Atmos. Chem. Phys. Discuss., 9, 20145-20194, 2009
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Impact of dust on tropospheric photochemistry over polluted regions: a case study of the Beijing megacity
S. Zhu1, T. Butler2, R. Sander2, J. Ma1, and M. G. Lawrence2
1Chinese Academy of Meteorological Sciences, Beijing, China
2Max Planck Institute for Chemistry, Mainz, Germany

Abstract. The box model MECCA (Module Efficiently Calculating the Chemistry of the Atmosphere) is updated by incorporating detailed heterogeneous chemistry occurring on mineral aerosol surfaces. The model is used to investigate the impact of dust on tropospheric photochemistry, when the dust is transported to a polluted region together with other trace gases. The impacts of dust via heterogeneous removal of gases are analyzed for different hypothetical transport rates which are described by four different exchange rate coefficients Kt in the model. Substantial impacts are found for many gases, including Ox (O3+O(3P), NOx (NO+NO2) and OH. The Ox daily average mixing ratio decreases due to heterogeneous reactions on dust. The change ranges from –2.5 to –18.4 nmol/mol, and is larger for faster mixing with upwind air masses (i.e. greater Kt). This translates into a large relative change in Ox, ranging from –44% to –55%, which changes slightly with increasing values of Kt. By assuming an artificial 50% decrease of all photolysis rates, the impacts of dust via photolysis perturbation are also estimated. Furthermore, the uncertainties in the results due to the uncertainties in the uptake coefficients are evaluated. It is found that for all gases which are heterogeneously removed, the self-removal results in the largest uncertainty (e.g. –49% for O3, –76% for NO2, –47% for HNO3, –92% for HCHO, –64% for CH3OH and –93% for SO2). The heterogeneous removal of NO2 is found to be particularly important, because it results in significant uncertainties not only in itself, but also in OH (340%) and HO2 (365%). Moreover, the heterogeneous removal of HCHO and O3 also has farther-reaching effects on the OH concentration (resulting in changes of –55% and 45%, respectively), and the heterogeneous removal of HCHO results in an uncertainty of –38% in the HO2 concentration.

Citation: Zhu, S., Butler, T., Sander, R., Ma, J., and Lawrence, M. G.: Impact of dust on tropospheric photochemistry over polluted regions: a case study of the Beijing megacity, Atmos. Chem. Phys. Discuss., 9, 20145-20194, doi:10.5194/acpd-9-20145-2009, 2009.
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