Measurement of Ozone in the Air


Regional Ozone Monitors

Mid-Atlantic state and local air pollution control agencies operate an extensive network of air quality monitors to measure the concentration of various air pollutants. The location of ozone monitors is shown in the following map. Monitoring site locations and characteristics are tabulated in Appendix C.

Map: Ozone Monitoring Sites Operated During 1995



Number of Days Exceeding the Ozone Standard

The number of days on which monitored air quality fails to meet the federal health standard for ozone is one measure of how serious the air pollution problem is in an area. The following table records the number of days exceeding the standard for the Region as a whole, for each state in the Region, and for each current or former ozone nonattainment area in the Region.

In the Mid-Atlantic Region, the areas surrounding New York City, Philadelphia, Baltimore, and Washington, D.C. usually record the highest number of days with unhealthy air quality. These areas also usually experience the highest levels of ozone, as refle cted in their designations as serious or severe ozone nonattainment areas.

Year-to-year variations reflect both efforts to control pollution and the impact of the weather. Over a long period of time, a general decline in the number of days with unhealthy air quality indicates pollution control measures are working.

Table: Number of Days Above the Ozone Standard in the Mid-Atlantic Regions, 1986 to 1995


Ozone and Temperature

High temperature is one indicator that weather patterns favor ozone production, although cloud cover, winds, humidity, solar radiation intensity, and other factors are also important. In the central part of the Mid-Atlantic Region , ozone is likely to be high on days that are hot, with upper-air winds from the north or northwest, light or stagnant surface winds, a strong inversion (which acts as a lid to contain pollutants), and clear skies (Ryan, 1995).

These graphs track ten years of ozone data. The first chart reflects data from all ozone monitors in the Mid-Atlantic Region that reported at least 50% complete data for at least eight of the past ten years. Ninety ozone monitors met this criterion and collected data on at least half of the days during the ozone season.

The following graphs show ten-year trends in the ratio of the number of high ozone days to the number of high temperature days. If this ratio falls, it indicates that ozone levels are improving and violations are less frequent than high temperature days. In the region as a whole, the ratio has stayed close to 0.5 most of the past ten years, indicating that an ozone violation has occurred about half the time that the high temperature was 90 degrees or more. In the Philadelphia and Greater New York areas , the ratio has dropped significantly from levels of 1.0 or greater. In some areas, 1988, 1990, and 1995 recorded upturns in the number of ozone violations per hot day.

Looking at these ratios helps to understand how many ozone violations would have occurred if the temperatures were similar from one year to the next, but because other factors are also important in helping to form ozone, this technique does not completely remove the effect of weather from ozone trends.

Sources: WF Ryan (1995), "Comments on O3 Local and Regional Trends," University of Maryland Department of Meteorology, December 7.

Chart: Ratio of Ozone Violations to 90 Degree Days in the Mid-Atlantic Region

Chart: Ratio of Ozone Violations to 90 Degree Days in the Baltimore Nonattainment Area

Chart: Ratio of Ozone Violations to 90 Degree Days in the Philadelphia Nonattainment Area

Chart: Ratio of Ozone Violations to 90 Degree Days in the Greater New York Nonattainment Area

Chart: Ratio of Ozone Violations to 90 Degree Days in the Washington D.C. Nonattainment Area


Trends in Hourly Ozone Concentrations

The following charts summarize trends in hourly ozone concentrations for the Mid-Atlantic Region and for each state in the Region.

The charts track the second-highest daily ozone concentrations measured at selected monitoring sites for the past ten years. The sites chosen were those with a continuous data record for at least eight years from 1986 through 1995.

The dotted line at .12 ppm is the current ozone standard. The shaded area includes all but the top and bottom 10% of the peak ozone data for each year.

Compliance with the standard is measured at each individual site in order to protect public health throughout the area. Although it does not indicate compliance at individual sites, the averages presented on these charts are one way to look at the genera l trend in an area.

The highest ozone levels in the past ten years were recorded in 1988. Nationally, 1988 was the third hottest summer in the last 100 years, and high ozone levels were recorded throughout the country.

Both weather patterns and emissions controls have helped reduce the severity of ozone pollution. Since 1988 peak ozone levels have been lower and there have been fewer days with ozone levels above the health standard. Average peak ozone levels in the Mi d-Atlantic Region have been below the .12 ppm ozone standard in most states since 1988. However, 1995 was also a record hot summer, and ozone levels in most of the Mid-Atlantic Region were higher in 1995 than they had been in many years. Cleaner cars, r eformulated gasoline, and reductions in nitrogen oxide emissions at some major sources have improved air quality.

Sources: These charts were prepared by Mark Schmidt of EPA's Office of Air Quality Planning and Standards using data from the national AIRS database as provided by state and local agencies through May 1996. Interpolation of m issing data for a site with eight or nine years of data was performed using the same logic used in preparing EPA's 1995 Trends Report.

Chart: Mid-Adtlantic Ozone Trends

Chart: Delaware Ozone Trends

Chart: Washington D.C. Ozone Trends

Chart: Maryland Ozone Trends

Chart: New Jersey Ozone Trends

Chart: North Carolina Ozone Trends

Chart: Pennsylvania Ozone Trends

Chart: Virginia Ozone Trends

Chart: West Virginia Ozone Trends


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Last revised: 11/17/98