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Ann. Geophys., 26, 1345-1354, 2008
www.ann-geophys.net/26/1345/2008/
© European Geosciences Union 2008
Fog-induced variations in aerosol optical and physical properties over the Indo-Gangetic Basin and impact to aerosol radiative forcing
S. K. Das1, A. Jayaraman2, and A. Misra1
1Physical Research Laboratory, Ahmedabad, 380 009, India
2National Atmospheric Research Laboratory, Gadanki, 517112, India
Abstract. A detailed study on the changes in aerosol physical and optical properties
during fog events were made in December 2004 at Hissar (29.13° N,
75.70° E), a city located in the Indo-Gangetic basin. The visible aerosol
optical depth was relatively low (0.3) during the initial days, which,
however, increased (0.86) as the month progressed. The increasing aerosol
amount, the decreasing surface temperature and a higher relative humidity
condition were found favoring the formation of fog. The fog event is also
found to alter the aerosol size distribution. An increase in the number
concentration of the nucleation mode (radius<0.1 μm) particles, along
with a decrease in the mode radius showed the formation of freshly nucleated
aerosols. In the case of accumulation mode (0.1 μm<radius<1.0 μm)
an increase in the mode radius was observed showing the hygroscopic and
coagulation growth of particles. The observed aerosol optical depth spectra
are model fitted to infer the aerosol components which are further used to
compute the aerosol radiative forcing. The top of the atmosphere forcing is
found to increase during foggy days due to large backscattering of radiation
back to space. It is also shown that during foggy days, as the day
progresses the RH value decreases, which reduces the forcing value while the
increasing solar elevation increases the forcing value. Thus the fog event
which prolongs longer into the daytime has a stronger effect on the diurnally
averaged aerosol radiative forcing than those events which are confined
only to the early morning hours.
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