Ozone: Monitoring of ambient air ozone concentration began in April 2008 with the installation of a Model 202 Ozone Monitor at the Santee Headquarters weather station. 15-minute average ozone (parts per billion by volume, or ppbv), 15-minute maximum ozone (ppbv), 15-minute minimum ozone (ppbv), and the standard deviation of the 15-minute average ozone measurement (ppbv) data are available.