Final Report: BC and Other Light-Absorbing Impurities in North American Great Plains Snow: Sources, Impacts, and a Comparison with North China Snow

EPA Grant Number: R835038
Title: BC and Other Light-Absorbing Impurities in North American Great Plains Snow: Sources, Impacts, and a Comparison with North China Snow
Investigators: Doherty, Sarah , Fu, Qiang , Hegg, Dean A. , Warren, Stephen
Institution: University of Washington
EPA Project Officer: Wilson, Wil
Project Period: July 1, 2011 through June 30, 2014 (Extended to June 30, 2015)
Project Amount: $825,483
RFA: Black Carbon's Role In Global To Local Scale Climate And Air Quality (2010) RFA Text |  Recipients Lists
Research Category: Global Climate Change , Climate Change , Air

Objective:

Model studies indicate that black carbon (BC) in snow may be responsible for a significant fraction of northern hemisphere warming due to its ability to lower snow’s very high albedo and because of the feedback processes that follow, which further reduce surface albedo. The highest concentrations of BC in snow are expected at the northern mid-latitudes, but the climate impacts of snow BC at these latitudes are relatively under-studied. Areas where the surface snow is not masked by tree cover (e.g., open plains) are especially susceptible to the reduction of planetary albedo. Further, model-observation comparisons indicate that model deposition rates are the largest source of uncertainty in atmospheric BC distributions. Measurements show that light-absorbing aerosols (LAAs) other than BC, such as mineral dust, soil, and organic carbon co-emitted with BC, also may play a significant role in snow albedo reductions; therefore, constraining their contributions to snow albedo reduction also is important. 

Objectives:

  1. Investigate the concentrations, sources and regional climate impacts of BC and other LAAs in snow in the North American Great Plains.
  2. Compare the concentrations, sources and regional climate impacts of LAAs in snow for the North American Great Plains vs. the steppes of North Asia.
  3. Improve our understanding of (a) the deposition rates of BC to snow, which affects both atmospheric and snowpack BC concentrations, and (b) consolidation of BC and other LAAs at the snowpack surface during melting, a potentially strong positive feedback mechanism.
  4. Test and improve our ability to measure BC and other LAAs in snow by conducting a comparison of three methods for measuring BC: our ISSW spectrophotometer, the single particle soot photometer, and the thermo-optical method.
  5. Use snow BC concentrations extending from the northern United States to the North Pole to make a first-order estimate of the contribution by North American sources to BC in Arctic snow. 

Summary/Accomplishments (Outputs/Outcomes):

Approach

This project included field sampling and analysis of snow samples for the mixing ratios and sources of BC and other light-absorbing particles in snow from broad surveys across central North America and northern China. Mixing ratios and sources of BC and other LAAs were estimated using an instrument (the ISSW spectrophotometer; Grenfell, et al., 2010) that measures spectral absorption. The absorption Ångström exponent is used to apportion absorption to BC and non-BC components, allow an estimate of BC mass. A subset of the snow samples also were analyzed for chemical compostion, and the combined chemical and ISSW data were input to a Positive Matrix Factorization (PMF) analysis of the sources of BC and other LAAs to snow. The same collection and analysis techniques as had been applied in an earlier survey of the Arctic were used (Doherty, et al., 2010), making the data sets amenable to direct comparison. Data from these surveys are being used by climate modelers to test model representation of BC and (in some cases) dust in snow; two such comparisons were supported by this grant.

Two focused studies quantifying the impact of processes following wet deposition were conducted. One analyzed the effect of melt on BC and other LAAs from samples collected in the percolation zone of Greenland and on seasonal snow near Barrow, Alaska. In a second study, we analyzed samples collected regularly over 2-3 months at three sites in Idaho and one site in north-central Utah. We are quantifying how the concentrations and sources of LAAs were affected by dry deposition, sublimation and melt.

In addition, we tested our method of estimating the mixing ratio specifically of BC in snow. Our estimates are based on measuring light absorption, attributing the absorption to BC and non-BC components, and converting absorption to mass. This method was tested against more direct measures of BC in liquid and snow samples. 

Summary of Accomplishments (Outputs/Outcomes)

  • North China snow survey: We analyzed samples collected by our colleagues at Lanzhou University (China) in January and February 2010 from north-central and north-east China. A post-doc from Lanzhou University visited our group to collaborate on this analysis. The lowest concentrations of BC were in the remote northeast on the border of Siberia, with a median concentration in surface snow of 117 ng/g. South of this, in the industrial northeast, the median snow BC concentration was 1220 ng/g. In the northeast, snow particulate light absorption was dominated by BC. Across the grassland of Inner Mongolia, OC, most likely from local soil, dominates light absorption, with median BC concentrations of 340 ng/g responsible for only about one-third of total particulate light absorption. In the Qilian Mountains, at the northern boundary of the Tibetan Plateau, snow particulate light absorption is dominated by local soil and desert dust. A preliminary comparison of snow BC concentrations with that estimated in one climate model show large differences at any given sampling site, but concentrations are generally consistent in terms of latitudinal and meridional gradients and in their overall range across north China (see Wang, et al., 2013). 
  • 2013 western North America snow survey:  More than 500 samples were collected from 67 sites from January 28 - March 21, 2013. Vertical profiles were collected across the northwest United States and the U.S. and Canadian Great Plains regions. Our northern-most site was Churchill, Manitoba on Hudson Bay. Sites in Canada tended to have the lowest BC mixing ratios (typically ~5-35 ng/g; surface snow median 15 ng/g), with somewhat higher mixing ratios in the Pacific Northwest (typically ~5-40 ng/g; surface snow median 22 ng/g) and Intra-mountain Northwest (typically 10-50 ng/g; surface snow median 24 ng/g). The Northern U.S. Plains sites, directly south of the Canadian sites, were the dirtiest, with BC mixing ratios typically ~15-70 ng/g (surface snow median 30 ng/g); multiple sample layers in this region had >100 ng/g BC in snow. The leading sources of snow particulate light absorption were soil, biomass burning and pollution. Here, “biomass burning” includes all biomass combustion, including biofuels such as wood stove smoke, and the “pollution” particulate absorption is essentially due to fossil fuel combustion aerosol. For some Intra-mountain Northwest sites up to one-half of absorption was due to non-BC components, and for many of the Northern U.S. Plains sites, 50-100% of absorption was due to non-BC components. Chemical and PMF analysis indicates that the non-BC absorption for most of these sites was by soil. In Canada, non-BC absorption varied 10%-60%, and was positively correlated with the fraction of absorption allocated to soil in the PMF analysis (see Doherty, et al., 2014).
  • Quantifying increased concentrations of particulate light absorbers with surface snow melt: Under this grant we analyzed samples collected during two field campaigns funded under another grant, with the goal of studying how black carbon and other light-absorbing particles are redistributed in the snowpack with melt. Previous anecdotal observations indicate that black carbon and other particles tend to remain at the snow surface once the snow starts to melt, because they are scavenged with <100% efficiency with melt water. Modeling studies indicate this may be an important positive feedback to the positive radiative forcing resulting from BC in snow. Our study shows that only about 10-30% of black carbon and other light-absorbing particles is washed down through the snowpack with melted snow water. This is very consistent with values used in one leading model (Flanner, et al., 2007; Flanner, et al., 2009). The snow studied did not have significant amounts of coarse soil/dust. However, we expect coarse particles to have a lower scavenging efficiency than the smaller combustion particles that dominated absorption in our snow samples; if so, the positive feedback with melting would be even stronger for dust than for BC (see Doherty, et al., 2013).
  • Quantifying the roles of wet deposition vs. dry deposition and other in-snow processes in the mixing ratios of BC and other LAAs in snow: From January 27 - March 25, 2014, we made repeat measurements at three sites on a north-south transect between McCall and Boise, Idaho.  Sample profiles were gathered every few days to monitor 1) variations in the particulate content of newly fallen snow at a given location; 2) post-deposition evolution of the snowpack and its particulate content; and 3) how these two factors varied at the three sites.  In total, 320 samples were gathered.  In addition, we analyzed samples collected by colleagues January 28 - February 21, 2013 and January 17 - February 13, 2014, at a site south of Vernal, Utah. Spatial and temporal variability of BC and other absorbers in snow was quantified for these samples. Spatial variability in the concentrations of particulate absorbers in snow at 1 m and 10 m scales was comparable and was in good agreement with that found in our previous studies: about 30%. In contrast, temporal variability at a given site was much larger. At the Utah site, most of this variability was driven by periodic significant deposition of dust to the snowpack, both via dry deposition and in new snowfall, alternately with cleaner snowfall events. At the Idaho sites, newly fallen snow was generally cleaner than aged snow and most of the variability at a given site was driven by dry deposition and post-depositional processes in the snowpack following snowfall. At both sites, these processes categorically led to increased mixing ratios of BC and other light-absorbing particles (i.e., BrC, dust and soil organics). About a factor of 2.5-3 increase in total particulate absorption can be attributed to a combination of the sublimation of snow water, dry deposition of aerosol that has the same absorption Ångström exponent (i.e., likely the same composition) as the wet-deposited aerosol, and melt scavenging that does not preferentially remove one absorbing component over another. Dry deposition of, in particular, soil, enhancement of surface particulate mixing ratios with melt, and preferential scavenging of combustion particles (versus soil/dust) from the snow with meltwater were responsible for approximately an order of magnitude increase in total particulate absorption. At two of the three Idaho sites and at the Utah site dust and soil were significant contributors to total particulate light absorption in the snowpack. Both of the Idaho mountain valleys were predominantly farmland but the farms are dormant in winter and the valleys are surrounded by tree-covered hillsides, so this finding was somewhat surprising. This source of light-absorbing particles to snow is very likely not accounted for in global and regional models, which generally only account for combustion sources and dust from desert regions (see Doherty, et al., 2015, nearing submission to JGR-Atmospheres).
  • Organics’ contributions to absorption determined via serial extractions: For a sub-set of our 2013 field samples, we extracted different organic components using a range of solvents, measuring spectral light absorption after each extraction step. This was combined with estimates of absorption by iron oxides to estimate total non-BC absorption. On average, humic-like substances (sodium hydroxide [NaOH]-soluble), polar OCs (methanol-soluble) and iron oxides were responsible for 9%, 4%, and 14%, respectively, of 300-750 nm absorption by snow particulate (with a great deal of variance about the means). The light absorption due to non-BC particulate components estimated by chemical methods was about 10% lower than that estimated by optical methods alone. We show that physically realistic, reasonable changes in the assumed absorption Ångström exponent of the BC and the non-BC particulate components in the optical analysis allow us to reach agreement between the chemically-determined and optically-determined estimates of non-BC absorption (see Dang and Hegg, 2014).
  • Test of our estimates of BC in snow: The ISSW spectrophotometer we use to measure light-absorbing particles in snow is a unique instrument that measures spectrally-resolved light absorption by particles deposited onto a filter. Estimates of BC are based on apportioning measured absorption to BC and non-BC absorbers, then conversion of absorption to BC loading on the filter. In collaboration with colleagues at NOAA, we tested the ISSW estimate of the mixing ratios of BC in snow. A first round of tests was conducted for solutions containing gravimetrically determined mixes of a BC standard (fullerene soot), a dust standard, and non-absorbing polystyrene latex spheres (PSLs). ISSW estimates of BC mixing ratios were quite good for pure fullerene, but they were biased high when the BC was mixed with the dust standard. In a second set of tests, a set of newly fallen snow samples (from January - February 2014 in central Idaho) were analyzed for BC with the ISSW and, by our colleagues at NOAA, with an SP2. These samples likely contained mostly combustion-sourced particles. A similar high bias was found. In both cases the bias was correlated with both the concentration of particles and with filter loading (because those two are correlated). A loading-based correction factor was generated and applied to the 2014 Idaho data and the 2013/2014 Utah data (see Schwarz, et al., 2012 and Doherty, et al., 2015, nearing submission to JGR-Atmospheres).
  • Test of BC in snow in one global model: In a collaborative effort with colleagues at the Pacific Northwest National Laboratory we used the estimates of the mixing ratio of BC in snow from our broad-area survey of January - March 2013 across central North America to test modeled BC in snow in the CESM global climate model. We find that the model has a significant low bias in predicted mixing ratios of BC in snow but only a small low bias in predicted atmospheric concentrations over the Northwest United States and West Canada. This is largely attributed to a low bias in biomass burning emissions in the model. The model also is missing soil–and BC in soil–as a source of BC and other LAAs to the snowpack (see Zhang, et al., 2015).
  • Trends in BC and other LAAs in snow on sea ice north of Greenland: As part of a multi-year study, samples of snow from the sea ice north of Greenland collected in spring 2013 also were analyzed. Samples from this region had been collected and analyzed for each spring 2008-2013. We found that over this period there were significant spatial and inter-annual variations, but there was no significant trend in the mixing ratios of BC and other particulate absorbers in the snow in this region (see Doherty, et al., 2015).

References:

Grenfell, T. C., S. J. Doherty, A. D. Clarke, and S. G. Warren (2011), Light absorption from particulate impurities in snow and ice determined by spectrophotometric analysis of filters, Appl. Opt., 50(14), 2037–2048. 

Doherty, S. J., S. G. Warren, T. C. Grenfell, A. D. Clarke, and R. E. Brandt (2010), Light-absorbing impurities in Arctic snow, Atmos. Chem. Phys., 10(23), 11,647–11,680. 

Flanner, M. G., Zender, C. S., Randerson, J. T., and Rasch, P. J (2007), Present-day climate forcing and response from black carbon in snow, Journal of Geo. Res., 112(D11). DOI: 10.1029/2006JD008003 

Flanner, M. G. (2009), Integrating anthropogenic heat flux with global climate models, Geo. Res. Letters, 36(2). 


Journal Articles on this Report : 10 Displayed | Download in RIS Format

Other project views: All 10 publications 10 publications in selected types All 10 journal articles
Type Citation Project Document Sources
Journal Article Dang C, Hegg DA. Quantifying light absorption by organic carbon in Western North American snow by serial chemical extractions. Journal of Geophysical Research-Atmospheres 2014;119(17):10247-10261. R835038 (2014)
R835038 (Final)
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  • Other: University of Washington-Full Text PDF
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  • Journal Article Doherty SJ, Grenfell TC, Forsstrom S, Hegg DL, Brandt RE, Warren SG. Observed vertical redistribution of black carbon and other insoluble light-absorbing particles in melting snow. Journal of Geophysical Research–Atmospheres 2013;118(11):5553-5569. R835038 (2012)
    R835038 (2013)
    R835038 (Final)
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  • Journal Article Doherty SJ, Dang C, Hegg DA, Zhang R, Warren SG. Black carbon and other light-absorbing particles in snow of central North America.Journal of Geophysical Research-Atmospheres 2014;119(22):12807-12831. R835038 (2014)
    R835038 (Final)
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  • Journal Article Doherty SJ, Hegg DA, Johnson JE, Quinn PK, Schwarz JP, Dang C, Warren SG. Causes of variability in light absorption by particles in snow at sites in Idaho and Utah. Journal of Geophysical Research-Atmospheres 2016;121(9):4751-4768. R835038 (Final)
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  • Journal Article Qian Y, Yasunari TJ, Doherty SJ, Flanner MG, Lau WKM, Ming J, Wang H, Wang M, Warren SG, Zhang R. Light-absorbing particles in snow and ice: measurement and modeling of climatic and hydrological impact. Advances in Atmospheric Sciences 2015;32(1):64-91. R835038 (2014)
    R835038 (Final)
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  • Journal Article Schwarz JP, Doherty SJ, Li F, Ruggiero ST, Tanner CE, Perring AE, Gao RS, Fahey DW. Assessing single particle soot photometer and integrating sphere/integrating sandwich spectrophotometer measurement techniques for quantifying black carbon concentration in snow. Atmospheric Measurement Techniques 2012;5(11):2581-2592. R835038 (2012)
    R835038 (2013)
    R835038 (Final)
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  • Journal Article Wang X, Doherty SJ, Huang J. Black carbon and other light-absorbing impurities in snow across Northern China. Journal of Geophysical Research–Atmospheres 2013;118(3):1471-1492. R835038 (2012)
    R835038 (2013)
    R835038 (Final)
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  • Journal Article Zhang R, Hegg DA, Huang J, Fu Q. Source attribution of insoluble light-absorbing particles in seasonal snow across Northern China. Atmospheric Chemistry and Physics 2013;13(12):6091-6099. R835038 (2013)
    R835038 (Final)
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  • Journal Article Zhang R, Wang H, Hegg DA, Qian Y, Doherty SJ, Dang C, Ma P-L, Rasch PJ, Fu Q. Quantifying sources of black carbon in western North America using observationally based analysis and an emission tagging technique in the Community Atmosphere Model. Atmospheric Chemistry and Physics 2015;15(22):12805-12822. R835038 (Final)
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  • Journal Article Doherty SJ, Steele M, Rigor I, Warren SG. Interannual variations of light-absorbing particles in snow on Arctic sea ice. Journal of Geophysical Research-Atmospheres 2015;120(21):11391-11400. R835038 (Final)
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