Much attention has been focused on the transport of ozone (O) to the Western U.S., particularly given the latest revision of the National Ambient Air Quality Standard (NAAQS) to 70 parts per billion by volume (ppbv) of O. This makes defining a "background" O amount essential so that the effects of stratosphere-to-troposphere exchange and pollution transport to this region can be quantified. To evaluate free-tropospheric and surface O in the Western U.S., we use self-organizing maps to cluster 18 years of ozonesonde profiles (940 samples) from Trinidad Head, CA. Two of nine O mixing ratio profile clusters exhibit thin laminae of high O above Trinidad Head. A third, consisting of background (~20 - 40 ppbv) O, occurs in ~10% of profiles. The high O layers are located between 1 and 4 km amsl, and reside above a subsidence inversion associated with a northern location of the semi-permanent Pacific subtropical high. Several ancillary data sets are examined to identify the high O sources (reanalyses, trajectories, remotely-sensed carbon monoxide), but distinguishing chemical and stratospheric influences of the elevated O is difficult. There is marked and long-lasting impact of the elevated tropospheric O on high-altitude surface O monitors at Lassen Volcanic and Yosemite National Parks, and Truckee, CA. Days corresponding to the high O clusters exhibit hourly surface O anomalies of +5 - 10 ppbv compared to a climatology; the anomalies can last up to four days. The profile and surface O links demonstrate the importance of regular ozonesonde profiling at Trinidad Head.
