Air Monitoring Station CAP Data Rules and Calculations


The ECATT Air Monitoring Stations Search for Criteria Air Pollutants (CAPs) captures data from a network of U.S. ambient air monitoring stations for Criteria Air Pollutants. Criteria air pollutants are six common air pollutants (particulate matter, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead), that have National Ambient Air Quality Standards (NAAQS) set by EPA, as required by the U.S. Clean Air Act. An air monitoring station is a physical monitor permanently placed at a location to collect air quality data.

Data owners (typically states, local, and federal agencies) are responsible for maintaining the air monitoring stations and uploading the data to EPA’s official database for ambient monitoring data known as the Air Quality System (AQS). EPA encourages state, local, and tribal air agencies to upload all monitoring data, but only programs receiving EPA funding are required to submit data to AQS. These programs include EPA’s National Air Toxics Trends System (NATTS), the Urban Air Toxics Monitoring Program (UTAMP), and community-scale air toxics monitoring grant sites.

EPA uses CAPs data from AQS for the ECATT Air Monitoring Stations Search Results and Report.

AQS Data Acquisition

Annual Statistics and Handling Non-Detects

EPA uses annual statistics CAPs data from AQS pre-generated annual data files. These files have an established method for handling non-detected measurements. The calculation methodology for these statistics and non-detects are found in the Annual Summary File Documentation.

Measurements are selected for inclusion in the air monitoring stations calculations based on the following criteria (in order of priority). A unique record (i.e., annual measurement in ECATT for CAPs data) is defined by:

  • AQS Site Code: A 9-digit unique identifier for the air monitoring station created based on a combination of the State code + County code + Site code.
  • Parameter Code: Identifies the pollutant and type of measurement taken by the monitor
  • Pollutant Standard: Identifies the pollutant-specific NAAQ standard with which the measurement was taken by the monitor
  • Reporting Year: The calendar year when the sample was taken
  • Parameter Occurrence Code (POC): Distinguishes measurements of the same parameter at the same site using multiple instruments. Take the measurements with lower POC for multiple measurements taken for a parameter at the same location for the same year.
  • Completeness Indicator: Indicates whether the regulatory data completeness criteria for valid summary data had been met by the monitor for the year. ECATT only uses AQS annual summary data where AQS has indicated that the data meet the completeness criteria for the parameter.

Multi-Year Statistics

A Spearman Correlation is run across all years for a single AQS site, parameter, and POC. It is used to indicate whether there is an overall increasing or decreasing trend in average concentration for the selected year range.

Daily Data

Data from the AQS Daily API are pulled to create the time series chart in the ECATT Air Monitoring Station report.