Interaction of Ozone and Sulfate in Air Pollution and Climate Change
In two recent studies, we describe how emission of ozone precursor gases (gases which react to form ozone) can dramatically affect both air quality and climate forcing by increasing the levels of tropospheric sulfate. Like many of their precursors, ozone and sulfate are pollutants that can detrimentally affect climate, agriculture, and human health. However, they act differently on the climate, as ozone tends to warm the planet while sulfate cools it.
Ozone and sulfate aerosol are formed in the atmosphere from chemical reactions involving gases such as sulfur dioxide, carbon monoxide and methane, which are emitted by both natural and human sources, the latter including automobile traffic, power generation, industry and agriculture.
Many of the reactions and molecules involved in the formation of sulfate and ozone overlap. Sulfate is generated by the oxidation of sulfur dioxide by the hydroxyl radical or by hydrogen peroxide, both of which can be derived from ozone. Likewise, ozone production requires the presence of nitrogen oxides, which sulfate can remove by conversion to nitric acid.
In the future, man-made emissions of the precursor gases will change as more nations industrialize, other nations implement emissions control strategies, and world population grows, leading to changes in the amount of pollution that people are exposed to. We used the GISS ModelE to simulate a future Earth atmosphere based on a middle-of-the-road projection of man-made precursor emissions to simulate levels of air pollution in the future and to investigate how the interaction between sulfate and ozone might affect future climate changes.
Figure 1 shows the percentage change in annual average sulfate aerosol and ozone air pollution at the Earth's surface by 2030. There are large increases in pollution in subtropical regions, especially Asia. Over the Indian subcontinent the surface sulfate aerosol amount changes from around 400 pptv in the present day to around 2000 pptv at 2030 and the surface level ozone increases from around 35 ppbv to 60 ppbv. The potential consequences of such large increases in the sulfate aerosol and ozone pollution may have serious social and economic impacts across the Indian subcontinent.
Next we calculated how much of the sulfate aerosol increase is due to the change in ozone precursor emissions alone and find the influence to be surprisingly large. Figure 2 shows the amount of the future surface sulfate aerosol that comes from changes in ozone precursor emissions only. Increases in ozone precursor emissions contribute about 10% to surface sulfate increases over the Middle East, North Africa and the most developed parts of South America, but the largest influence occurs over the Indian subcontinent, where the surface sulfate is 20% greater as a result of the future emissions-driven increases in ozone precursors.
Moreover, ozone precursor emissions also contribute 20% of the negative sulfate forcing over India, which is more than twice the direct positive forcing of ozone itself. In contrast, changes in sulfate precursor emissions do not significantly affect future ozone levels.
This new insight, that ozone precursors have a surprisingly large influence on air quality via sulfate and that their overall climate impact may be opposite to the conventional view, is of direct relevance to regulatory policy. The interconnection between ozone and sulfate can complicate environmental efforts, as a reduction of ozone precursors would improve surface air quality, but also impose additional positive forcing via sulfate reduction. Our results suggest that future regulations should address ozone and sulfate simultaneously, which they do not currently do, as well as consider both air quality and climate.
Unger, N., D.T. Shindell, D.M. Koch, and D.G. Streets 2006. Cross influences of ozone and sulfate precursor emissions changes on air quality and climate. Proc. Natl. Acad. Sci. 103, 4377-4380, doi:10.1073pnas.0508769103.
Unger, N.B., D.T. Shindell, D.M. Koch, M. Amann, J. Cofala, and D.G. Streets 2006. Influences of man-made emissions and climate changes on tropospheric ozone, methane and sulfate at 2030 from a broad range of possible futures. J. Geophys. Res. 111, D12313, doi:10.1029/2005JD006518.