A simulation based upon emissions projections by the Intergovernmental Panel on Climate Change (IPCC) predicts a longer and more intense ozone season in the United States by 2030, despite domestic emission reductions, the researchers note. Mitigation should therefore be considered on a global scale, the researchers say, and must take into account a rising global background level of ozone. Currently, the U.S. standard is based upon 84 parts per billion by volume of ozone, not to be exceeded more than three times per year, a standard that is not currently met nationwide. In Europe, the standard is much stricter, 55-65 parts of ozone per billion by volume, but these targets are also exceeded in many European countries.
Writing this month in the journal Geophysical Research Letters, Arlene M. Fiore and her colleagues say that one way to simultaneously decrease ozone pollution and greenhouse warming is to reduce methane emissions. Ozone is formed in the troposphere by chemical reactions involving methane, other organic compounds, and carbon monoxide, in the presence of nitrogen oxides and sunlight. Methane is known to be a major source of ozone throughout the troposphere, but is not usually considered to play a key role in the production of ozone smog in surface air, because of its long lifetime.
Sources of manmade methane include, notably, herds of cattle and other ungulates, rice production, and leaks of natural gas from pipelines, according to the IPCC. In addition, natural sources of methane include wetlands, termites, oceans, and gas hydrate nodules on the sea floor.
In a baseline study in 1995, 60 percent of methane emissions to the atmosphere were the result of human activity. The IPCC's A1 scenario, which Fiore characterizes as "less optimistic in terms of anticipated emissions than a companion B1 scenario," posits economic development as the primary policy influencing future trends of manmade emissions in most countries. Under A1, emissions would increase globally from 1995 to 2030, but their distribution would shift. Manmade nitrogen oxides would decline by 10 percent in the developed world, but increase by 130 percent in developing countries. During the same period, methane emissions would increase by 43 percent globally, according to the A1 scenario.
The researchers find that a reduction of manmade methane by 50 percent would have a greater impact on global tropospheric ozone than a comparable reduction in manmade nitrogen oxide emissions. Reducing surface nitrogen oxide emissions does effectively improve air quality by decreasing surface ozone levels, but this impact tends to be localized, and does not yield much benefit in terms of greenhouse warming. Reductions in methane emissions would, however, help to decrease greenhouse warming by decreasing both methane and ozone in the atmosphere world-wide, and this would also help to reduce surface air pollution.
Both in the United States and Europe, aggressive programs of emission controls aimed at lowering ozone-based pollution may be offset by rising emissions of methane and nitrogen oxides from developing countries, the researchers write. Pollution could therefore increase, despite these controls, and the summertime pollution season would actually lengthen, according to the simulation under the A1 scenario.
The study was funded by the Environmental Protection Agency (EPA), National Aeronautics and Space Administration (NASA), and the National Science Foundation (NSF).