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This study examines the effect of a typical pre-monsoon season dust storm ontropospheric chemistry through a case study in northern India. Dust canalter photolysis rates by scattering and absorbing solar radiation andprovide surface area for heterogeneous reactions. We use the WeatherResearch and Forecasting model coupled with Chemistry (WRF-Chem) to simulatethe dust storm that occurred during 17–22 April 2010 and investigate thecontribution of different processes on mixing ratios of several key tracegases including ozone, nitrogen oxides, hydrogen oxides, methanol, aceticacid and formaldehyde. We revised the Fast Troposphere Ultraviolet Visible(F-TUV) photolysis scheme to include effects of dust aerosols on photolysisrates in a manner consistent with the calculations of aerosol opticalproperties for feedback to the meteorology radiation schemes. In addition,we added 12 heterogeneous reactions on the dust surface, for which 6reactions have relative-humidity-dependent reactive uptake coefficients(γ). The inclusion of these processes in WRF-Chem is found to reducethe difference between observed and modeled O3 from 16 ± 9 to2 ± 8 ppbv and that in NOy from 2129 ± 1425 to372 ± 1225 pptv compared to measurements at the high-altitude siteNainital in the central Himalayas, and reduce biases by up to 30% intropospheric column NO2 compared to OMI retrievals. The simulated duststorm acted as a sink for all the trace gases examined here andsignificantly perturbed their spatial and vertical distributions. Thereductions in these gases are estimated as 5–100%, and more than 80% ofthis reduction was due to heterogeneous chemistry. The RH dependence ofγ is also found to have substantial impact on the distribution oftrace gases, with changes of up to 20–25% in O3 and HO2, 50%in H2O2 and 100% in HNO3. A set of sensitivity analysesrevealed that dust aging could change H2O2 and CH3COOH levelsby up to 50% but has a relatively small impact on other gases

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