Tropospheric ozonesonde profiles at long-term US monitoring sites: 2. Links between Trinidad Head, CA, profile clusters and inland surface ozone measurements

Much attention has been focused on the transport of ozone (O-3) to the western U.S., particularly given the latest revision of the National Ambient Air Quality Standard to 70 parts per billion by volume (ppbv) of O-3. This makes quantifying the contributions of stratosphere-to-troposphere exchange, local pollution, and pollution transport to this region essential. To evaluate free-tropospheric and surface O-3 in the western U.S., we use self-organizing maps to cluster 18 years of ozonesonde profiles from Trinidad Head, CA. Three of nine O-3 mixing ratio profile clusters exhibit thin laminae of high O-3 above Trinidad Head. The high O-3 layers are located between 1 and 6 km above mean sea level and reside above an inversion associated with a northern location of the Pacific subtropical high. Ancillary data (reanalyses, trajectories, and remotely sensed carbon monoxide) help identify the high O-3 sources in one cluster, but distinguishing mixed influences on the elevated O-3 in other clusters is difficult. Correlations between the elevated tropospheric O-3 and surface O-3 at high-altitude monitors at Lassen Volcanic and Yosemite National Parks, and Truckee, CA, are marked and long lasting. The temporal correlations likely result from a combination of transport of baseline O-3 and covarying meteorological parameters. Days corresponding to the high O-3 clusters exhibit hourly surface O-3 anomalies of +5-10 ppbv compared to a climatology; the positive anomalies can last up to 3 days after the ozonesonde profile. The profile and surface O-3 links demonstrate the importance of regular ozonesonde profiling at Trinidad Head.