Abstract
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 NOy 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.
Highlights
In light of the above conditions, this manuscript examines the effects of dust aerosols on the distribution of many key trace gases including O3, nitrogen oxides, hydrogen oxides, methanol, acetic acid and formaldehyde by incorporating the updated information on heterogeneous reactive uptake of trace gases in MOZCART chemical mechanism of Weather Research and Forecasting model coupled with Chemistry (WRF-Chem)
Dust storms in northern India are characterized by large increase in aerosol optical depth (AOD) (> 50 %) and decrease in α (> 70 %) (Dey et al, 2004; Prasad and Singh, 2007)
The effects of a typical pre-monsoon season dust storm on tropospheric chemistry are analyzed for a case study in northern India, using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), which is further developed to enhance its ability to simulate tropospheric chemistry in the presence of dust particles
Summary
Dust aerosols have gained considerable attention in the recent years because they constitute a major fraction of the particulate matter in the troposphere and because they have important implications for air quality, visibility, the earth’s radiation budget (e.g., Haywood and Boucher, 2000; Seinfeld et al, 2004), biogeochemistry (e.g., Jickells et al, 2005), hydrological cycles (e.g., Miller et al, 2004; Zhao et al, 2011), and atmospheric chemistry (e.g., Dentener et al, 1996; Wang et al, 2012). Global modeling studies have suggested that heterogeneous chemistry on dust aerosols can reduce surface O3 in northern India by 4–10 % (e.g., Dentener et al, 1996; Bauer et al, 2004). We revise the Fast Troposphere Ultraviolet Visible (F-TUV) scheme to include effects of dust aerosols on photolysis rates This extended configuration of WRF-Chem is used to simulate the impact of a typical pre-monsoon season dust storm on the regional tropospheric chemistry in northern India. We conclude that heterogeneous chemistry mostly, partly dust-modified photolysis rates, improves the agreement between model results and observations, and that these processes lead to a reduction in O3, H2O2, and HNO3
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