Air Pollution as a Climate Forcing: A Workshop
Day 2 Presentations
Absorbing Aerosol: What Does it Comprise?
Lawrence Berkeley National Laboratory, Berkeley, CA, U.S.A.
This contribution addresses the following questions about absorbing aerosols: 1) what is causing absorption, 2) where does it come from, and 3) how is it changing.
Earlier work at LBNL has defined Black Carbon (BC) as "combustion produced black carbonaceous material having graphite-like microstructure". This definition was based on applications of Raman, photo-acoustic and optical spectroscopy combined with chemical and thermal analyses. These results have shown that BC is the principal light absorbing carbonaceous species in the spectral region above 630 nm. Recent results obtained at our laboratory have quantified the relative contributions of BC and organic carbon (OC) to aerosol absorption. These results show that the absorption by OC, and especially by biomass OC, can be discerned. The OC absorption increases with decreasing wavelengths and becomes distinguishable from BC below 500 nm. The OC absorption cross section (m2/g), however, is only 12-25% of that of BC even at the lowest measured wavelength of 350 nm.
As early as 1880 it was concluded that coal combustion in low-temperature domestic and semi-domestic furnaces, compounded by inadequate air supply and over-stoking, was a major source of black smoke (i.e. soot or BC). This view remains largely valid today. Such sources were prevalent in the U.S. and Western Europe until about mid 20th century, and in developing countries throughout most of the second half of the past century. Additionally, diesel engines became significant sources worldwide during this period. However, improvements in engine technology in the west resulted in decreasing diesel BC emissions factors.
Estimates of historical BC emissions show that these were large in industrialized countries until about mid-century but radically decreasing afterwards. In contrast the BC emissions from developing countries increased concomitantly with decreasing emissions in the West. The estimated and observed decrease in BC emissions from these countries was largely the consequence of availability of cleaner petrochemical fuels which replaced the use of coal in polluting domestic, commercial and industrial sectors. Total coal consumption, however, did not necessarily decrease because most of it was used in more combustion-efficient power plants. As such changes in coal utilization technologies did not occur in developing countries BC emissions increased with increasing total coal consumption.
Because the dependence of BC and SO2 emissions on fuel consumption is different, the temporal changes in sulfate and BC concentrations, and therefore the aerosol single scatter albedo, are expected to exhibit regional differences. An estimate of these differences will be presented.
Finally, the feasibility of, and problems with, developing BC emission trends from 1850 to present will be illustrated by using the U.S. and Great Britain as examples.