Quantifying Sources,Transport,Deposition, And Radiative Forcing Of Black Carbon In The Northern Hemisphere Mid-latitudes

Black carbon (BC) is a distinct type of carbonaceous particulate matter mainly emitted from the incomplete combustion of fossil fuels, biofuels and biomass burning. It is the dominant light-absorbing particles, both in the atmosphere and deposited on snow/ice. In addition to its impact on air quality, BC plays a unique and important role in the climate system through its effect on radiation, clouds and snow-albedo, and associated feedbacks that modify atmospheric circulation patterns and/or accelerate the snowmelt and glacier retreat both in the Arctic and across the mid-latitudes of the Northern Hemisphere. An important aspect of the BC-climate connection is the source attribution of BC in the atmosphere and deposited on snow/ice. In this thesis, I introduce three epic surveys of light-absorbing particles in snow, which have been conducted across Northern China and North America during three winters (in 2010,2012 and 2013), and the studies using a receptor model along with observational analyses and a state-of-the-science global climate model equipped with a novel explicit source tagging technique to characterize the fate of BC particles emitted from various geographical regions and sectors and their transport pathways to receptors in the understudied mid-latitude regions, including Northern China, the Himalayas, Tibetan Plateau, and North America.In Chapter 3, seasonal snow samples obtained at 46 sites in 6 provinces of China in January and February of 2010 were analyzed for a suite of chemical species, and these data are combined with previously determined concentrations of insoluble light-absorbing particles (ILAP) to use as input to a Positive Matrix Factorization (PMF) receptor model to explore the sources of ILAP in the snow. The PMF analysis for ILAP sources is augmented with backward trajectory cluster analysis and the geographic locations of major source areas for the three source types. The two analyses consistently indicate that three factors/sources were responsible for the measured light absorption of snow: a soil dust source, an industrial pollution source, and a biomass and/or biofuel burning source. Soil dust was the main source of the ILAP in snow over Northern China, accounting for 53% of ILAP on average.In Chapter 4, we use the Community Atmosphere Model version 5 (CAM5) with a newly developed source-tagging technique to characterize the transport of BC particles originating from various geographical regions and sectors to the Himalayas and Tibetan Plateau (HTP) and estimate the resultant radiative forcing. The results show that the relative contributions from different geographical regions and source sectors depend on seasons and the locations in the HTP. The largest contribution to annual mean BC burden and surface deposition in the entire HTP region is from biofuel and biomass (BB) emissions in South Asia, followed by fossil fuel (FF) emissions from South Asia, then FF from East Asia. The same roles hold for all the seasonal means except for the summer when East Asia FF becomes more important. For finer receptor regions of interest, South Asia BB and FF have the largest impact on BC in Himalayas and Central Tibetan Plateau, while East Asia FF and BB contribute the most to Northeast Plateau in all seasons and Southeast Plateau in the summer. Central Asia and Middle East FF emissions have relatively more important contributions to BC reaching Northwest Plateau, especially in the summer. Although local emissions only contribute about 10% of BC in the HTP, this contribution is extremely sensitive to local emission changes. Lastly, we show that the annual mean radiative forcing (0.42 W m-2) due to BC in snow outweighs the BC dimming effect (-0.3 W m-2) at the surface over the HTP. We also find strong seasonal and spatial variation with a peak value of 5 W m-2 in the spring over Northwest Plateau. Such a large forcing of BC in snow is sufficient to cause earlier snow melting and potentially contribute to the acceleration of glacier retreat.In Chapter 5, we combined the methods used in chapters 3 and 4 to establish source-receptor relationships for atmospheric BC and its deposition to snow over Western North America, including the Northwest USA and West Canada. 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. It is found 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. 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. 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 BB contribution is larger than FF. The observationally based PMF analysis of the snow-impurity chemistry is also 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.