Urban ozone

Urban ozone. Electric power plants contribute heavily to NOx emissions, which are precursor chemicals that (along with VOCs) react in the atmosphere in the presence of sunlight to form ozone. Strong concentrations of ozone often occur in and downwind of large urban areas.

During cardiovascular exercise, human exposure to ozone at concentrations both above and below the 120-part-per-billion maximum allowed under the NAAQS has been shown to result in transient respiratory problems.(19) Ozone can also seriously irritate the eyes and mucous membranes. The effects of elevated ozone levels are not known for all types of vegetation, but such levels are harmful to many types of trees and crops. High ozone concentrations seem to be more detrimental than low-level extended exposure.(20)

The assessment of the impact of NOx controls on ozone concentrations is complex and must be studied carefully in developing ozone abatement strategies, according to a 1992 report (21) from a National Research Council committee. The committee found that ambient measurements of VOC/NOx ratios--which, as they vary, have different effects on ozone formation--were larger than expected from an assessment of emission inventories. The committee also determined that the effectiveness of efforts to control VOC and NOx emissions depends on ambient VOC/NOx ratios. Generally, at ratios of 10 or less, VOC control is more effective and NOx control may be counterproductive. At ratios greater than 20, NOx control is generally more effective. Hence, if VOC emission inventories have been understated, past ozone control strategies may have been misdirected. Tighter controls on NOx may be more effective in controlling ozone under certain circumstances.

The committee also found that combinations of biogenic VOCs and anthropogenic NOx can significantly affect ozone formation in some urban and rural regions of the United States and concluded, again, that the appropriate strategy may be to monitor and control NOx emissions.

Global climate change. Greenhouse gases are necessary for life on Earth because they keep ambient temperatures well above what they would otherwise be. Many scientists believe that anthropogenic additions (some from electric power plants) to the Earth's natural complement of greenhouse gases are augmenting this greenhouse effect and thus raising global temperatures.

The principle greenhouse gases are water vapor, CO2, CH4, N2O, and chlorofluorocarbons (CFCs).(22) The levels of CO2 and N2O in the atmosphere can be influenced by the amount of electricity generated and the fuel used. Of the fossil fuels, coal has the highest carbon content. Oil and natural gas have approximately 80 percent and 60 percent of the carbon content of coal, respectively, on an energy-equivalency basis.(23)

Although CO2 is not a regulated pollutant, the reduction of greenhouse gas emissions in general, including those of CO2, is the focus of several international efforts. The United States signed the Framework Convention on Climate Change during the 1992 United Nations Conference on Environment and Development. President Clinton reaffirmed the U.S. commitment to control greenhouse gases by developing the Climate Change Action Plan. This largely voluntary plan is intended to stabilize greenhouse gases at 1990 levels by 2000. In 1994, electric utility groups signed a memorandum of understanding with DOE to pursue voluntary reductions in emissions of greenhouse gases and DOE completed draft guidelines for utilities to report emissions reductions voluntarily.