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About the Monitor Locator Map - Criteria Air Pollutants
What does the map tell me?
A Monitor Locator Map shows the locations of air monitoring sites that measure ambient concentrations of criteria air pollutants that you choose. The map has a symbol at the latitude-longitude coordinates of each air monitor. The shape of the map symbol indicates the type of monitor, and the map legend tells how many monitors of each type are in the map.
What does the map look like?
This image shows a Monitor Locator Map of California. Each shaded county has at least one monitoring site for the criteria air pollutants selected. Marker symbols on the map indicate where sites are located and what pollutant they measure. The map legend shows the symbol-pollutant associations, and how many monitors there are for each pollutant. This map also shows optional geographic features: Class I areas (outlined in green) and major cities.
How can I customize the map?
To generate a Monitor Locator map, choose at least one pollutant. You may also choose optional criteria to refine data selection and add geographic features to the map. The selection criteria are described below.
- Geographic Area
- The name of the geographic area previously selected is displayed
here. You can click on "change" to choose a different area.
- Pollutant (required)
- Selects monitoring sites based on the criteria air pollutants they
measure and report to EPA. Choose at least one pollutant. (You may
choose more than one.)
- Monitor Type (optional)
- Selects air monitors based on the purpose for which they were
established or the monitoring network to which they belong. You may select
multiple monitor types. If you do not select this option, monitor type
plays no role in determining which monitoring sites to include in a map.
- Year Monitor Active (optional)
- Selects monitors that reported data in a particular year. You may select up
to eleven years. If you do not select a year, the map includes monitors that
reported data in the latest year for which data are available.
- Above Air Standard (optional)
- Selects only monitoring sites that reported an air pollutant
concentration exceeding the level of a national ambient air quality standard
in the year indicated by Year Monitor Active. (You must select a Year Monitor
Active in conjunction with this option.) If you do not select this option,
monitoring sites are included in the map irrespective of reported pollutant
concentrations.
National ambient air quality standards have two forms: concentration and frequency. Concentration-based standards commonly apply to a long-duration value such as yearly average pollutant concentration. The average value may not exceed a specified level. Frequency-based standards usually apply to a short-duration measure such as pollutant concentration for one hour or one day. No more than one value per year may exceed a specified level. The above air standard option selects monitoring sites that failed either type of standard for a single year. This criterion is not the same as determining attainment of an air quality standard, which involves multi-year averaging of concentrations and exceedance counts. A monitoring site selected by the above air standard option is not necessarily failing to attain national air quality standards.
- Exceptional Data (optional)
- "Exceptional" data are high pollution levels caused by an unusual
event such as a wildfire. EPA regulations allow these data, if properly
documented and approved by EPA, to be excluded from the determination
whether a community met Federal air quality standards. About 6% of monitoring
sites report exceptional data in a given year, and EPA approves (excuses)
about one-third of the exceptional data, or about 2% of all sites.
You may select this option only in conjunction with the above air standard option. Together, the two options select monitors whose data exceeded the level of an air quality standard, including exceptional data approved (excused) by EPA. If you do not select this option, exceptional data approved by EPA are not used to select monitors whose data exceeded the level of an air quality standard. In either case, monitors are included whose "normal" data and unapproved exceptional data exceeded the level of an air quality standard.
- Geographic Features (optional)
- Enhances a map with features such as city locations, highways,
and national park outlines. Without this option selected, the map
includes standard features: markers indicating monitor locations,
and outlines of geographic areas.