Collaborative Research: Direct radiative forcing over central Greenland; Assessment of the coupled effect of light absorbing aerosols and snow albedo variability

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The PIs will make key measurements needed to accurately estimate the direct radiative forcing by aerosols over the central Greenland ice sheet. Sampling will take place over a field season that will occur from the spring through fall of 2011. It will include real-time measurements of aerosol physical and optical properties (size distribution, multi-wavelength scattering, σsp, and backscattering, σbsp, coefficients as well as the multi-wavelength absorption coefficient, σap) needed to estimate the aerosol single scattering albedo, ω, and asymmetry parameter, g. Additional measurements of the wavelength-dependent optical depth, τλ, as well as the spectral surface reflectance, Rs will also be made. These data will serve as input to a radiative transfer model that will be used to estimate the direct aerosol radiative forcing at the surface and top of the atmosphere over Greenland. Sources and source regions of the direct radiative forcing will be determined by the chemical composition of the aerosols sampled. It is hypothesized that dust, biomass burning and fossil fuel combustion aerosols often exert a positive direct radiative forcing many times greater than that of anthropogenic greenhouse gases. Additional characterization of snow albedo, which varies occurs on timescales of hours to days, will be made to determine whether it, too, exerts a significant influence on the direct aerosol forcing over Summit. Surface snow grain properties and surface snow chemistry will be determined at high temporal frequency to explore the link between the variability in these properties and the surface albedo.