Abstract:
In the present study, we illustrate for the first time that direct aerosol radia-
tive forcing has large uncertainty due to diversity in simulated vertical profile
of aerosol over Manora Peak, Nainital (considered to be free troposheric site).
In order to have a comprehensive picture, we choose March and October months
as representative months of high and low aerosol mass loading over the site, respec-
tively. Monthly averaged aerosol optical depths (AODs) at 0.5 μm are ∼0.30
(±0.02) and 0.13 (±0.01), respectively, during the above months. The derived
aerosol extinction profile showed an elevated aerosol layer with maximum extinc-
tion of ∼0.10 ± 0.01 km ⁻¹ (0.08 ± 0.02 km ⁻¹ ) at ∼1.12 km (0.75 km) during
March (October) month. The elevated aerosol layer contributed 44% and 68%
to the total AOD during March and October months, respectively. The observed
AODs at different wavelengths and black carbon (BC) measurements were used to
estimate the other aerosol optical parameters, which are crucial in aerosol radiative
forcing. The derived aerosol extinction profile has been used in radiative trans-
fer (RT) model in addition to the standard aerosol extinction profile of RT model
along with the aerosol optical properties. Our results indicate that there is an incre-
ment in surface radiative forcing, which is ∼10% (25%) due to the insertion of
derived aerosol extinction profile for the same columnar properties of aerosols
during March (October). Moreover, we found that higher the aerosol layer con-
tribution to the total AOD, the more the uncertainty in aerosol radiative forcing.
Apart from this, significant differences were also found in atmospheric forcing at
each altitude due to variation in vertical profile of aerosol extinction, which leads
the modification of the thermal structure of the atmosphere. Hence, our study has
emphasized the importance of proper selection of aerosol vertical profile to obtain
more realistic values of radiative forcing.
