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.
