Black carbon aerosol plays a unique and important role in Earth's climate system. Black carbon is a type of carbonaceous material with a unique combination of physical properties. This assessment provides an evaluation of black-carbon climate forcing that is comprehensive in its inclusion of all known and relevant processes and that is quantitative in providing best estimates and uncertainties of the main forcing terms: direct solar absorption; influence on liquid, mixed phase, and ice clouds; and deposition on snow and ice. These effects are calculated with climate models, but when possible, they are evaluated with both microphysical measurements and field observations. Predominant sources are combustion related, namely, fossil fuels for transportation, solid fuels for industrial and residential uses, and open burning of biomass. Total global emissions of black carbon using bottom-up inventory methods are 7500 Gg yr−1 in the year 2000 with an uncertainty range of 2000 to 29000. However, global atmospheric absorption attributable to black carbon is too low in many models and should be increased by a factor of almost 3. After this scaling, the best estimate for the industrial-era (1750 to 2005) direct radiative forcing of atmospheric black carbon is +0.71 W m−2 with 90% uncertainty bounds of (+0.08, +1.27) W m−2. Total direct forcing by all black carbon sources, without subtracting the preindustrial background, is estimated as +0.88 (+0.17, +1.48) W m−2. Direct radiative forcing alone does not capture important rapid adjustment mechanisms. A framework is described and used for quantifying climate forcings, including rapid adjustments. The best estimate of industrial-era climate forcing of black carbon through all forcing mechanisms, including clouds and cryosphere forcing, is +1.1 W m−2 with 90% uncertainty bounds of +0.17 to +2.1 W m−2. Thus, there is a very high probability that black carbon emissions, independent of co-emitted species, have a positive forcing and warm the climate. We estimate that black carbon, with a total climate forcing of +1.1 W m−2, is the second most important human emission in terms of its climate forcing in the present-day atmosphere; only carbon dioxide is estimated to have a greater forcing. Sources that emit black carbon also emit other short-lived species that may either cool or warm climate. Climate forcings from co-emitted species are estimated and used in the framework described herein. When the principal effects of short-lived co-emissions, including cooling agents such as sulfur dioxide, are included in net forcing, energy-related sources (fossil fuel and biofuel) have an industrial-era climate forcing of +0.22 (−0.50 to +1.08) W m−2 during the first year after emission. For a few of these sources, such as diesel engines and possibly residential biofuels, warming is strong enough that eliminating all short-lived emissions from these sources would reduce net climate forcing (i.e., produce cooling). When open burning emissions, which emit high levels of organic matter, are included in the total, the best estimate of net industrial-era climate forcing by all short-lived species from black-carbon-rich sources becomes slightly negative (−0.06 W m−2 with 90% uncertainty bounds of −1.45 to +1.29 W m−2). The uncertainties in net climate forcing from black-carbon-rich sources are substantial, largely due to lack of knowledge about cloud interactions with both black carbon and co-emitted organic carbon. In prioritizing potential black-carbon mitigation actions, non-science factors, such as technical feasibility, costs, policy design, and implementation feasibility play important roles. The major sources of black carbon are presently in different stages with regard to the feasibility for near-term mitigation. This assessment, by evaluating the large number and complexity of the associated physical and radiative processes in black-carbon climate forcing, sets a baseline from which to improve future climate forcing estimates.

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Bounding the role of black carbon in the climate system: A scientific assessment

The optical properties of the light-absorbing, carbonaceous substance often called “soot,” “black carbon,” or “carbon black" have been the subject of some debate. These properties are necessary to model how aerosols affect climate, and our review is targeted specifically for that application. We recommend the term light-absorbing carbon to avoid conflict with operationally based definitions. Absorptive properties depend on molecular form, particularly the size of sp 2-bonded clusters. Freshly-generated particles should be represented as aggregates, and their absorption is like that of particles small relative to the wavelength. Previous compendia have yielded a wide range of values for both refractive indices and absorption cross section. The absorptive properties of light-absorbing carbon are not as variable as is commonly believed. Our tabulation suggests a mass-normalized absorption cross section of 7.5 ± 1.2 m2/g at 550 nm for uncoated particles. We recommend a narrow range of refractive indices for s...

https://www.tandfonline.com/doi/pdf/10.1080/02786820500421521

Light Absorption by Carbonaceous Particles: An Investigative Review

Air pollution in mega cities in China

Although the definition and measurement techniques for atmospheric "black carbon" ("BC") or "elemental carbon'' ("EC") have long been subjects of scientific controversy, the recent discovery of light-absorbing carbon that is not black ("brown carbon, C brown ") makes it imperative to reassess and redefine the components that make up light-absorbing carbonaceous matter (LAC) in the atmosphere. Evidence for the atmospheric presence of C brown comes from (1) spectral aerosol light absorption measurements near specific combustion sources, (2) observations of spectral properties of water extracts of continental aerosol, (3) laboratory studies indicating the formation of light-absorbing organic matter in the atmosphere, and (4) indirectly from the chemical analogy of aerosol species to colored natural humic substances. We show that brown carbon may severely bias measurements of "BC" and "EC" over vast parts of the troposphere, especially those strongly polluted by biomass burning, where the mass concentration of C brown is high relative to that of soot carbon. Chemical measurements to determine "EC" are biased by the refractory nature of C brown as well as by complex matrix interferences. Optical measurements of "BC" suffer from a number of problems: (1) many of the presently used instruments introduce a substantial bias into the determination of aerosol light absorption, (2) there is no unique conversion factor between light absorption and "EC" or "BC" concentration in ambient aerosols, and (3) the difference in spectral properties between the different types of LAC, as well as the chemical complexity of C brown , lead to several conceptual as well as practical complications. We also suggest that due to the sharply increasing absorption of C brown towards the UV, single-wavelength light absorption measurements may not be adequate for the assessment of absorption of solar radiation in the troposphere. We discuss the possible consequences of these effects for our understanding of tropospheric processes, including their influence on UV-irradiance, atmospheric photochemistry and radiative transfer in clouds.

/pdf/black-carbon-or-brown-carbon-the-nature-of-light-absorbing-3er7h504pj.pdf

Black carbon or brown carbon? The nature of light-absorbing carbonaceous aerosols

Although organic compounds typically constitute a substantial fraction of the fine particulate matter (PM) in the atmosphere, their molecular composition remains poorly characterized. This is largely because atmospheric particles contain a myriad of diverse organic compounds, not all of which extract in a single solvent or elute through a gas chromatograph; therefore, a substantial portion typically remains unanalyzed. Most often the chemical analysis is performed on a fraction that extracts in organic solvents such as benzene, ether or hexane; consequently, information on the molecular composition of the water-soluble fraction is particularly sparse and incomplete. This paper investigates theoretically the characteristics of the water-soluble fraction by splicing together various strands of information from the literature. We identify specific compounds that are likely to contribute to the water-soluble fraction by juxtaposing observations regarding the extraction characteristics and the molecular composition of atmospheric particulate organics with compound-specific solubility and condensibility for a wide variety of organics. The results show that water-soluble organics, which constitute a substantial fraction of the total organic mass, include C2 to C7 multifunctional compounds (e.g., diacids, polyols, amino acids). The importance of diacids is already recognized; our results provide an impetus for new experiments to establish the atmospheric concentrations and sources of polyols, amino acids and other oxygenated multifunctional compounds.

Water-soluble organics in atmospheric particles: A critical review of the literature and application of thermodynamics to identify candidate compounds

In the spring of 1988 an interagency consortium of Federal Land Managers and the Environmental Protection Agency initiated a national visibility and aerosol monitoring network to track spatial and temporal trends of visibility and visibility-reducing particles. The monitoring network consists of 36 stations located mostly in the western United States. The major visibility-reducing aerosol species, sulfates, nitrates, organics, light-absorbing carbon, and wind-blown dust are monitored as well as light scattering and extinction. Sulfates and organics are responsible for most of the extinction at most locations throughout the United States, while at sites in southern California nitrates are dominant. In the eastern United States, sulfates contribute to about two thirds of the extinction. In almost all cases, extinction and the major aerosol types are highest in the summer and lowest during the winter months.

Spatial and seasonal trends in particle concentration and optical extinction in the United States

The fine particle (<2.5 μm) composition data from seven National Park Service locations in Alaska for the period from 1986 to 1995 was performed using a new type of factor analysis, positive matrix factorization (PMF). This method uses the estimates of the error in the data to provide optimum data point scaling and permits a better treatment of missing and below detection limit values. Eight source components were obtained for data sets from the Northwest Alaska Areas and the Bering Land Bridge sites. Five to seven components were obtained for the other Alaskan sites. The solutions were normalized by using aerosol fine mass concentration data. Squared correlation coefficients between the reconstructed mass obtained from aerosol composition data for the sites and the measured mass were in the range of 0.74–0.95. Two factors identified as soils were obtained for all of the sites. Concentrations for these factors for most of the sites have maxima in the summer and minima in the winter. A sea-salt component was found at five locations. A factor with the highest concentrations of black carbon (BC), H+, and K identified as forest fire smoke was obtained for all data sets except at Katmai. Factors with high concentrations of S, BC-Na-S, and Zn-Cu were obtained at all sites. At three sites, the solutions also contained a factor with high Pb and Br values. The factors with the high S, Pb, and BC-Na-S values at most sites show an annual cycle with maxima during the winter-spring season and minima in the summer. The seasonal variations and elemental compositions of these factors suggest anthropogenic origins with the spatial pattern suggesting that the sources are distant from the receptor sites. The seasonal maxima/minima ratios of these factors were higher for more northerly locations. Four main sources contribute to the observed concentrations at these locations: long-range transported anthropogenic aerosol (Arctic haze aerosol), sea-salt aerosol, local soil dust, and aerosol with high BC concentrations from regional forest fires or local wood smoke. A northwest to southeast negative gradient suggesting long-range transport of air masses from regions north or northwest of Alaska dominated the spatial distribution of the high S factor concentrations.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98JD01212

Atmospheric aerosol over Alaska: 2. Elemental composition and sources

The relative importance of soluble aerosols to spatial and seasonal trends of impaired visibility in the United States

During the winter and summer months of 1990 a special study called Project MOHAVE (measurement of haze and visual effects) was carried out with the principle objective of attributing aerosol species to extinction and scattering and the aerosol species to sources and/or source regions. The study area included much of southern California and Nevada, Arizona, and Utah; however, the intensive monitoring sites and primary focus of the study was on the Colorado Plateau of northern Arizona, southern Nevada, and Utah. This paper reports on the apportionment of various aerosol species to measured fine and coarse mass concentrations and these species to scattering and extinction. The study is unique in that a number of “ambient” integrating nephelometers were operated to measure the ambient scattering coefficient, while transmissometers were used to measure atmospheric extinction. Comparison of measured scattering, extinction, and aerosol species concentration, both statistically and theoretically, allows for an estimate of scattering and absorption efficiencies. Analysis suggests that using elemental carbon, derived from thermal optical techniques, to estimate absorption may significantly underestimate absorption. Using elemental carbon, absorption is estimated to be 5% of extinction, while direct measurements of absorption suggest that it is about 30% of measured extinction. Furthermore, because light absorption by soil is usually not accounted for, soil extinction is underestimated by about 30%.

Examining the relationship among atmospheric aerosols and light scattering and extinction in the Grand Canyon area

Under the IMPROVE visibility monitoring network, federal land managers have monitored visibility and fine particle concentrations at 29 Class I area sites (mostly national parks and wilderness areas) and Washington, DC since 1988. This paper evaluates trends in reconstructed visibility and fine particles for the 10th (best visibility days), 50th (average visibility days), and 90th (worst visibility days) percentiles over the nine-year period from 1988-96. Data from these sites provides an indication of regional trends in air quality and visibility resulting from implementation of various emission reduction strategies.

https://www.tandfonline.com/doi/pdf/10.1080/10473289.2000.10464127

James F. Sisler

Papers

Spatial and seasonal trends in particle concentration and optical extinction in the United States

Atmospheric aerosol over Alaska: 2. Elemental composition and sources

The relative importance of soluble aerosols to spatial and seasonal trends of impaired visibility in the United States

Examining the relationship among atmospheric aerosols and light scattering and extinction in the Grand Canyon area

Interpretation of trends of PM25 and reconstructed visibility from the IMPROVE network.