About AirData

Overview

The AirData Web site gives you access to yearly summaries of United States air pollution data, taken from EPA's air pollution databases. The data include all fifty states plus District of Columbia, Puerto Rico, and the U. S. Virgin Islands. AirData has information about where air pollution comes from (emissions) and how much pollution is in the air outside our homes and work places (monitoring).

Types of Data

Emissions

Emissions -- the quantity of pollutants released into the air during a year -- normally are estimated from amounts of material consumed or product produced. Most emissions estimates are provided to EPA by state environmental agencies. Some estimates are for individual sources, such as factories, and some estimates are county totals for classes of sources, such as vehicles. Emissions estimates for individual sources are based on their normal operating schedules, and take into account the effects of installed pollution control equipment and of regulatory restrictions on operating conditions. This is called "estimated emissions with rule effectiveness."

Monitoring

Ambient concentrations of pollutants in outdoor air are measured at more than 4000 monitoring stations owned and operated mainly by state environmental agencies. They forward the hourly or daily measurements of pollutant concentration to EPA's database, and EPA computes a yearly summary for each monitoring station (maximum value, average value, number of measurements, etc.). AirData has the yearly summary values only, and not the individual hourly or daily measurements.

Types of Air Pollutants

Criteria Air Pollutants

The Clean Air Act of 1970 defined six criteria pollutants and established ambient concentration limits to protect public health. EPA periodically has revised the original concentration limits and methods of measurement, most recently in 1997.

Monitoring sites report data to EPA for these six criteria air pollutants:

Carbon monoxide (CO)
Nitrogen dioxide (NO2)
Ozone (O3)
Sulfur dioxide (SO2)
Particulate matter (PM10 and PM2.5)
Lead (Pb)

(PM10 and PM2.5 are acronyms for particulate matter consisting of particles smaller than 10 and 2.5 micrometers, respectively.)

You might expect that EPA would track emissions of the same six criteria air pollutants. But ozone is not emitted directly; it forms by chemical reactions of organic compounds with nitrogen oxides in the air, mediated by sunlight. Lead is both a criteria air pollutant and a hazardous air pollutant, and EPA tracks emissions of lead only as a hazardous air pollutant. Ammonia reacts with nitric and sulfuric acids in the atmosphere to form fine particulate matter, so EPA tracks ammonia emissions.

Thus, EPA collects emissions data for three criteria air pollutants:

Carbon monoxide (CO)
Sulfur dioxide (SO2)
Particulate matter (PM10 and PM2.5)

and three precursors/promoters of criteria air pollutants:

Volatile organic compounds (VOC)
Nitrogen oxides (NOx)
Ammonia (NH3)

Hazardous Air Pollutants (HAPs)

The Clean Air Act amendments of 1990 listed 189 pollutants known or suspected to cause serious health problems, and directed EPA to establish emission limits for them. The Act also provided a mechanism for amending the original list of pollutants, based on new information about health and environmental effects. There are now 188 hazardous air pollutants, which also are known as toxic air pollutants or air toxics.

Monitoring of ambient concentrations of HAPs is not mandated by the Clean Air Act, and monitoring is not the norm. EPA is developing regulations to limit HAP emissions, thereby preventing ambient HAP concentrations from reaching levels that would pose significant health risks.

AirData has reports of HAP emissions, but no HAP monitoring reports. The Monitor Query section of AirData has HAP monitoring data, but monitoring stations are sparsely distributed geographically, compared with criteria air pollutant monitoring.

Air Pollution Databases

AirData draws information from two EPA databases.

NEI Database - Emissions Data

EPA's National Emission Inventory (NEI) database contains detailed information about sources that emit criteria air pollutants and their precursors, and hazardous air pollutants. An extract of the database for AirData includes estimates of annual air pollutant emissions from point, nonpoint, and mobile sources in the 50 States, the District of Columbia, Puerto Rico, and the Virgin Islands. EPA conducts a national inventory of air pollutant emissions at three-year intervals, and adds the new data to the National Emission Inventory database. Between inventories, EPA refines and corrects the emissions data, and updates the database several times. Data are extracted for use in AirData approximately once per year. The database status page gives the date of the latest AirData update.

The National Emission Inventory database is the successor of two separate databases for criteria air pollutant emissions (the National Emission Trends database, or NET) and hazardous air pollutants (the National Toxics Inventory database, or NTI). With the National Emission Inventory database, EPA changed the terminology for some categories of air pollution sources, and changed the methodology for estimating some emissions. The NEI database page has more details about those changes. AirData reports include some of the old terminology.

AQS Database - Monitoring Data

The Air Quality System (AQS) database is EPA's repository of criteria air pollutant monitoring data since the 1970s. Considerably less data for hazardous air pollutants have been collected, mostly after 1995. AirData reports and maps present a subset of criteria air pollutant data extracted from AQS. The Monitor Query section of AirData has a wider array of air monitoring values for both criteria and hazardous air pollutants, but in a more technical format.

The AQS database is updated at least weekly. Most states submit their air monitoring data monthly, with different groups of states submitting data during any given week. After the individual measurements of hourly of daily pollutant concentrations have been stored in the AQS database, yearly summary values (the highest values, the average, etc.) are computed and stored for each air monitor. Only these summary values are extracted from the AQS database and presented in AirData reports and queries. The individual hourly and daily monitoring values are not available at this AirData Web site. You may obtain compressed files of hourly and daily monitoring values for some air pollutants at the AQS Web site. The related links Web page lists other sources of hourly and daily air monitoring values.

Around the beginning of each month, criteria air pollutant summary values for the current year and ten previous years are extracted from the AQS database for presentation in AirData reports. The database status page gives the date of the latest AirData update.

Criteria and hazardous air pollutant monitoring summary data for 1971 through the current year have been extracted from the AQS database for presentation in the Monitor Query section of AirData. Current and revised data are extracted monthly. The last extraction date is listed at the bottom of the main Monitor Queries Web page.