Effects of dust aerosols on tropospheric chemistry during a typical pre-monsoon season dust storm in northern India

Abstract. This study examines the effect of a typical pre-monsoon season dust storm on tropospheric chemistry through a case study in northern India. Dust can alter photolysis rates by scattering and absorbing solar radiation and provide surface area for heterogeneous reactions. We use the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) to simulate the dust storm that occurred during 17–22 April 2010 and investigate the contribution of different processes on mixing ratios of several key trace gases including ozone, nitrogen oxides, hydrogen oxides, methanol, acetic acid and formaldehyde. We revised the Fast Troposphere Ultraviolet Visible (F-TUV) photolysis scheme to include effects of dust aerosols on photolysis rates in a manner consistent with the calculations of aerosol optical properties for feedback to the meteorology radiation schemes. In addition, we added 12 heterogeneous reactions on the dust surface, for which 6 reactions have relative-humidity-dependent reactive uptake coefficients (γ). The inclusion of these processes in WRF-Chem is found to reduce the difference between observed and modeled O₃ from 16 ± 9 to 2 ± 8 ppbv and that in NO_y from 2129 ± 1425 to 372 ± 1225 pptv compared to measurements at the high-altitude site Nainital in the central Himalayas, and reduce biases by up to 30% in tropospheric column NO₂ compared to OMI retrievals. The simulated dust storm acted as a sink for all the trace gases examined here and significantly perturbed their spatial and vertical distributions. The reductions in these gases are estimated as 5–100%, and more than 80% of this reduction was due to heterogeneous chemistry. The RH dependence of γ is also found to have substantial impact on the distribution of trace gases, with changes of up to 20–25% in O₃ and HO₂, 50% in H₂O₂ and 100% in HNO₃. A set of sensitivity analyses revealed that dust aging could change H₂O₂ and CH₃COOH levels by up to 50% but has a relatively small impact on other gases.