Abstract
Abstract. The Community Atmosphere Model (CAM5), equipped with a technique to tag black carbon (BC) emissions by source regions and types, has been employed to establish source–receptor relationships for atmospheric BC and its deposition to snow over western North America. The CAM5 simulation was conducted with meteorological fields constrained by reanalysis for year 2013 when measurements of BC in both near-surface air and snow are available for model evaluation. We find that CAM5 has a significant low bias in predicted mixing ratios of BC in snow but only a small low bias in predicted atmospheric concentrations over northwestern USA and western Canada. Even with a strong low bias in snow mixing ratios, radiative transfer calculations show that the BC-in-snow darkening effect is substantially larger than the BC dimming effect at the surface by atmospheric BC. Local sources contribute more to near-surface atmospheric BC and to deposition than distant sources, while the latter are more important in the middle and upper troposphere where wet removal is relatively weak. Fossil fuel (FF) is the dominant source type for total column BC burden over the two regions. FF is also the dominant local source type for BC column burden, deposition, and near-surface BC, while for all distant source regions combined the contribution of biomass/biofuel (BB) is larger than FF. An observationally based positive matrix factorization (PMF) analysis of the snow-impurity chemistry is conducted to quantitatively evaluate the CAM5 BC source-type attribution. While CAM5 is qualitatively consistent with the PMF analysis with respect to partitioning of BC originating from BB and FF emissions, it significantly underestimates the relative contribution of BB. In addition to a possible low bias in BB emissions used in the simulation, the model is likely missing a significant source of snow darkening from local soil found in the observations.
Highlights
Black carbon (BC) is the most light-absorbing component of anthropogenic aerosols, and it has been assessed to be responsible for a significant fraction of the climate warming in the Northern Hemisphere (Bond et al, 2013)
The Community Atmosphere Model version 5 (CAM5) global model, implemented with an explicit BC source tagging technique, has been employed to establish source–receptor relationships for atmospheric BC and its deposition to snow over a large receptor area encompassing a substantial portion of the Great Plains of North America
We found that CAM5 had a small low bias (11 %) but a substantial random error in the estimates of monthly-mean near-surface atmospheric BC concentrations
Summary
Black carbon (BC) is the most light-absorbing component of anthropogenic aerosols, and it has been assessed to be responsible for a significant fraction of the climate warming in the Northern Hemisphere (Bond et al, 2013). R. Zhang et al.: Quantifying sources of black carbon in western North America surface, thereby increasing absorbed solar radiation (Flanner et al, 2007, 2009). Zhang et al.: Quantifying sources of black carbon in western North America surface, thereby increasing absorbed solar radiation (Flanner et al, 2007, 2009) The latter effect is of special interest due to the strong positive feedbacks it can trigger (e.g., Hansen and Nazarenko, 2004; Flanner et al, 2007; Bond et al, 2013). Some recent studies (e.g., Flanner et al, 2009; Shindell and Faluvegi, 2009; Bond et al, 2013) have pointed out that the climatic effect of BC might be greater at mid-latitudes, a relatively understudied region, from the standpoint of global mean forcing
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