Light-absorbing particulates in seasonal snow in western North America

Thesis (Master's)--University of Washington, 2014Commonly found light-absorbing particulates (LAPs) in snow are black carbon (BC), organic carbon (OC), and mineral dust (MD). These LAPs can reduce the very high albedo of snowpack and trigger positive feedback processes, eventually accelerate the snowmelt and hence influence the climate and hydrology. From the January to March of 2013, a field campaign was conducted to study the LAPs in seasonal snow across 13 American states and 3 Canadian Provinces in western North America. We collected and filtered more than 600 snow samples from 67 sites to extract the water-insoluble LAPs in snow, and saved melted snow samples. More than 500 LAP nuclepore samples were analyzed in a spectrophotometer to estimate the light absorption due to LAP samples. This optical analysis also allow us to calculate the absorption Ångström exponent (Å) of LAPs, estimate the BC mixing ratio, and partition the light absorption by BC and non-BC LAPs. About 100 LAP GHP samples were extracted by a serial of chemical solvents to remove OC; then measured in the spectrophotometer to estimate the light absorption changes. The iron concentration was derived from ICP-MS (Inductively Coupled Plasma - Mass Spectroscopy), and was used to estimate the light absorption due to MD. The BC mixing ratio varies from 4 - 510 ng/g (ng of BC/g of snow), with regional medians vary from 14 ng/g in the Pacific Northwest to 65 ng/g in the Northern Plains. This amount of BC is lower than that found in China, and the LAP in the cleanest sites is as low as that found in the Arctic snow. The regional medians of Å vary from 1.6 to 2.6, indicating that BC is not the only LAP in snow. Chemical extractions suggest that methanol-soluble OC (polar OC) and base-soluble HULIS are responsible for 3% and 8% of light absorption by all LAP respectively. They are likely generated from biomass burning or soil. The fractional light absorption produced by OC and HULIS in the Northern Plains is a factor of two higher than that of the other regions. No non-polar OC was found to contributing to light absorption. Based on the analysis of iron, on average 14% of the light absorption is produced by MD. The optical-estimated light absorption due to non-BC is in agrees with the sum of fractional absorption due to OC and MD across sampling regions