Ground-level ozone is measured at CASTNET sites following the guidelines in 40 Code of Federal Regulations (CFR) Parts 50 and 58. Continuous measurements are averaged to hourly concentrations. Data are reported to AirNow and EPA's Air Quality System (AQS) database routinely. There are more than 80 CASTNET sites reporting hourly ozone concentrations throughout the United States. Site location information is included with this dataset. The QAPP and associated SOPs can be found on the CASTNET website: www.epa.gov/castnet/documents-reports. Citation information for this dataset can be found in the EDG's Metadata Reference Information section and Data.gov's References section.