Impact of dust on tropospheric chemistry over polluted regions: a case study of the Beijing megacity

The box model MECCA (Module Efficiently Calculating the Chemistry of the Atmosphere) is extended 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, focusing on the example of Beijing. 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 K_t in the model. Along with the dust, airmasses with trace gas levels characteristic for regions upwind of Beijing are transported with the same rate ( K_t). Substantial impacts are found for many gases, including O _x (O ₃+O( ³P)), NO _x (NO+NO ₂) and OH. The O _x daily average mixing ratio decreases due to heterogeneous reactions on dust. The change ranges from −2.5 to −18.4 nmol mol ⁻¹, and is larger for faster mixing with upwind air masses (i.e. greater K_t). This translates into a large relative change in O _x, ranging from −44% to −55%, depending on K_t. By assuming an artificial 50% decrease of all photolysis rates, the impacts of dust via perturbation of the photolysis rates in the polluted region 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 O ₃, −76% for NO ₂, −47% for HNO ₃, −92% for HCHO, −64% for CH ₃OH and −93% for SO ₂). The heterogeneous removal of NO ₂ is found to be particularly important, because it results in significant levels of uncertainty not only for itself, but also for OH (340%) and HO ₂ (365%). Moreover, the heterogeneous removal rates of HCHO and O ₃ also have 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 HO ₂ concentration. The limitations of MECCA due to its missing oxidation mechanism for aromatics and other higher VOC species has also been considered, and shown to be potentially important in the quantitative results, though not likely to change the qualitative results of this study.