2014 Progress 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: Hunt, Sherri
Project Period: July 1, 2011 through June 30, 2014 (Extended to June 30, 2015)
Project Period Covered by this Report: July 1, 2013 through August 31,2014
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


  1. Investigate the concentrations, sources and regional climate impacts of BC and other light-absorbing aerosols (LAAs) in snow in the North American Great Plains. 
  2. Compare the concentrations, sources and regional climate impacts of LAA 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. 

Progress Summary:

Analysis of the 2013 field season samples was completed, and a paper was submitted to Journal of Geophysical Research summarizing the results. 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. PMF analysis found three leading sources of snow particulate light absorption: Soil, Biomass Burning and Pollution. Here, “biomass burning” includes all biomass combustion, including, for example, 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 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. Thus, even in regions far from deserts, dust/soil can play a significant or dominant role in snow particulate absorption.

Serial chemical extraction of sub-groups of organic carbon (OC) on a sub-set of the samples also was completed, and the analysis reported in a paper by Dang and Hegg (2014). The results suggest that humic-like substances (NaOH-soluble), polar OCs (methanol-soluble), and iron oxides are responsible for 9%, 4% and 14% (sample means) of the total visible light absorption, respectively, in our samples. The fraction of light absorption due to non-BC light absorbing particles (LAPs) estimated by the chemical methods is about 10% lower than that estimated from optical analysis alone, averaged across all sampling regions. 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.

A second set of field measurements was conducted January 27 to March 25, 2014. This year, all fieldwork was in central Idaho, making 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.  ISSW (optical) and chemical analysis of the samples is complete, as is an initial PMF analysis. We currently are jointly analyzing these three analyses and working on writing up the results. 

Future Activities:

  • Analysis of the soil samples from the three 2014 sites for the spectral “fingerprint” of their absorption. The results then will be used, along with the PMF analysis results, to determine how best to separate BC and non-BC absorption in the ISSW measurements of the samples from the 2014 field season. We then will re-process 2014 field sample ISSW measurements accordingly.
  • Jointly analyzing the ISSW (optical), chemical data and Positive Matrix Factorization (PMF) output for 2014 data.
  • Publish results from 2014 field samples – likely combined with analysis of samples gathered by colleagues also in 2014 in Vernal, Utah, also at regular intervals in one location.
  • Finalize 2013 Summary paper, currently in revision stage with Journal of Geophysical Research.
  • In collaboration with our colleagues at NOAA, we will compare the ISSW and SP2 estimates of snow BC content for the sub-set of samples gathered during the 2013 and 2014 field campaigns for this purpose. Previous comparisons (reported by Schwarz, et al., 2012) indicated the ISSW has a high bias in estimated BC when the snow contains a mix of BC and dust, but this was for samples generated using synthetic BC and dust. Initial comparisons of the 2013 samples also show significantly lower mixing ratios of BC in snow from the SP2 than from the ISSW. However, the samples included in this comparison were generally very heavily loaded with dust/soil. This can lead to high biases in estimated BC with the ISSW (per Schwarz, et al., 2012) and likely low biases in the SP2 measurements. The latter is because the BC may stick to the dirt/soil particles, which do not make it from the water sample into the SP2 detection chamber. We still are working on understanding this. The 2014 samples were specifically chosen to capture for newly fallen snow, where snow particulates are dominated by BC and brown carbon from fossil fuel and biomass combustion. We expect this will allow for a more well-constrained comparison of the ISSW and SP2.
  • Graduate Student Cheng Dang will spend October and November 2014 as a visiting scientist at Pacific Northwest National Laboratory (PNNL) in Richland, WA. She will be trained in regional climate modeling (WRF-Chem) and will run the model with the goal of identifying the sources of model biases. She will bring to PNNL her in-depth understanding of our data set, facilitating comparisons between our field measurements and both their regional and global (CESM) model runs. 

Journal Articles on this Report : 3 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)
  • Full-text: Wiley-Full Text PDF
  • Abstract: Wiley-Abstract & Full Text HTML
  • Other: University of Washington-Full Text PDF
  • 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)
  • Full-text: Wiley-Full Text PDF
  • Abstract: Wiley-Abstract & Full Text HTML
  • Other: ResearchGate-Full Text PDF
  • 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)
  • Full-text: ResearchGate-Full Text PDF
  • Abstract: SpringerLink-Abstract
  • Supplemental Keywords:

    black carbon, climate, cryosphere, radiative forcing, light absorbing particles

    Relevant Websites:

    http://www.atmos.washington.edu/sootinsnow/ Exit
    http://www.atmos.washington.edu/~sarahd/ Exit

    Progress and Final Reports:

    Original Abstract
  • 2012 Progress Report
  • 2013 Progress Report
  • Final Report