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not uniform; at some parts it washes out the aerosols from the atmosphere
through wet deposition, while in other parts aerosol loading remains high.
This results in wide bi- and tri-modal distributions. In the post-monsoon
season, the AOD distribution tends to readjust to unimodality and the
frequencies of large AOD start decreasing. The ADRF estimated in the
clear-sky condition using our model are compared with the independent
estimates over some locations in the IGB to test the validity of our model
(Fig. 2(b)), which shows that the expansion of our model is able to produce
the ADRF within the current uncertainty levels. The cloudy-sky ADRF
is calculated incorporating the cloud parameters from MODIS (for details
see Ref. 23).
The critical part of Eq. (1) is AF, as no such measurements exist in IGB.
We employ MODIS-aerosol fine mode fraction (AFMF) product to infer
the AF. Earlier, researchers have utilized MODIS-AFMF data to assess
the anthropogenic contribution over the oceanic region,
3
as in general,
anthropogenic aerosols dominate in the fine mode fraction. The model-
derived AF was compared with the MODIS-AFMF product over Kanpur
for the entire 5 year period and a statistically significant relationship
(correlation of 0.96) was found at 95% confidence level:
AF = AFMF
∗
0
.
65 + 0
.
1534
.
(2)
The equation suggests that MODIS has a bias in determining the AF for this
region, and thus using the above relation, AF was estimated for each 1
◦
×
1
◦
grid of IGB. Subsequently, the anthropogenic ADRF at the surface and the
atmosphere were estimated for each grid using Eq. (1). The anthropogenic
ADRF at the TOA was calculated from the anthropogenic surface and
atmospheric ADRF values.
2.3
. Uncertainty in anthropogenic ADRF
The estimations of anthropogenic ADRF over the IGB involve some
assumptions leading to uncertainty in our estimates, which we have
quantified so that our results can be compared with other studies. There are
two basic assumptions involved here, first all the water-soluble components
are considered to be of anthropogenic origin, and second, the aerosol forcing
eciency over Kanpur is considered to be representative over the entire
IGB. Uncertainties in the measurements and AERONET-retrievals, which
are used to formulate and constrain the model, also propagate to increase
