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 pho- tolysis 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 oc- curred during 17–22 April 2010 and investigate the contri- bution of different processes on mixing ratios of several key trace gases including ozone, nitrogen oxides, hydrogen ox- ides, methanol, acetic acid and formaldehyde. We revised the Fast Troposphere Ultraviolet Visible (F-TUV) photol- ysis scheme to include effects of dust aerosols on photol- ysis 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 NOy from 2129 ± 1425 to 372 ± 1225 pptv compared to measurements at the high- altitude site Nainital in the central Himalayas, and reduce bi- ases 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 per- turbed 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 H₂ O₂ and 100 % in HNO₃ . A set of sensitivity analyses revealed that dust aging could change H₂ O₂ and CH₃ COOH levels by up to 50 % but has a relatively small impact on other gases.