CASTNET Ozone Monitoring
CASTNET measures rural, ground-level ozone at more than 85 locations throughout the United States. The hourly data are used by the Agency and external stakeholders to answer important scientific and policy questions. Examples of how the CASTNET ozone data are used include:
- evaluating and assessing compliance with the National Ambient Air Quality Standard (NAAQS),
- providing real-time air quality information through AirNow,
- validating chemical transport models that are then used to evaluate proposed regulations,
- validating satellite measurements,
- determining air quality impacts from wildfires and other exceptional events,
- assessing damage to trees, vegetation, and crops based on exposure (W126), and
- establishing linkages between air quality and climate change.
CASTNET ozone data fill spatial gaps in the nation's air quality networks by providing measurements in rural locations where monitoring is often sparse. These data are used to determine if an area meets or exceeds the NAAQS, offering rural and tribal communities with information to make informed decisions about policies that will protect people and the environment. The figure below shows the most recent 3-year design value for CASTNET sites that met the data completeness requirements and the W126 values which provide an estimate of whether ozone exposure has likely caused damage to vegetation.
The map contains two layers that can be turned on and off in the legend (top left corner). Only one ozone layer will be visible at a time. The circles represent the CASTNET 2019-2021 three-year average of the 4th highest 8-hour daily maximum ozone concentration (ppb). The diamonds represent the cumulative W126 exposure index (ppm-hours) from 2021. Ozone concentrations at individual site locations are calculated from ambient measurements. The pop-ups at the individual site locations include an arrow in the top right corner to see more information about the location depending on which layers are selected.
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. Every three years the program participates in a TSA conducted by an independent auditor. The final audit reports and contractor responses to the findings can be found on the Documents page.