Ground-level ozone is created by chemical reactions between emissions of nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. Emissions from motor vehicles, industrial facilities, electric utilities, construction equipment, locomotives, ships, gasoline vapors, and chemical solvents are some of the major sources of NOx and VOCs. Currently, electric utilities emit only about 5 percent of the nation’s combined emissions of NOx and VOCs.
The Clean Air Act (CAA) requires the U.S. Environmental Protection Agency (EPA) to establish national ambient air quality standards (NAAQS)—at levels that are protective of public health with an adequate margin of safety—for six pollutants, including ozone. Breathing elevated ozone concentrations can trigger adverse health effects, such as respiratory inflammation, particularly for sensitive groups like asthmatics. Ground-level ozone also can reduce crop yields and tree growth.
EPA reviewed its ozone standards in 1997 and set a tighter limit of 0.08 parts per million (ppm) for a new measuring time period, the 8-hour average (with rounding, the standard actually was 0.084). The CAA requires EPA to complete a review of each NAAQS, including ozone, at five-year intervals and make any necessary revisions. In March 2008, EPA tightened the standard again to 0.075 ppm.
EPA is scheduled to propose to retain or revise the 2008 standard in late 2013.
What are Electric Utilities Doing to Help Reduce Ozone?
The electric power industry nationally has reduced its national total NOx emissions by 76 percent over the period of 1990 to 2012. NOx reductions from electric utilities have been reduced through the CAA’s Acid Rain Program, the “NOx SIP Call," and the Clean Air Interstate Rule (CAIR). NOx emissions will be further reduced under new Ozone NAAQS, the CAA's fine particle NAAQS, regional haze requirements, and 2015 Phase 2 CAIR requirements and/or a revised Crss-State Air Pollution Rule.
The power sector currently contributes only about 10 percent of manmade NOx emissions and five percent of national man-made ozone precursors.