Trends in Ozone Adjusted for Weather Conditions

Variations in weather conditions play an important role in determining ozone concentrations. 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, or windy. EPA uses a statistical model to adjust for the variability in seasonal ozone concentrations due to weather to provide a more accurate assessment of the underlying trend in ozone caused by emissions.

The graph below shows the May - September average of the daily maximum 8-hour ozone concentrations from 2001 to 2010. The dotted red line shows the trend in observed ozone concentrations at selected monitoring sites, while the solid blue line shows the underlying ozone trend at those sites after removing the effects of weather. The solid blue line represents ozone levels anticipated under average weather conditions and serves as a more accurate assessment of the trend in ozone due to changes in emissions. For example, above average temperatures and below average humidity across the most of the US in 2007 contributed to increased ozone formation while below average temperatures and above average humidity across much of the US in the summer of 2009 contributed to decreased ozone formation. The statistical model accounts for these effects by adjusting the seasonal average ozone trend downward in 2007 and upward in 2009.

The national ozone trend is the average of the trends calculated independently at 180 ozone monitoring sites using paired ambient ozone monitoring data from EPA and meteorology data from the National Weather Service. The map below shows the locations of the 180 monitoring sites.

Examining the ozone trend separately for the Eastern and Western US (as defined on the map above) reveals two distinct patterns. After adjusting the ozone trend for weather, the Eastern US shows a steady decline in summertime ozone levels between 2002 and 2009. This decline may be partially due to EPA sponsored programs designed to reduce summertime emissions of ozone precursors in the Eastern US. Meanwhile, summertime ozone levels in the Western US have also started to decrease over the past few years.

The ozone trends in urban and rural areas both resemble the national trend with generally lower ozone concentrations measured in rural areas, as one might expect. Atmospheric circulation can cause ozone to be transported over long distances so that rural ozone concentrations may be elevated by ozone produced in regional urban centers.

The statistical model used by EPA to determine the adjustments to the ozone trend is fit independently in each location using local ozone and meteorology data. However, the results tend to have a strong geographic coherence. The maps below illustrate the spatial pattern in the adjustments to the observed ozone trend across the nation in 2007 and 2009.

Click here to download maps of metoeorological adjustments for each year from 2001 to 2010

To see the trend in ozone near where you live before and after adjusting for weather effects, select your state from the list below.
(Note: You will need WinZip or other software capable of opening compressed .zip files to view the graphics.)

USA (180 locations)

Additional Resources

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

Technical Approach - Louise Camalier, William Cox, and Pat Dolwick. The Effects of Meteorology on Ozone in Urban Areas and their use in Assessing Ozone Trends. Atmospheric Environment, Volume 41, Issue 33, October 2007, pages 7127-7137.

Reference - William Cox and Shao-Hang Chu. "Assessment of Interannual Ozone Variation in Urban Areas from a Climatological Perspective." Atmospheric Environment, Volume 30, Issue 14, July 1996, pages 2615-2625.

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

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

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

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

Trends in Ozone Adjusted for Weather Conditions

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