Black carbon and other light-absorbing particulates in seasonal snow in western North America

Small amounts of light-absorbing particulates (LAPs) in snow can decrease the surface albedo, especially at visible and ultraviolet wavelengths. Commonly found LAPs in snow are black carbon (BC), organic carbon (OC), and iron oxides (hematite and goethite). Their presence reduces the albedo of snow and triggers positive feedback processes; both of these advance the melting of snowpack and hence influence the climate and hydrological cycle.

A large-area geographical survey of LAPs in snow was conducted in western North America from January to March of 2013. We collected more than 600 snow samples from 67 sites across 13 American states and 3 Canadian provinces, spanning from Nevada to Manitoba. Then we melted and filtered these samples. The filters were analyzed with a spectrophotometer to estimate the BC mixing ratio and (from the absorption Angstrom exponent) the fraction of the light absorption by BC and non-BC LAPs. In addition, chemical analyses of the samples were used to obtain independent estimates of the light absorption due to OC and to iron oxides, and to subdivide the OC absorption into contributions by different types of OC.

The BC mixing ratio of most samples ranges from 2 to 50 ng BC/g snow, with the following regional medians: Pacific Northwest 19, Rocky Mountains 23, Northern Plains 36 and Canada 17. Using the optical analysis alone, we estimate that, in these regions, BC accounts for 82%, 78%, 60% and 70% respectively of total particulate light absorption in the snowpack. The remainder is due to non-BC LAPs (mostly OC and iron oxides). The fraction of light absorption due to non-BC estimated from the optical analysis is generally ~10% higher than that quantified by the chemical analysis.

The mixing ratio of BC in snow of western North America is much lower than was found in Northern China, where it ranged from 100 to 1200 ng/g (Wang, Doherty, and Huang, 2013). Snow particulate light absorption in Inner Mongolia and the Qilian Mountains was dominated by non-BC LAPs. In those regions, the snow was thin and patchy, so the soil was exposed in places; the non-BC LAPs in those regions were likely iron oxides and OC from local soil. The snow was also thin and patchy at some of our sites in North and South Dakota, and here local soil also appears to play a dominant role in snow particulate light absorption. However, if the snow is patchy, the decrease of albedo caused by LAPs plays a smaller role in modulating the surface radiation budget, as the surface albedo is already low due to the exposure of darker underlying ground.