About the County Air Quality Map - Criteria Air Pollutants

What does the map tell me?

A County Air Quality Map shows which counties conducted monitoring for an air pollutant, and the peak values they measured during a year. Map shading indicates value ranges; positioning the mouse pointer on a county shows its specific value as a tool tip. A bar chart beside the map shows the aggregate population of counties in each value range and the total population within the geographic area of the map. Positioning the mouse pointer on a bar segment shows the aggregate population of counties in that value range as a tool tip.

Air monitoring data for the map come from EPA's Air Quality System (AQS database). The data pertain to ambient concentrations of criteria air pollutants. County population data are from the 2000 census.

To generate a County Air Quality map, choose a monitoring value and a year. Monitoring value choices are discussed below.

What does the map look like?

The example County Air Quality map shows 2004 ozone concentrations for California counties. Shading colors indicate whether the highest ozone value reported by monitoring sites in each county is below the level of the national air quality standard (blue and green shades) or above it (orange). The value that determines shading is the second-highest 1-hour average ozone concentration measured at each site. The bar chart beside the map shows how the population of California counties is distributed in the ranges of ozone concentrations listed in the map legend. Most people live in counties in which the peak ozone value was below the level of the air quality standard.

How can I customize the map?

Geographic Area: The name of the geographic area previously selected is displayed here. You can click on "change" to choose a different area.
Monitoring Value: This option determines which air pollution value is displayed in a map. The first choice (AQI) is an overall indicator or air quality. The other choices pertain to criteria air pollutants and their National Ambient Air Quality Standards (NAAQS). See Monitoring values and related air quality standards, below, for descriptions of these choices.
Year: The calendar year of monitoring data displayed in the map. Choose one of the available years.
Exceptional Data: Include "exceptional" data, which 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. In other words, this option generally affects 2% of monitoring sites. This option does not affect AQI values, which always include exceptional data.

Monitoring values and related air quality standards

EPA uses annual summary measures of air pollution to gauge compliance with air quality standards established by the Clean Air Act. The standards are framed in terms of different summary measures for each criteria air pollutant. Additionally, EPA defined the Air Quality Index (AQI) as an indicator of daily overall air quality, based on ambient concentrations of the criteria air pollutants. AQI associates a numerical value and a health category with each day's air pollution levels.

The County Air Quality map displays each county's value of a standards-related air pollution annual summary measure or an AQI-related value. A county's value is the highest of all monitoring sites in the county reporting data during the year. Map shading patterns indicate whether a county's value is below or above the level of the applicable air quality standard.

Several of the County Air Quality map's monitoring value choices involve a "2nd highest" value. Why not show the highest value? The second-highest value is used when the applicable air quality standard allows the measured pollutant concentration to be higher than the value set in the standard one time per year. In these cases, the second-highest value indicates whether the level of the standard was exceeded.

The units of measure for pollutant concentrations are:

Parts per million by volume (ppm)
Micrograms per cubic meter of air (µg/m³)

The monitoring value choices for a County Air Quality map are listed below, grouped by pollutant.

AQI - Air Quality Index

Number of days Air Quality Index (AQI) was above 100: Total number of days in the year when the AQI category was unhealthy for sensitive groups, unhealthy, very unhealthy, or hazardous. For more information about AQI categories, see the AQI brochure.

CO - Carbon Monoxide

CO - 2nd highest 1-hour average concentration: Second-highest 1-hour average concentration for the year. This value, rounded to the nearest 1 ppm, should not exceed the 1-hour standard (35 ppm).
CO - 2nd highest 8-hour average concentration: Second-highest nonoverlapping 8-hour concentration for the year. This value, rounded to the nearest 1 ppm, should not exceed the 8-hour standard (9 ppm). AQS software computes an 8-hour average concentration for each hour of the day as a moving average of eight 1-hour values. Nonoverlapping means that the highest and second-highest 8-hour values do not have any hours in common - they are separated in time by at least eight hours.

NO2 - Nitrogen Dioxide

NO2 - Annual mean of 1-hour average concentrations: Arithmetic average of all 1-hour values for the year. This value should not exceed the annual standard (0.053 ppm).

O3 - Ozone

O3 - 2nd highest 1-hour average concentration: The second-highest "daily max value" -- take the highest 1-hour value of each day, pick the second-highest of those values. This value, rounded to the nearest 0.01 ppm, should not exceed the 1-hour standard (0.12 ppm).
In June 2005 EPA revoked the ozone 1-hour standard except in 13 Early Action Compact areas.
O3 - 4th highest 8-hour average concentration: The fourth-highest "daily max value" -- take the highest 8-hour value of each day, pick the fourth-highest of those values. This value, rounded to the nearest 0.001 ppm, should not exceed the 8-hour standard (0.075 ppm). AQS software computes an 8-hour value for each hour of the day as a moving average of eight 1-hour values.

In March 2008 EPA strengthened the ozone 8-hour standard. The 2008 standard applies retrospectively to monitoring data for prior years. Some locations that previously met the ozone standard may exceed the level of the 2008 standard.

SO2 - Sulfur Dioxide

SO2 - 2nd highest 24-hour average concentration: Second-highest 24-hour average concentration for the year. This value, rounded to the nearest 0.01 ppm, should not exceed the 24-hour standard (0.14 ppm). AQS software computes a midnight to midnight 24-hour average value for each day from 1-hour values.
SO2 - Annual mean of 24-hour average concentrations: Arithmetic average of all 1-hour values for the year. This value should not exceed the annual standard (0.030 ppm).

PM2.5 - Particulate Matter smaller than 2.5 micrometers

PM2.5 - 98th percentile of 24-hour average concentrations: The 98th percentile 24-hour value. This value, rounded to the nearest 1 microgram, should not exceed the 24-hour standard (35 µg/m³). The 98th percentile value is higher than 98 percent of 24-hour values for the year.

In December 2006 EPA strengthened the PM2.5 24-hour standard. The 2006 standard applies retrospectively to monitoring data for prior years. Some locations that previously met the PM2.5 standard may exceed the level of the 2006 standard.
PM2.5 - Annual Mean of 24-hour average concentrations: Arithmetic mean of 24-hour values for the year. This value, rounded to the nearest 0.1 microgram, should not exceed the annual standard (15.0 µg/m³).

PM10 - Particulate Matter smaller than 10 micrometers

PM10 - 2nd highest of 24-hour average concentration: Second-highest 24-hour value for the year. This value, rounded to the nearest 10 µg/m³, should not exceed the 24-hour standard (150 µg/m³).
PM10 - Annual mean of 24-hour average concentrations: Weighted arithmetic mean of 24-hour values for the year. The method of calculation compensates for scheduled sampling that did not occur.

In December 2006 EPA revoked the PM10 annual standard due to a lack of evidence linking health problems to long-term exposure to coarse particle pollution. Before its revocation, the level of the annual standard was 50 micrograms per cubic meter of air.

Pb - Lead

Pb - Highest quarterly mean of 24-hour average concentrations: Highest of the quarterly mean values. This value, rounded to the nearest 0.1 µg/m³, should not exceed the quarterly standard (1.5 µg/m³). Each quarterly mean is the arithmetic average of 24-hour values for a calendar quarter: January-March (1), April-June (2), July-September (3), and October-December (4).