Ian Delaney, Central Washington University Master’s research. Recipient of MBVRC research grant, 2013.
Black carbon (also called soot, from the incomplete combustion of fossil and biofuels) deposition on snow and ice darkens the surface of glaciers and snowpack, reducing albedo or reflectivity, causing additional absorption of solar radiation by the snowpack thus accelerating snowmelt and changing the timing of runoff. This is particularly important in Washington State and Mount Baker, as glaciers and seasonal snowpack have shrunk considerably in recent years and are integral to the region’s water resources. Little data exists regarding the concentration of black carbon in Mount Baker’s snow, necessary to determine if enough black carbon is present to substantially accelerate snowmelt. To obtain this data, snow samples were collected in the early and late season (June 8th and September 12th, 2013) from the Boulder and Easton glaciers of Mount Baker and analyzed for black carbon. Analysis of this data suggests that black carbon concentrations are quite low during the early season, but increase considerably during the later summer- enough to reduce albedo by up to 21%. Increases in black carbon concentration found in the snowpack later in the season coincide with increased atmospheric concentrations of black carbon. Also during the later part of the season, large amounts of runoff come from glacial melt. As a result, black carbon contributes to glacier melt late in the summer season, as opposed to melt of the seasonal snow earlier in the season. As black carbon contributes to albedo reduction and accelerated snowmelt, future work is needed to determine if black carbon in the region comes from anthropogenic activity or natural processes such as forest fires. Should large amounts of black carbon come from anthropogenic activities, efforts to reduce regional emissions, can improve the state of the regions water resources and glacial environments.
Mount Baker glaciers lost 12% to 20% of their volume from 1990 to 2010, resulting in the terminus of glaciers on the mountain retreating up to 520 m from the mid 1980’s [Pelto and Brown, 2012]. This trend of glacial retreat is evident through out the Cascades [Riedel and Larrabee, 2011] and is also correlated with reductions in the region’s seasonal snowpack [Mote et al., 2005]. As Washington’s water resources during the dry summer months are dependent on snow and glacial melt, retreating glaciers could greatly impact the regions hydrologic regime [Elsner et al., 2010; Granshaw and Fountain, 2006; Vano et al., 2010]. Although temperature increases in the region have been documented and contribute to snowpack reduction [Mote, 2003], the contribution of light absorbing impurities such as black carbon (also called soot, from the incomplete combustion of fossil and biofuels) and dust has not been extensively documented. Light absorbing impurity deposition on snow and ice darkens the surface of glaciers and snowpack, reducing albedo or reflectivity, causing radiation absorbed by black carbon in the snowpack to accelerate snowmelt and even change the timing of runoff [Painter et al., 2007; Warren and Wiscombe, 1980]. This research was focused on black carbon deposition on Mount Baker’s snowpack as it may originate from anthropogenic sources [Ramanathan and Carmichael, 2008].
In order to assess the concentrations of black carbon in the snow on Mount Baker, as well as seasonal changes and spatial variability, snow samples from surface snow were collected from the Boulder and Easton glaciers. The Boulder glacier was sampled on June 8th, 2013, while the Easton glacier was sampled on September 12th, 2013. Samples were collected in 50 mL polypropylene vials from a variety of elevations along the glacier. Unfortunately, samples collected on June 8th melted during their transportation from the field, causing substantial underestimation of black carbon concentration using our analytical procedure. Additionally, due to weather and avalanche conditions sampling from high points on the mountain (above roughly 2700m) was not possible during the June 8th sampling event. In late September 2013 samples were analyzed for black carbon concentration using the Single Particle Soot Photometer housed at Central Washington University. This method for determining black carbon concentration is minimally affected by the presence of other impurities such as dust [Schwarz et al., 2012], which can create uncertainties in other methods.
As found in samples collected during the previous summer (2012) black carbon concentrations were far lower during the early summer (June 8th sampling event) compared to the later season sampling event. However, the samples collected on June 8th underestimate black carbon concentrations substantially due to premature melt during transport from the field. Concentrations from June 8 averaged 4.85 μg/L, while the concentrations from September averaged 89.57 μg/L, with a maximum of 320.39 μg/L, showing a large increase over the course of the summer (Figure 1). Elevated concentrations in the snowpack are coincident with increased atmospheric concentrations of black carbon [Jenkins, 2011], and snowmelt, which can concentrate black carbon on the surface of the snowpack [Conway et al., 1996]. Additionally, the average BC concentration found in September could reduce albedo between 14 and 21%, a substantial reduction, which could affect snowmelt [Yasunari et al., 2010].
Black carbon concentrations in samples collected during the spring and summer of 2012 show an elevational gradient early in the season (April to June, 2012) with lower black carbon concentrations at higher elevations. However, samples collected from a variety of locations later in season (July to September, 2012), do not show the same gradient (Figure 2). We attributed the disappearance of the gradient to an elevated planetary boundary layer during the summer season exposes higher elevation snow to black carbon [Venzac et al., 2009]. The planetary boundary layer is the lowest level of the atmosphere, over which air moves turbulently and is responsible for the transportation of local pollutants, such as black carbon. However, during the 2012 field season, the elevation ranges over which the samples could be collected were limited and the assessment relied on data collected over a variety of glaciers covering a prolonged period of time, thus poorly constraining temporal and spatial variation and thoroughly assessing the elevational gradient. Samples collected in September from the Easton Glacier provided an opportunity to examine the potential role of the elevated planetary boundary layer in destroying the elevational gradient evident earlier in the season (seen April to June, 2012). Samples collected between 2828 m and 1912 m showed a poor relationship between black carbon concentration and elevation. This observation is consistent with our hypothesis and the data collected during 2012. However, as data collected during 2013 is of a far higher quality as it is well constrained temporally and spatially.
Because black carbon concentrations peak later in the summer and are low during the early spring, its effect on snowmelt is greatest during the late season when glacier melt takes place, thus contributing to glacial retreat. During this time, water resources in the region also rely on snow and glacial melt [Elsner et al., 2010], and would be greatly affected by reduced late season, high alpine snow and disappearance of glaciers [Granshaw and Fountain, 2006]. To compound the effects of higher black carbon concentration over this period of time, the black carbon can also reach the high alpine snow in more considerable quantities (likely due to the elevated planetary boundary layer) during the later season, compared to the early season.
As snow albedo reduction due to black carbon likely contributes to accelerated snowmelt on Mount Baker during the later season, future work needs to assess the sources of black carbon (natural or anthropogenic) and better constrain its effect on regional hydrology. Should large amounts of black carbon on Mount Baker’s snow originate from anthropogenic sources, controls on regional emissions could reduce glacial retreat and help maintain the current hydrologic regime. Thank you to the Mount Baker Volcano Research Center for supporting this project and contributing to the better understanding of the mountain’s glaciers and their melt.
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