Report on the Environment
Ambient Concentrations of Ozone
What are the trends in outdoor air quality and their effects on human health and the environment?
The above question pertains to all 'Outdoor Air' Indicators, however, the information on these pages (overview, graphics, references and metadata) relates specifically to "Ambient Concentrations of Ozone". Use the right side drop list to view the other related indicators on this question.
- Avoid spilling gasoline, and don't "top off" the tank.
- Check daily air quality forecasts
- Use paints, stains, finishes, and paint strippers that are water-based or low in volatile organic compounds.
- On ozone action days, refuel your vehicle after dusk.
- Avoid burning leaves, trash, and other materials that cause particle pollution when incinerated.
- Acid Deposition
- Air Toxics Emissions
- Ambient Concentrations of Benzene
- Ambient Concentrations of Carbon Monoxide
- Ambient Concentrations of Lead
- Ambient Concentrations of Manganese Compounds in EPA Region 5
- Ambient Concentrations of Nitrogen Dioxide
- Ambient Concentrations of Ozone
- Ambient Concentrations of Particulate Matter
- Carbon Monoxide Emissions
- Concentrations of Ozone-Depleting Substances
- Lake and Stream Acidity
- Lead Emissions
- Mercury Emissions
- Nitrogen Oxides Emissions
- Ozone Injury to Forest Plants
- Ozone Levels over North America
- Ozone and Particulate Matter Concentrations for U.S. Counties in the U.S./Mexico Border Region
- Particulate Matter Emissions
- Percent of Days with Air Quality Index Values Greater Than 100
- Regional Haze
- Sulfur Dioxide Emissions
- Volatile Organic Compounds Emissions
Click to enlarge exhibit
Ozone is a gas found in different parts of the atmosphere. Ozone in the upper atmosphere, or stratosphere, helps protect the Earth from the sun’s harmful rays. (The Ozone Levels over North America indicator describes trends in stratospheric ozone levels over the U.S.) In the lowest level of the atmosphere, the troposphere, ozone is harmful to both human health and the environment. For this reason, ozone is often described as being “good up high and bad nearby” (U.S. EPA, 2003a). Although some industrial sources release ozone directly into the environment, most ground-level ozone forms in the air from chemical reactions involving nitrogen oxides (NOx), volatile organic compounds (VOCs), and sunlight. Ozone levels are typically highest during the afternoon hours of the summer months, when the influence of direct sunlight is the greatest. These highest levels occur during what is known as the “ozone season,” which typically occurs from May 1 to September 30 but whose time frame varies by state (U.S. EPA, 2003b).
Variations in weather conditions play an important role in determining ozone levels. Daily temperatures, relative humidity, and wind speed can affect ozone levels. In general, warm dry weather is more conducive to ozone formation than cool wet weather. Wind can affect both the location and concentration of ozone pollution. NOx and VOC emissions can travel hundreds of miles on air currents, forming ozone far from the original emissions sources. Ozone also can travel long distances, affecting areas far downwind. High winds tend to disperse pollutants and can dilute ozone concentrations. However, stagnant conditions or light winds allow pollution levels to build up and become more concentrated.
Inhalation exposure to ozone has been linked to numerous respiratory health effects, including acute reversible decrements in lung function, airway inflammation, cough, and pain when taking a deep breath. Ozone exposure can aggravate lung diseases such as asthma, leading to increased medication use and increased hospital admission and visits to emergency rooms. In addition, evidence is highly suggestive that ozone directly or indirectly contributes to non-accidental and cardiopulmonary-related mortality, but the underlying mechanisms by which such effects occur have not been fully established (U.S. EPA, 2006). Although people with lung disease are most susceptible to the effects of ozone, even healthy people who are active outdoors can suffer from ozone-related health effects. Further, evidence suggests that older adults (more than 65 years old) appear to be at excess risk of ozone-related mortality or hospitalization (U.S. EPA, 2006). Elevated concentrations of ozone can also affect vegetation and ecosystems, as the Ozone Injury to Forest Plants indicator describes further (U.S. EPA, 2006).
This indicator presents ambient ground-level ozone concentrations in parts per million (ppm) from 1978 to 2009. Data are shown for 8-hour averaging times, based on continuous ozone monitoring data and consistent with this pollutant’s National Ambient Air Quality Standard (NAAQS). The 8-hour standard is indicative of exposures occurring over a sustained period of time (e.g., an outdoor worker’s exposure over the course of a work day). Trends for this indicator represent 173 sites in 135 counties nationwide that have data for the period of record in the State and Local Air Monitoring Stations network or by other special purpose monitors. The indicator also displays trends in ozone measurements in each EPA Region. This indicator’s exhibits display the corresponding NAAQS as a point of reference, but the fact that the national or regional concentrations fall below the standard does not mean that all monitoring sites nationally or in any EPA Region also are below the standard. The indicator displays trends in the number of the 173 sites nationwide at which ozone concentrations exceeded the 8-hour standard, but this statistic is not displayed for each EPA Region.
Trends in ozone concentrations can be difficult to discern because of the year-to-year variations in the concentrations. By presenting data for rolling 3-year time periods, this indicator smoothes out the “peaks” and “valleys” in the trend, making it easier to see the long-term trend. Three years is consistent with the 3-year period used to assess compliance with the ozone standards. For the 8-hour trends in this report, a 3-year average of the fourth highest daily maximum 8-hour concentration in each year is used to be consistent with the 8-hour ozone standard. The 3-year statistic is assigned to the last year in each 3-year period. For example, 1980 is based on 1978-1980 and 2006 is based on 2004-2006. Thus, when endpoint comparisons are used in this report to describe long-term changes (i.e., 1978-2006), they are based on the first 3-year period and the last 3-year period.
Between the 1978-1980 and 2007-2009 averaging periods, nationwide fourth highest daily maximum 8-hour ambient ozone concentrations decreased by 28 percent (Exhibit 2-13, panel A). Although the 8-hour ozone levels in 2007-2009 were the lowest on record and the number of trend sites measuring ozone concentrations above the level of the 8-hour NAAQS decreased by 45 percent over the time frame covered in this indicator (Exhibit 2-13, panel B), ambient air monitoring data collected up through 2009 and reported to EPA’s Air Quality System indicate that approximately 61.5 million people lived in counties where 8-hour average ozone concentrations are above the level of the primary ozone NAAQS (U.S. EPA, in press). Among the ten EPA Regions, the most substantial declines in 8-hour levels were observed in EPA Regions that originally had the highest ozone concentrations (EPA Regions 1 and 9) (Exhibit 2-14). Over nearly the entire period of record, Region 10 showed the lowest Regional ozone levels.
Also shown in Exhibit 2-13 (panel A) are the 90th and 10th percentiles based on the distribution of statistics at the monitoring sites. This provides additional graphical representation of the variability of measured concentrations across the monitoring sites for a given 3-year period. Thus, the graphic displays the concentration range where 80 percent of measured values occurred for that 3-year period.
In summary, despite reductions in ambient concentrations of ozone over the past quarter century and decreases in the emissions of ozone precursors since 1990 (the Nitrogen Oxides Emissions indicator; the VOC Emissions indicator), ozone remains one of the most persistent and ubiquitous air pollution issues in the U.S.
- Short-term trends in ozone concentrations are often highly dependent on meteorological conditions. This complicates efforts to interpret data for any given year. Air quality trends over the longer term are far less likely to be influenced by unusual meteorological conditions.
- Because most of the monitoring sites are located in urban areas, the trends might not accurately reflect conditions outside the immediate urban monitoring areas.
- Because of the relatively small number of trend sites in some EPA Regions, the regional trends are subject to greater uncertainty than the national trends. Some EPA Regions with low average concentrations may include areas with high local concentrations, and vice versa.
- To ensure that long-term trends are based on a consistent set of monitoring sites, selection criteria were applied to identify the subset of ozone monitoring sites with sufficient data to assess trends since 1978. Monitoring sites without sufficient data are not included in the trend analysis. Some excluded monitoring sites reported ozone concentrations above the level of the ozone standard over the time frame covered by this indicator. In 2009, for example, 148 sites recorded ozone concentrations above the level of the NAAQS: this includes the 86 trend sites shown in Exhibit 2-13, panel B, and 62 sites that did not have sufficient long-term data to be included in this indicator.
Summary data in this indicator were provided by EPA’s Office of Air Quality Planning and Standards, based on ozone ambient air monitoring data in EPA’s Air Quality System (U.S. EPA, 2010) (http://www.epa.gov/ttn/airs/airsaqs/). National and regional trends in this indicator are based on the subset of ozone monitoring stations that have sufficient data to assess trends since 1978.
U.S. EPA (United States Environmental Protection Agency). In press. Our nation’s air: status and trends through 2009. Research Triangle Park, NC.
U.S. EPA. 2010. Data from the Air Quality System. Accessed 2010. http://www.epa.gov/ttn/airs/airsaqs/
U.S. EPA. 2006. Air quality criteria for ozone and related photochemical oxidants. EPA/600/R-05/004aF-cF. Research Triangle Park, NC. http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=149923
U.S. EPA. 2003a. Ozone: Good up high, bad nearby. EPA/451/K-03/001. Washington, DC. http://www.epa.gov/oar/oaqps/gooduphigh/
U.S. EPA. 2003b. Latest findings on national air quality—2002 status and trends. EPA/454/K-03/001. Research Triangle Park, NC. http://www.epa.gov/air/airtrends/aqtrnd02/2002_airtrends_final.pdf
|Ambient Concentrations of Ozone|
|2.||ROE Question(s) This Indicator Helps to Answer|
|This indicator is used to help answer one ROE question: "What are the trends in outdoor air quality and their effects on human health and the environment?"|
This indicator presents nationwide ambient concentration trends for ground-level (tropospheric) ozone from 1978 to 2009. Tracking these trends is important because ozone, a primary component of smog, can trigger a variety of health effects and damage vegetation and ecosystems.
This indicator is based on 1978-2009 ambient air quality data retrieved from EPA's Air Quality System (AQS) in 2010. AQS data are direct measurements of pollutant concentrations at monitoring stations operated by tribes and state and local governments throughout the nation. EPA and other federal agencies also operate some air quality monitoring sites. For more information about the AQS, see http://www.epa.gov/ttn/airs/airsaqs/.
The complete set of ozone monitoring data used to prepare this indicator can be queried from the publicly available AQS database. Summary data in this indicator were provided by EPA's Office of Air Quality Planning and Standards, based on queries run on the raw ozone ambient air monitoring data in AQS (http://www.epa.gov/ttn/airs/airsaqs/). Underlying data are available at http://www.epa.gov/air/data/index.html. Information about AirData and AQS can be found at http://www.epa.gov/air/data/info.html and http://www.epa.gov/air/data/aqsdb.html.
This indicator is based on directly measured concentrations of ground-level (tropospheric) ozone using a set of standard methods (e.g., various ultraviolet photometric instruments). Thousands of monitoring sites report air quality data for one or more of the six criteria pollutants to the EPA's Air Quality System (AQS). The sites consist of National Air Monitoring Stations (NAMS), State and Local Air Monitoring Stations (SLAMS), and other special-purpose monitors. NAMS provide systematic and consistent urban area-oriented ambient monitoring whereas SLAMS allow state or local governments to develop networks tailored for their immediate monitoring needs. The NAMS/SLAMS network's focus is on providing data for assessing public health consequences of criteria pollutants. Pollutant-specific guidance for establishing NAMS/SLAMS networks is provided in 40 CFR 58, Appendix D. This monitoring network conforms to uniform criteria for monitor siting, instrumentation, and quality assurance. The results of EPA’s criteria pollutant monitoring program, from which this indicator originates, have been peer reviewed to assess that the underlying data are scientifically correct and technically adequate. The most recent review was as a part of U.S. EPA (2003).
This indicator reflects only those sites that met specific criteria for data completeness over the period of record (1978-2009). In all, 173 sites met the criteria for this analysis (see "Indicator Derivation"). Spatially, these sites are distributed across the nation, although they tend to be more heavily representative of larger urban areas, where monitoring is generally more extensive.
The ambient monitoring network is not designed to specifically represent sensitive populations or ecosystems. It is designed to represent specific areas, many of which are urban areas with a large population. Many of the sampled areas could be considered "sensitive" to ozone, because monitoring focuses on areas believed to have some of the highest ozone concentrations nationwide (e.g., non-attainment areas).
Ambient air monitoring methods are officially documented in (1) 40 CFR 50 — National ambient air quality standards (NAAQS) and reference methods for determining criteria air pollutant concentrations in the atmosphere; and (2) 40 CFR 53 — Process for determining reference or equivalent methods for determining criteria air pollutant concentrations in the atmosphere. For access to these documents and other information about ambient air monitoring, see http://www.epa.gov/ttn/amtic/. Physical methods for sampling and monitoring are specifically described in EPA’s Air Quality Criteria for Ozone and Related Photochemical Oxidants (http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_cr_cd.html) and in EPA's most recent list of approved reference and equivalent methods for criteria pollutant monitoring, which are posted and periodically updated on the AMTIC Web site (http://www.epa.gov/ttn/amtic/criteria.html).
The NAMS/SLAMS monitoring network and other aspects of survey design are documented in the following:
Additional information about monitoring methods and trends in ambient air concentrations of ozone can be found in various reports available from EPA's Office of Air and Radiation (see http://www.epa.gov/air/airtrends/reports.html) (e.g., U.S. EPA, 2003, 2004, 2008).
The conceptual model used to derive these indicators has been used and thoroughly reviewed as part of the Agency's national report on air quality trends for 31 years. For this indicator, monitoring sites were included in the trend analysis if they had at least 20 valid years of data during the 1978-2009 time frame and were not missing more than 2 consecutive years of data. A "valid" year was a year in which data were available for at least 50 percent of the days during the "ozone season," which varies by state but typically runs from May 1 to September 30.
Once sites were selected, missing annual summary statistics were estimated by simple linear interpolation from the surrounding years. Missing end points were replaced with the nearest valid year of data. The resulting data sets are statistically balanced.
This indicator reports ozone concentrations as fourth highest daily maximum 8-hour concentrations, which is based on the NAAQS. As described in EPA's Air Quality Criteria for Ozone and Related Photochemical Oxidants (http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_cr_cd.html), this is a standard averaging method which was determined to be appropriate for ozone based on a variety of factors (e.g., toxicological knowledge). To compute concentrations for display in the indicator, hourly values from a given site were combined into 8-hour running averages, and the fourth highest daily maximum 8-hour concentration for each year was selected for analysis. To be consistent with EPA's 8-hour NAAQS for ozone, the indicator reports the average of the fourth highest daily maximum 8-hour concentrations for three consecutive calendar years. Conceptual methods are documented in several EPA publications, including U.S. EPA (2003).
All sites were weighted equally in calculating the composite average trend statistics. No attempt was made to portray data beyond the temporal bounds of the data set. The statistical procedures used for this indicator comply with the recommendations of the Intra-Agency Task Force on Air Quality Indicators (U.S. EPA., 1981). This specific indicator could be reproduced by obtaining the publicly available data ("Data Availability") and then applying the data selection criteria and analytical methods outlined in this question.
|9.||Quality Assurance and Quality Control|
Comprehensive information on quality assurance and quality control (QA/QC) procedures is available on the Internet (http://www.epa.gov/ttn/amtic/qaqcrein.html) and through EPA's Quality Assurance Handbook (EPA-454/R-98-004, Section 15). There is a Quality Assurance Project Plan (QAPP) from each state or local agency operating a NAMS/SLAMS monitor meeting the EPA Requirements for Quality Assurance Project Plans, EPA QA/R-5. The quality assurance plans for specific sites are publicly available by request to the reporting agency or the corresponding EPA Regional Office; some of these may also be accessed online (see: http://www.epa.gov/ttn/amtic/plans.html). The plans are audited at least once every 3 years as required in 40 CFR 58, Appendix A, Section 2.5. In addition, the data repository (i.e., AQS) provides direct access to information about precision and accuracy, which are two of the more prominent quality assurance indicators (http://www.epa.gov/ttn/airs/airsaqs/padata/padata.htm).
The Clean Air Act (CAA) requires EPA to set NAAQS for widespread pollutants from numerous and diverse sources considered harmful to public health and the environment (general information about NAAQS can be found at http://www.epa.gov/ttn/naaqs/). The NAAQS standard for ground-level ozone used in this indicator is 0.075 parts per million (ppm) over an 8-hour averaging period (not to be exceeded by the 3-year average of the fourth highest daily maximum 8-hour average per year). This is considered a "primary" standard, meaning it is designed to protect public health including the health of "sensitive" populations (e.g., asthmatics, children, elderly persons). The same value also serves as the "secondary" standard, which is established to protect public welfare, including protection against visibility impairment, and damage to animals, crops, vegetation, and buildings. This standard is designed to protect against unhealthy exposures over the course of a workday (8-hour). The scientific basis for these particular standards is described in EPA's Air Quality Criteria for Ozone and Related Photochemical Oxidants (http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_cr_cd.html). The CAA requires periodic review of the science upon which the standards are based and the standards themselves. For this indicator, the fourth maximum 8-hour average for each 3-year time frame was compared against the level of the 8-hour NAAQS.
|11.||Comparability Over Time and Space|
The data presented in this indicator are viewed as highly comparable over both time and space because all data summarized in this indicator were collected using federal reference or equivalent methods, monitoring locations were selected according to strict and consistent siting criteria, and extensive quality assurance protocols must be followed.
|12.||Sources of Uncertainty|
Content under review.
|13.||Sources of Variability|
Year-to-year variability may be influenced by meteorological factors and changes in emission sources. However, this indicator has a long (32-year) period of record, which makes it a useful indicator of long-term, general trends in ambient ozone concentrations across the U.S. that are not overly influenced by shorter term fluctuations in meteorological conditions or emission events. By presenting data for rolling 3-year time periods, this indicator smoothes out the "peaks" and "valleys" in the short-term ozone signal, making it easier to see the long-term trend.
The indicator presents a time series of concentrations averaged across 173 monitoring stations. No special statistical techniques or analyses were used to characterize the long-term trends and their statistical significance.
Limitations to this indicator include the following:
U.S. EPA (United States Environmental Protection Agency). 2008. Latest findings on national air quality: status and trends through 2007. EPA-454/R-08-006. http://www.epa.gov/air/airtrends/2008/.
U.S. EPA. 2004. The ozone report—Measuring progress through 2003. EPA/454/K-04/001.
U.S. EPA. 2003. National air quality and emissions trends report, 2003 special studies edition. EPA/454/R-03/005. http://www.epa.gov/air/airtrends/aqtrnd03/.
U.S. EPA. 1981. Intra-Agency Task Force report on air quality indicators. EPA/450/4-81/015.