Regional Differences in Ozone
Introduction
Ozone requires the right mix of two essential groups of gases: volatile organic compounds (VOCs) and nitrogen oxides (NOx). When those gases are combined in sunlight with the right amount of heat, ozone forms. However, the sources of these gases, success at reducing them and the complications of ozone transported by the wind vary from region to region. The following analysis looks at the sources, trends and transport of ozone in each of the ten regions that EPA uses to group the states.
National Sources of Ozone
All the data on emissions of VOCs and NOx in this appendix were obtained from the US EPA’s National Emissions Trends Tier reports for 1999 inventoried data. Those data include emissions not only from individual facilities (called point sources), but also from so-called area sources which include many small, individual sources (like cars or residences) and sources that cover a large geographic area, such as wildfires. The data are estimated annually, but the sources are inventoried only every three years. Click here for the 1999 data, the most current based on inventories of sources.
The National Emissions Trend Tier data were sorted by region, by major source category, and by pollutant for this discussion. A brief description from EPA follows to explain each of the major source categories.
What Do These Categories Mean? 
Ozone Trends
Nationally, we have seen a significant improvement in the past 20 years in monitored ozone levels with a decline of 11 percent between 1982 and 2001. The EPA map below shows that the success has varied greatly by region. From the steepest drop of 24 percent in Region 9, led by California’s stringent controls, to the significant lack of progress in the lower Midwest in Region 7, where monitored levels have remained stagnant. More ominously, many regions actually saw a significant increase in ozone levels during the 1990s.
Region 7 had the largest increase in that decade, 13 percent increase in monitored ozone levels, while three other regions (3, 4, and 5) also increased monitored 8-hour ozone emissions. For more discussion of the differences in 8-hour ozone trends, see the descriptions below for each region .
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Note on Regional Trends
Because air quality monitors are concentrated in urban locations, it is not possible, strictly speaking, to accurately describe average ozone concentrations across as large an area as an EPARegion. EPAincludes this reminder in its discussion of the trend data: "These trends are influenced by the distribution of monitoring locations in a given region and, therefore, can be driven largely by urban concentrations. For this reason, they are not indicative of background regional concentrations."i For more discussion on regional trends, see EPA’s annual Air Trendsreport . |
The Transport of Ozone
By its nature, ozone is created in the atmospheric mixing bowl and carried by prevailing winds to areas often far beyond its source. Section 110 of the Clean Air Act recognizes the impact of ozone transported across political boundaries, by requiring communities to prevent sources from "contributing significantly" to downwind areas. When that doesn’t work, Section 126 of the Act allows downwind states to petition EPA to step in and act to reduce industrial pollution from upwind sources.
The most comprehensive effort to reduce transported ozone is currently in progress. Years of study in the 1990s had identified significant sources of NOx, largely from electric power plants, which were contributing to the ozone levels in much of the Northeast. At the request of 8 Northeastern states, EPA issued a rule in September 1998 targeting most of the eastern United States, a requirement commonly referred to as the NOx SIP call. 2 This rule required 22 states and the District of Columbia to significantly reduce NOx emissions by May 1, 2003, a date that was later extended to May 31, 2004, by court action for most of the states.3 The states included in the requirement are: Alabama, Connecticut, Delaware, Georgia, Illinois, Indiana, Kentucky, Massachusetts, Maryland, Michigan, Missouri, North Carolina, New Jersey, New York, Ohio, Pennsylvania, Rhode Island, South Carolina, Tennessee, Virginia, Wisconsin, and West Virginia.
Ozone is also transported across national borders. For example, ozone produced south and west of the New England states in the Ohio Valley and in the Canadian "Windsor-Quebec Corridor." In those two areas, heavy concentrations of power plants and transportation corridors produce ozone, which is carried into New England and the Mid-Atlantic States (EPA Regions 2 & 3), as well as into New Brunswick and other Canadian provinces. 4
Not only does ozone move into a state from the outside, it also moves within the state. For example, some air pollution episodes have been followed hour-by hour as they moved downwind from city to city within Pennsylvania. 5 In many cases, the highest levels of ozone will show up in suburban areas to downwind of larger communities. For example, even though an area such as San Francisco County in California may not be experiencing high ozone readings, it may be contributing to poor air quality in outlying areas such as the Sacramento and San Joaquin Valley areas to the East and other parts of the Bay Area to the south.6
There are some regions that are notable as sources of transported ozone affecting cities and states within the region and outside it. The Southeast (EPA Region 4) and the Midwest (EPA Region 5) are two. The Southeast is home to some of the most polluting power plants in the nation 7 and to cities with extremely high driving rates. Atlanta residents average 37.6 miles per day; Birmingham, 35.6; and Asheville, North Carolina, 47.5—all of them much higher than the traditionally car-dependent Los Angeles, whose residents average only 22.2 miles each day.8 As a result, the Southeast produces more NOx emissions (5.4 million tons in 1999) and VOC emissions (4.15 million tons) than any other section of the country.
The Midwest (EPA Region 5) is another region with many of the nation’s most polluting coal-fired power plants, including 8 of the top 20 NOx emitting facilities in the nation in 1999. 9 This region produces the second highest NOx emissions—4.98 million tons—and the second highest VOC emissions—3.5 million tons—in 1999.
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