Overview
Smoke or soot was the first pollutant to be recognized as having significant environmental impact yet one of the last to be studied by the contemporary atmospheric research community. Faraday recognized that soot was composed of carbon and that it was produced by the incomplete combustion of carbon-containing fuels. Soot is composed of a complex mixture of organic compounds which are weakly absorbing in the visible spectral region and a highly absorbing black component which is variously called “elemental”, “graphitic” or “black carbon”. The term elemental carbon has been used in conjunction with thermal and wet chemical determinations and the term graphitic carbon suggests the presence of graphite-like micro-crystalline structures in soot as evidenced by Raman Spectroscopy of Rosen and Novakov. The term black carbon is used to imply that this soot component is primarily responsible for the absorption of visible light as shown by Yasa et al. and Rosen et al. For consistency, in this paper, we will use the term black carbon (BC) as a synonym for both the elemental and graphitic component of soot as suggested by Novakov.
The disastrous effects of coal pollution on human health and mortality in the early 1950s in London led to the UK Clean Air Act of 1956. This act led to dramatic reductions of soot concentrations in the United Kingdom which were followed by similar reductions in US cities like Pittsburg and St. Louis. These reductions were largely achieved by the decreased use of soft coal for domestic heating by switching either to “smokeless” coals or other forms of fuel, such as fuel oil and natural gas. The steady reduction of smoke pollution in the industrial cities of Europe and United States caused a shift in research emphasis away from soot emissions and the almost complete neglect of black carbon as a significant aerosol constituent, at least in the United States. In the 1970s, however, a series of studies largely conducted by the Novakov group at Lawrence Berkeley Lab substantially changed this picture and demonstrated that black carbon as well as the organic soot components continued to be a large component in urban aerosols across the United States and Europe which led to improved controls of these emissions. Obviously, in the less-developed regions of the world where there were very limited or no controls on soot emissions the air quality continued to degrade as the population increased. It was not generally realized until many years later that from the perspective of global effects the emissions from these regions were extremely important.
Most of the developments mentioned above relate to air quality in urban atmospheres. The first indications of the role of black carbon in a larger, global context came from studies of the Arctic Haze phenomena, first described by Mitchell. Black carbon was identified in the Arctic haze aerosols by Rosen et al. and in the Arctic snow by Clarke and Noone.
In general, aerosol particles can affect the radiation balance leading to a cooling or heating effect with the magnitude and sign of the temperature change largely dependent on aerosol optical properties, aerosol concentrations, and the albedo of the underlying surface. A purely scattering aerosol will reflect energy that would normally be absorbed by the earth-atmosphere system back to space and leads to a cooling effect. As one adds an absorbing component the aerosol, it can lead to a heating of the earth-atmosphere system if the reflectivity of the underlying surface is sufficiently high. Early studies of the effects of aerosols on atmospheric radiative transfer on a global scale assumed a dominantly scattering aerosol with only a small absorbing component, since this appears to be a good representation of naturally occurring aerosols. However, as discussed above, urban aerosols have a large black carbon component and if these particles can be transported on a global scale then one would expect a heating effect over surfaces with a high surface albedo like snow or ice. Furthermore, if these particles are deposited in the snow an additional heating effect would occur due to reductions in the surface albedo.
In the late 1970s and early 1980s surprisingly large ground level concentrations of black carbon were observed throughout the western Arctic by Rosen et al. and modeling studies indicated that they could lead to heating over polar ice. One of the major uncertainties in modeling the effects of the Arctic haze on the solar radiation balance was limited knowledge of the vertical distributions of black carbon. During 1983 and 1984 as part of the NOAA AGASP program, the first measurements of such distributions in the Arctic atmosphere were obtained with an aethalometer which had the capability of measuring black carbon on a real-time basis. These measurements showed substantial concentrations of black carbon found throughout the western Arctic troposphere including the North Pole. The vertical profiles showed either a strongly layered structure or an almost uniform distribution to 8 kilometers with concentrations within layers as large as those found at ground level in typical mid-latitude urban areas in the United States as shown by Rosen et al. The absorption optical depths associated with these vertical profiles were large as evidenced by a vertical profile over the Norwegian arctic where absorption optical depths of 0.023 to 0.052 were calculated respectively for external and internal mixtures of black carbon with the other aerosol components. The average optical depths over all the AGASP flights which included eight vertical profiles over the Alaskan, Canadian, and Norwegian Arctic was also large with values respectively of 0.013 and 0.030 for an external and internal mixture as shown by Rosen and Hansen. Optical depths of these magnitudes lead to a substantial change in the solar radiation balance over the highly reflecting Arctic snow surface during the March–April time frame of these measurements. This is illustrated by the calculations of Porch and McCracken and Cess, who have modeled the Arctic aerosol for an absorption optical depth of 0.021 (which is close to the average of an internal and external mixtures for the AGASP flights), under cloud-free conditions, and a surface albedo of 0.8 and obtain an increase in the surface-atmosphere energy absorption of 7 Wm−2 averaged over the day for March 15 at 70 °N. These heating effects were viewed at the time as potentially one of the major causes of Arctic warming trends as described in Archives of Dept. of Energy, Basic Energy Sciences Accomplishments.
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