Aerosol vertical profiles strongly affect their radiative forcing uncertainties: study by using ground-based lidar and other measurements

In the present study, we illustrate for the first time that direct aerosol radiative 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, respectively. 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 extinction 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 transfer (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 increment 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 contribution 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.