The 8-hour average values above were calculated using all monitoring data. In many instances, the highest hourly average concentrations occur outside of the EPA-defined "ozone season." The Yellowstone National Park site (see blue arrow above) is heavily influenced by natural sources of ozone. The highest hourly average concentrations occur at this site during the springtime. Many of the sites shown above are also affected by natural background ozone. Note that the 8-hour average values are much higher than EPA anticipated for those sites whose values are heavily influenced by natural background. For more information, please email us.