Air Pollution as a Climate Forcing: A Workshop
Day 1 Presentations
Overview of Gas and Aerosol Emissions
David G. Streets
Argonne National Laboratory, Argonne, IL, U.S.A.
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Knowledge of global emissions of gases and aerosols is important for an understanding of atmospheric chemistry as well as for identifying sources to be targeted for control policies. What is perhaps not generally appreciated is that the sources of major air pollutants in the world can be quite different for different species. Figure 1 shows the global distributions of ten of the most important emitted species by source type and world region, respectively. What we are most familiar with are emissions of the major air pollutants — SO2, NOx, and CO2, for example — from fossil-fuel-fired industrial, power, and transportation sources in the developed world. But Figure 1 clearly demonstrates that vegetation burning is by far the largest contributor to emissions of primary carbonaceous aerosols and developing countries are the largest contributing source regions (both shown in shades of blue) for these aerosols. Similarly, the major contributors to tropospheric ozone production — NOx, NMVOC, CO, and CH4 — are by no means closely associated with typical large combustion sources in the developed world.
Unfortunately, it is small, non-traditional sources in the developing world where we are most unsure about activity levels and emission factors. We estimate, for example, that more than 80% of black carbon (BC) and organic carbon (OC) emissions from anthropogenic sources in the world are due to small coal and biofuel stoves used in developing countries for cooking and space heating (Streets et al., 2001). What is urgently needed is a program of stove testing that will thoroughly investigate aerosol and particulate emissions to clarify (a) typical emission factors during operation in a developing country setting, (b) the representativeness of sampled sources for the entire population, (c) typical operating practices, (d) typical fuels and fuel characteristics, and (e) daily and seasonal operating cycles. Only then will we truly understand the emissions and global burden of primary aerosols. In support of this goal we need a measurement inter-comparison program to explain the large differences that appear to exist in measurements of BC and OC by different methods (chemical vs. absorption).
A confounding factor for aerosol emissions determination is that open biomass burning (forest fires, savanna burning, and crop-residue burning in fields after harvest) contributes about 50% of global BC. And, again, our knowledge of the extent of burning, inter-annual variability, and field-measured emission factors is weak. Here, we are hopeful that satellite observations can ultimately provide reliable characterization of biomass burning amounts.
Emissions of ozone precursors are better known than aerosols, but still inadequate. And while emissions of primary aerosol particles may diminish in the future, as particle controls are widely deployed, the outlook for ozone is pessimistic. NOx emissions will grow as transportation systems penetrate the developing world, and NMVOC emissions are likely to increase rapidly with the advent of chemicals and petroleum-based industries in developing countries. IPCC scenarios show this trend quite dramatically. Figure 2 presents projected future emissions of NOx and NMVOC under two IPCC scenarios: A1, which emphasizes globalization and economic growth, and B1 which emphasizes globalization, sustainability, and environmental protection. It is clear that under either of these scenarios emissions of both species are likely to grow in the future. Strikingly, there can be expected to be a major shift in the global pattern of emissions away from the developed world (Europe, North America, Japan, etc.), where growth is lower and environmental protection measures are implemented, and toward the developing world (Asia, Africa, South America, etc.), where growth rates are high and emission controls very weak. Thus the spatial distribution of emissions will shift from temperate latitudes in the northern hemisphere toward tropical latitudes. Speciation of NMVOC may shift from alkenes to alkanes, as well.
-- FIGURE 2 -- Fig. 2. Projected future emissions under IPCC A1 and B1 scenarios (blue = developing countries; red = developed countries)
Inevitably, these emission trends are likely to lead to increased ozone concentrations in the developing world. This will not only threaten human health and agricultural production in developing countries, but it will also increase the global background of ozone, making it more difficult for European and North American countries to comply with their ambient ozone standards. Figure 3 shows results from the Harvard GEOS-CHEM model (Fiore et al., 2002). It shows the trend in global ozone concentrations under the IPCC A1 scenario. The large increase in concentrations in India, China, and the Middle East (from increased extraction of oil and gas) is clearly evident.
The inescapable conclusion from any overview of global emissions as they affect the production of primary aerosol species and tropospheric ozone precursors is that we need to shift our focus away from traditional sources in the developed world and in the direction of distributed sources in the developing world, such as residential stoves, small industrial kilns and boilers, waste burning, unconventional two-stroke vehicles, and agricultural practices. We need to better understand their environmental emissions and, more importantly, target them for improvement or replacement as a means of alleviating their harmful effects at local, regional, and global scales.
- Bond, T.C., and D.G. Streets, 2002. A new global black carbon inventory, in preparation (see Bond abstract elsewhere in this collection).
- Fiore, A.M., D.J. Jacob, B.D. Field, D.G. Streets, S. Fernandes, and C. Jang, 2002. Linking air quality and climate change objectives: the case for controlling methane, in preparation.
- Streets, D.G., S. Gupta, S.T. Waldhoff, M.Q. Wang, T.C. Bond, and Y. Bo, 2001. Black carbon emissions in China, Atmos. Environ., 35, 4281-4296.
- Van Aardenne, J.A., F.J. Dentener, J.G.J. Olivier, C.G.M. Klein Goldewijk, and J. Lelieveld, 2001. A 1°x1° resolution data set of historical anthropogenic trace gas emissions for the period 1890-1990, Global Biogeochem. Cycles, 15, 909-928.