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Making Progress on Ground-Level Ozone

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Ground-Level Ozone

Ground-level ozone is a gas that forms when emissions of NOX and VOCs react with other chemicals in the air in the presence of strong sunlight. NOX and VOCs are emitted by combustion sources (such as vehicles and power plants). VOCs are also given off by solvents, cleaners, and paints. Ground-level ozone can cause or exacerbate respiratory illnesses and is especially harmful to young children, the elderly, and those suffering from chronic asthma and/or bronchitis. Ground-level ozone can affect leaves and roots of plants, especially trees, which can make them more susceptible to attack from insects and diseases and can reduce their ability to withstand droughts, windstorms, and manmade stresses such as acid rain.

Key Commitments of the Ozone Annex

The commitments to reduce NOX and VOCs apply to a defined region in both countries known as the Pollutant Emission Management Area (PEMA), which includes central and southern Ontario, southern Quebec, 18 U.S. states, and the District of Columbia. The states and provinces within the PEMA are the areas where emission reductions are most critical for reducing transboundary ozone.

CANADA:

UNITED STATES:

Figure 6. Ozone Season NOX Emissions under the NOX Budget Trading Program

Figure 6. Ozone Season NOX Emissions under the NOX Budget Trading Program
Source: EPA, 2008

Ambient Levels of Ozone

Under the Ozone Annex, the United States and Canada are required to report on the amount of ozone, NOX, and VOCs in the air we breathe (i.e., ambient concentrations) from all relevant monitors within 500 km of the border. Both countries have extensive networks to monitor ground-level ozone and its precursors, and both governments prepare routine reports summarizing measurement levels and trends. The latest reported data from both countries are for 2006.

Figure 7 illustrates that higher levels of ozone occurred in the Great Lakes and Ohio Valley regions, as well as downwind of urban areas.

Figure 7. Ozone Concentrations along the Canada–U.S. Border (Three-Year Average of the Fourth Highest Daily Maximum 8-Hour Average), 2004–2006

Figure 7. Ozone Concentrations along the Canada–U.S. Border (Three-Year Average of the Fourth Highest Daily Maximum 8-Hour Average), 2004–2006


Note: Data contoured are the 2004–2006 averages of annual fourth highest daily values, where the daily value is the highest running 8-hour average for the day. Sites used had at least 75 percent of possible daily values for the period.

Source: Environment Canada National Air Pollution Surveillance (NAPS) Network Database, 2008 (www.etc-cte.ec.gc.ca/naps/index_e.html); EPA Aerometric Information Retrieval System (AIRS) Database (www.epa.gov/air/data/index.html)

Ambient Concentrations of Ozone, NOX, and VOCs

Figure 8 illustrates that ozone levels within the PEMA have decreased over time with a notable decline in ozone levels since 2002. Figures 9 and 10 depict the average ozone season levels of ozone precursors NOX and VOCs in the eastern United States and Canada. Although NOX and VOC concentrations have fluctuated over recent years, these fluctuations are most likely attributable to changes in weather conditions. Overall, the data indicate a downward trend in the ambient levels of both NOX and VOCs.

Figure 8. Annual Average Fourth Highest Maximum 8-Hour Ozone Concentration for Sites within 500 km of the Canada–U.S. Border, 1995–2006

Figure 8. Annual Average Fourth Highest Maximum 8-Hour Ozone Concentration for Sites within 500 km of the Canada–U.S. Border, 1995–2006
Source: EPA and Environment Canada, 2008

Figure 9. Average Ozone Season 1-Hour NOX Concentration for Sites within 500 km of the Canada–U.S. Border, 1995–2006

Figure 9. Average Ozone Season 1-Hour NOX Concentration for Sites within 500 km of the Canada–U.S. Border, 1995–2006
Source: EPA and Environment Canada, 2008

Figure 10. Average Ozone Season 24-Hour VOC Concentration for Sites within 500 km of the Canada–U.S. Border, 1997–2006

Figure 10. Average Ozone Season 24-Hour VOC Concentration for Sites within 500 km of the Canada–U.S. Border, 1997–2006
Source: EPA and Environment Canada, 2008

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