Trends in Ozone Adjusted for Weather Conditions

Variations in weather conditions play an important role in determining ozone levels. Ozone is more readily formed on warm, sunny days when the air is stagnant. Conversely, ozone production is more limited when it is cloudy, cool, rainy, and windy. EPA uses a statistical model to account for the weather-related variability of seasonal ozone concentrations to provide a more accurate assessment of the underlying trend in the precursor emissions that cause ozone.

The graphic below shows 8-hour ozone concentrations for the summers (May-September) of 1997 through 2007. The dotted line shows the trend in observed values at monitoring sites, while the solid line illustrates the underlying ozone trend after removing the effects of weather. The solid line represents ozone levels anticipated under typical weather conditions and serves as a more accurate ozone trend for assessing changes in emissions. Typical weather conditions are determined by averaging conditions (e.g., temperature, humidity, etc.) for the time period presented. The information provided is useful for reviewing the weather influence for a particular ozone season.

Graph showing seasonal average ozone air quality, 1997 - 2006, for 85 urban and 48 rural sites, unadjusted and adjusted for weather

Seasonal average ozone is the average of daily maximum 8-hour ozone concentrations from May 1 through September 30 using 85 urban and 48 rural sites across the US (see map below).

Map of United States showing the locations of the 85 urban and 48 rural sites used to generate the information in the graph above

To get available information where you live, click on your region or select your state from the list below.

You will need Adobe Acrobat Reader to view the Adobe PDF files on this page. See EPA's PDF page for more information about getting and using the free Acrobat Reader.

Region 1 (PDF) (9 pp, 78 KB) Connecticut Maine Massachusetts Rhode Island	Region 2 (PDF) (5 pp, 56 KB) New York	Region 3 (PDF) (15 pp, 132 KB) Maryland Pennsylvania Virginia Washington, DC West Virginia	Region 4 (PDF) (23 pp, 191 KB) Alabama Florida Georgia Kentucky Mississippi North Carolina South Carolina Tennessee
Region 5 (PDF) (18 pp, 141 KB) Illinois Indiana Michigan Minnesota Ohio Wisconsin	Region 6 (PDF) (11 pp, 87 KB) Arkansas Louisiana Oklahoma Texas	Region 7 (PDF) (5 pp, 50 KB) Kansas Missouri Nebraska	Region 8 (PDF) (8 pp, 64 KB) Colorado Utah Wyoming
Region 9 (PDF) (10 pp, 94 KB) Arizona California Nevada	Region 10 (PDF) (5 pp, 50 KB) Oregon Washington

Additional Resources

Resources for Meteorological Adjustment of Ozone Trends (PDF) (2pp, 76k) - Memo from Air Quality Analysis Division, OAQPS to EPA Regional Air Division Directors, Regions 1-10. December 18, 2007

Technical Approach - Camalier, L., Cox, W., Dolwick, P., 2007. The effects of meteorology on ozone in urban areas and their use in assessing ozone trends. Atmospheric Environment 41, 7127-7137.

Example Code (txt) (8k) - R-code used to calculate trends in ozone adjusted for weather conditions.

Input Data (txt) (297k) - Data used in conjunction with example code to calculate trends for test case using data for Cincinnati, Ohio.

Output Data (txt) (2k) - Output data from test case using Cincinnati, Ohio.

Output Graphic (jpg) (119k) - Trends for test case using data for Cincinnati, Ohio.

Trends in Ozone Adjusted for Weather Conditions

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