24 hour simulation. The flux-based approach of the PRA framework accounts for dynamic changes in the uptake of gas species on particle surfaces, which are caused by changes of gas phase and particle composition and associated modification of surface properties. Thus, it is possible to assess in detail the effects of heterogeneous reactions between major atmospheric oxidants and PAH coated soot surfaces on gas phase composition, on uptake kinetics, and on degradation of particle-bound PAHs in atmospherically relevant scenarios. In contrast to previous modeling results we found no significant impact of these reactions on gas phase composition, regardless of the magnitude of soot emissions. Reactive uptake of O3 and NO2 is found to decrease by several orders of magnitude in the first minute of a particle's atmospheric lifetime but to stay relatively constant thereafter. This is in agreement with the results of previous applications of the PRA framework and experimental data. In case of OH and NO3, uptake coefficients vary with the degree of PAH degradation. They are higher than those for O3 and NO2 during day (&sim 10<super>-1</super> to &sim 10<super>-4</super> vs. &sim 10<super>-7</super> to &sim 10<super>-5</super>), but may be significantly lower at night (as low as &sim 10<super>-9</super>), when particle-bound PAHs are very efficiently depleted by reaction with NO3. PAH lifetime is on the order of minutes during day, when it is determined mainly by O3, which is about an order of magnitude lower than other laboratory and modeling studies suggested. During night, when NO3 levels are high, the PAH coating is oxidized within seconds, in agreement with experimental results. This study is the first to assess heterogeneous kinetics in atmospheric systems employing a particle-resolved aerosol model, and the complexity of the considered scenarios exceeds that of previous laboratory experiments and modeling studies. The results presented here allow for a much improved evaluation of the role of soot, one of the most ubiquitous types of atmospheric particles, on atmospheric gas phase composition and of its impact on health related issues and climate.