Note that the data published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution. For the most recent data, purchase the 2008 Report from Island Press.

The Indicator

The indicator reports the number of days per year when peak 8- hour average ozone concentrations exceed 0.08 parts per million (ppm). When a monitor exceeds this 8-hour average concentration four or more times per year, an area is likely to be out of compliance with the National Ambient Air Quality Standard (NAAQS) for ozone; this standard was chosen by the Environmental Protection Agency (EPA) to “protect the public health ... with an adequate margin of safety,” as specified by the Clean Air Act. Note that the actual calculation to determine compliance with the NAAQS involves calculation of a 3-year average of the annual fourth-highest daily maximum 8-hour average concentration; if this value exceeds 0.08 ppm an area is in violation.

Oxides of nitrogen (NO_x), which are byproducts of fossil fuel combustion, when in the presence of sunlight in the atmosphere, will break apart and generate nitric oxide (NO) and a single atom of oxygen (O). This oxygen atom quickly combines with molecular oxygen (O₂) forming ozone (O₃). Ozone can oxidize NO back to nitrous oxide (N0₂), which allows the cycle to start over again. Volatile organic compounds (VOCs), which come from paints and solvents, unburned fuel, and industrial sources, factor into the equation because they also can oxidize NO to NO₂. Hence, with both NO_x and VOCs present, ozone accumulates in the atmosphere and ultimately poses a threat to human health, wildlife, pets, and building materials.

The EPA Website http://www.epa.gov/airtrends/ozone.html provides a discussion of some factors that can contribute to year-to-year fluctuations in ozone levels.

The Data

Data Source: Data are maintained by EPA in the Aerometric Information Retrieval System (AIRS). The Clean Air Act requires every state to establish a network of air-monitoring stations for pollutants, including ozone, using criteria set by EPA for their location and operation; there are approximately 1800 ozone monitors in the network with some data between 1990 and 2004. The states must provide EPA with an annual summary of results from each monitor.

Data Collection Methodology: Ozone monitoring instruments are intended to produce a measurement every hour, for a possible total of 8,760 hourly measurements in a year. A monitor is considered operational if it reports a measurement for more than half the hours in a year.

Data Manipulation: For each of the 1800 ozone monitors nationwide, EPA provided The Heinz Center with 15 years of data on the number of days per year that peak 8-hour average ozone concentrations exceeded 0.08 ppm. Data were not reported for years missing more than half the daily peak concentrations during the ozone season (typically May through September). From these monitors, The Heinz Center selected the 751 monitors that are located in urban and suburban areas (as defined for this report; see Area of Urban/Suburban Lands and associated technical note). Following EPA’s protocol for trend analysis, the trend graphs include only those monitors with data for at least 12 of 15 years (at least 75% of the total number of years) between 1990 and 2004. In addition, monitors were excluded if they had more than two consecutive years without data. 323 monitors meet the criteria for data completeness. For the maps, which provide the locations of monitors according to their 2004 values, 494 monitors had data. In Hawaii, there are three ozone monitors; however, we do not have satellite data on the extent of urban and suburban areas for this state. Therefore, it was not possible to identify the urban and suburban monitors in Hawaii in the same fashion as in the lower 48 states. As is discussed in the technical note for Area of Urban/Suburban Lands, there is reasonable overlap between the urban and suburban areas defined using satellite data and Census Bureau Block Groups having at least 1000 people per square mile. For this reason, we identified Hawaiian monitors located in Block Groups having a density of at least 1000 people per square mile. Two of the Hawaiian monitors passed this screen; ultimately one of these was dropped due to insufficient data. There is a single monitor in Alaska and following the method used for Hawaii, it was excluded from our analysis because it is not within a Block Group having at least 1000 people per square mile.

Data Quality and Caveats: The monitors that make up this national network conform to uniform criteria for monitor siting, instrumentation, and quality assurance.

Data Access: Air quality data upon which this indicator is based are collected regularly by EPA and are available at http://www.epa.gov/airs/. EPA provided the specific data used in this analysis to The Heinz Center especially for this project. However, annual summary monitoring data are available at EPA’s AIRData Web site (http://www.epa.gov/air/data/index.html).

2003 Web Site Update: data were provided by the EPA. Because these data are continually updated and revised, the analysis was repeated for the entire time period 1990-2002, even though 1990-1999 had been done for the 2002 Report.

2005 Web Site Update: data were provided by the EPA for 1990 through 2004. As noted for Update 2003, data are continually revised for the entire time period; differences with previously published data are small. Note that the data presented as part of this update excluded data that had been flagged at the state level for being unreliable and the relevant regional EPA office concurred that the flagged data were, in fact, unreliable.