CASTNET Ozone Monitoring
CASTNET measures ground-level ozone at more than 85 rural sites throughout the United States. In 2011, EPA upgraded all CASTNET ozone equipment and enhanced the quality assurance program to comply with the requirements in 40 Code of Federal Regulations (CFR) Part 58. CASTNET ozone data are submitted to AIRNow Tech, posted on the CASTNET website, and submitted to the EPA's Air Quality System (AQS). These data are used to determine if an area meets, or exceeds, the National Ambient Air Quality Standard (NAAQS). In addition to supporting the NAAQS, CASTNET ozone data are frequently used to assess background ozone concentrations, determine impacts from wildfire and other exceptional events, validate satellite measurements, and evaluate damage to crops and vegetation due to ozone exposure.
CASTNET 2018-2020 three-year average of the 4th highest 8-hour daily maximum ozone concentration. Click here to open a larger version of the map. The map was created using inverse distance weighting (IDW) to create an interpolated surface between monitoring sites. The result provides a visual representation of spatially interpolated ozone concentrations. This is intended to illustrate regional patterns of ozone concentrations. Individual grid values between monitoring locations may not reflect actual concentrations and should not be used as a measurement surrogate. The interpolated layer can be turned off in the legend. Ozone concentrations at individual site locations are calculated from ambient measurements.
The CASTNET ozone analyzers undergo nightly zero, span, and precision checks to quickly diagnose any problems with the system. Data review is performed daily by an automatic screening system.
All on-site transfer standards are certified Level III, meaning they have been calibrated by a Level II standard. The Level II transfer standards are used to calibrate the on-site ozone transfer standards twice per year. The Level II transfer standards are calibrated once per year at NIST or at one of the EPA regional laboratories by a Standard Reference Photometer (SRP), otherwise known as a Level I standard.
Every CASTNET ozone monitor within the network is audited once per year by an independent auditor who completes a Performance Evaluation (PE). The PE results are required to be submitted to AQS before annual data can be certified. A monitoring agency may perform an independent PE at a CASTNET site if they follow the procedures outlined in the Third Party Audit Guidelines(2 pp, 68 K, About PDF) document. In addition, each year 20% of the network participates in the National Performance Audit Program (NPAP). State, local, and tribal agencies participate in the NPAP to provide consistency in the data across all monitoring organizations.
Technical Systems Audit
Every three years the reporting agency is required to participate in a Technical Systems Audit (TSA) to verify the monitoring program complies with the established regulations in the Code of Federal Regulations (40 CFR Part 50, 53, and 58). In late 2012, the EPA CASTNET contractor, Wood, participated in the first CASTNET program TSA for regulatory ozone monitoring. ARS, Inc., the NPS and BLM-WSO CASTNET contractor, participated in a TSA at their facility in 2013. The final audit reports and contractor responses to the findings can be found on the Documents page.