Cryospheric and Polar Processes Seminar

Quantifying sources, distribution, and processing of light absorbing aerosols in the cryosphere: A comparison of dissolved and refractory black carbon in polar and high mountain regions

Dr. Alia Lauren Khan - Postdoctoral Research Associate, National Snow and Ice Data Center; PhD, Civil Engineering, University of Colorado Boulder

Light absorbing aerosols (LAAs), such as black carbon (BC), in snow and ice are one of the least understood parameters in global climate models.  This is due to complicated physical processes within the cryosphere and too few in situ observations.  BC is derived from the incomplete combustion of biomass and fossil fuels and can enhance melt water generation and glacial recession when deposited on snow and ice surfaces. Measurements are limited due to the difficulty of collecting and preserving samples for analysis from remote environments. In order to help build a larger repository of ground observations, three state-of-the-art methods were applied for determination of BC concentration and composition in snow and glacial melt-water across the polar regions in the Arctic and Antarctic, as well as major mountain regions such as the Himalayas, Rockies, and Andes Mountains.  

In this seminar, 4 case studies will be presented. The first utilizes a DBC molecular marker method to identify chemical signatures of past and present sources of dissolved black carbon (DBC) in Antarctic lakes, where we found a wildfire signature is preserved in the deep, ancient brines of Antarctic lake bottom waters. In contrast, the surface waters are enriched in BC from fossil fuels.  The second also utilizes the DBC molecular marker technique to explore DBC concentration and composition across a global sample set from the cryosphere. Here we show the bottom waters of Antarctic lakes are surprisingly much more enriched in DBC compared to other regions of the cryosphere. Also, the DBC compositions from samples collected on the Greenland Ice Sheet are distinct from the rest of the dataset, containing a wildfire signature. The third and fourth studies utilize the Single Particle Soot Photometer to measure refractory black carbon (rBC).  The third study also applies spectral albedo measurements and the light absorption heating method to show that coal dust from an active mine in Svalbard, Norway significantly reduces the spectral reflectance of the surrounding Arctic surface snow. The fourth study reports aerosol rBC concentrations in the boundary layer of the McMurdo Dry Valleys, as well as in snow from the accumulation area of the Commonwealth Glacier.  Here we determine that aerosol concentrations increase during katabatic wind events, but there is no significant trend in deposition in the snow pit.

These findings support the importance of real in-situ observations in order to fully understand the role of BC in the global carbon cycle. Also, these ground-based measurements will likely serve as validation for future remote sensing of snow/ice impurities and LAAs deposition models.