Dr. Sachchida Nand Tripathi's Homepage

Scientific Objective

Atmospheric aerosols consist of small particles, some of which can be suspended in the troposphere for several days to several weeks. Their concentration has increased substantially due to human activities such as fossil fuel burning, power generation and land use. The short lifetime of the troposphere aerosols and the suggested dominance of anthropogenic sources suggest that radiative forcing (the change in surface-troposphere energy balance) is likely to be spatially very inhomogeneous in contrast to that due to carbon dioxide.

Radiative forcing due to atmospheric aerosols is one of the largest sources of uncertainty in assessing future climate change (Hansen et. al., 2000). Systematic characterization of aerosols is needed to understand the aerosol effect on climate. Aerosol optical thickness, single scattering albedo, asymmetric parameter are the key parameters used for calculating the direct radiative forcing due to aerosols.

Brief Description of Work

1. Aerosol Optical Properties:

An automatic CIMEL sun and sky scanning radiometer (CIMEL Electronique CE-318) deployed in Indian Institute of Technology Kanpur as part of Aerosol Robotic Network (AERONET) is located on top of a roof with no obstructions to the sun above 100 elevation. The CIMEL radiometer takes measurements of the direct sun radiance at eight spectral channels (340, 380, 440, 500, 670, 870, 940 and 1020 nm) with triplet observations per wavelength and diffuse sky radiances at four spectral channels (440, 670, 870 and 1020 nm) [Holben et al., 1998]. 940-nm channel is used to estimate the water vapor content (WVC) and the remaining seven channels are used to retrieve aerosol optical depth (AOD). The radiometer is powered by batteries charged from solar panels, without dependency on the local power supply. The monthly, seasonal and inter-annual variability of aerosol optical properties in Kanpur as retrieved by AERONET have been studied.

Spatial and temporal variability of aerosol direct forcing over the Ganga basin is investigated by coupling the ground-based measurements and satellite data into radiative transfer model (SBDART) for clear sky and all-sky conditions.

3. Aerosol Model:

Aerosol samples are collected in the winter season as part of the ISRO GBP Land Campaign II to study the size-segregated chemical compositions. The data are used to build an aerosol model for the region in the winter and it is extended for the entire Ganga basin. The model examines the relative share of different species to the observed aerosol optical depth. It has been found that the relative humidity is the key component in affecting the optical properties and hence the direct radiative forcing. The atmospheric absorption in the Ganga basin is significantly high (~23 W m-2).

4. Dust Events in Kanpur, source and radiative forcing:

Dust samples were collected and analyzed during May 2004 in Kanpur (in Ganga basin), Northern India for the first time. Chemical evidence along with the air mass trajectories suggests three major sources of the mineral dusts transported to the Ganga basin. High concentration of Pb and Cd indicates possible mixing with the anthropogenic pollution. Measured mean aerosol optical depth and Ångstrom wavelength exponent are 0.58 and 0.21. The composite aerosol model reveals low single scattering albedo (~0.74) due to the presence of black carbon. During the dust storms, shortwave (SW) clear-sky diurnally averaged top of the atmosphere (TOA) and surface forcing come out to be +11±0.7 and -26±3 W m-2, respectively. Corresponding forcing in the longwave (LW) region are +1.9±0.6 and +1.6±0.4 W m-2. Net atmospheric forcing (i.e. SW+LW, 36 W m-2) corresponds to heating rate of ~1.020 K/day in the lower atmosphere. Dust alone has resulted in the net TOA and surface forcing of +7 and -12 W m-2.

Column-integrated aerosol black carbon (BC) concentration ([BC]) has been retrieved over Kanpur during 2001-2003. [BC] is derived from BC volume fraction and AERONET-retrieved size distribution using Maxwell-Garnett and Bruggeman mixing rules in a 4 component mixture of water, BC, organic carbon (OC)/dust and (NH4)2SO4. The volume fraction of each component is retrieved by matching the mixture refractive index (real, n(l) and imaginary, k(l)) with AERONET-retrieved refractive index. [BC] shows seasonal variations with high values (> 8 mg m-2) observed during the post-monsoon and winter seasons and low values (< 6 mg m-2) during the monsoon season. Diurnal variation of [BC] is associated with boundary layer dynamics and the local anthropogenic activities. Specific absorption cross-section (aa) decreases exponentially with the increase in [BC]. Yearly averaged [BC] and aa are 8.18 ± 3.2, 5.56 ± 2.1, 7.72 ± 4.2 mg m-2 and 9.56 ± 4.1, 11.98 ± 5.2, 10.07 ± 2.4 m2 g-1 for 2001, 2002 and 2003, respectively with only 2% difference in the values retrieved by two different mixing rules. The sensitivity of [BC] to the BC refractive index is tested considering four major classes of BC, where the Maxwell-Garnett mixing rule is found to show more stability compared to Bruggeman mixing rule. aa does not change significantly for G-type BC with increase in OC/BC ratio indicating it as the predominant contributor to the absorption. k(λ) of dust has more impact on retrieval of [BC], as 10% increase in k(λ) increases the variation of [BC] by 100%. During the intense dust loading period (April-June), retrieval of [BC] is also affected by n(λ) of dust. [BC] along with the surface measurements provide information on the BC mixing height, which may have implication to the regional climate forcing.