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Impacts of aerosol direct effects on tropospheric ozone through changes in atmospheric dynamics and photolysis rates
Citation:
Xing, J., J. Wang, R. Mathur, S. Wang, G. Sarwar, Jon Pleim, C. Hogrefe, Y. Zhang, J. Jiang, David-C Wong, AND J. Hao. Impacts of aerosol direct effects on tropospheric ozone through changes in atmospheric dynamics and photolysis rates. Atmospheric Chemistry and Physics. Copernicus Publications, Katlenburg-Lindau, Germany, 17:9869-9883, (2017).
Impact/Purpose:
Photochemistry in the atmosphere is a well-known source of tropospheric ozone and is determined by ambient levels of O3 precursors (i.e., NOx and VOC) and photolysis rates, which are largely influenced by meteorological factors such as solar irradiance and temperature. It is well known that aerosols influence radiation through light scattering and absorption, thereby modulating atmospheric radiation and temperature. These aerosol direct effects (ADEs) can then impact thermal and photochemical reactions leading to the formation of O3. In this study, we apply the process analysis methodology (an advanced probing tool that enables quantitative assessment of integrated rates of key processes and reactions simulated in the atmospheric model) in the two-way coupled meteorology and atmospheric chemistry model, i.e., the Weather Research and Forecasting (WRF) model coupled with the Community Multiscale Air Quality (CMAQ) model developed by U.S. Environmental Protection Agency to examine the process chain interactions arising from ADEs and quantify their impacts on O3 concentration.
Description:
Aerosol direct effects (ADE), i.e., scattering and absorption of incoming solar radiation, reduce radiation reaching the ground and the resultant photolysis attenuation can decrease O3 formation in polluted areas. One the other hand, evidence also suggests that ADE associated cooling suppresses atmospheric ventilation thereby enhancing surface-level O3. Assessment of ADE impacts is thus important for understanding emission reduction strategies that seek co-benefits associated with reductions in both particulate matter and O3 levels. This study quantifies the impacts of ADE on tropospheric ozone by using a two-way online coupled meteorology and atmospheric chemistry model, WRF-CMAQ, instrumented with process analysis methodology. Two manifestations of ADE impacts on O3 including changes in atmospheric dynamics (∆Dynamics) and changes in photolysis rates (∆Photolysis) were assessed separately through multiple scenario simulations for January and July of 2013 over China. Results suggest that ADE reduced surface daily maxima 1h O3 (DM1O3) in China by up to 39 µg m-3 1 through the combination of ∆Dynamics and ∆Photolysis in January, but enhanced surface DM1O3 by up to 4 µg m-3 in July. Increased O3 in July is largely attributed to ∆Dynamics which causes a weaker O3 sink of dry deposition and a stronger O3 source of photochemistry due to the stabilization of atmosphere. Meanwhile, surface OH is also enhanced at noon in July, though its daytime average values are reduced in January. An increased OH chain length and a shift towards more VOC-limited condition are found due to ADE in both January and July. This study suggests that reducing ADE may have potential risk of increasing O3 in winter, but it will benefit the reduction of maxima O3 in summer.
