Tropospheric ozonesonde profiles at long-term U.S. 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 (O₃) 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 O₃. This makes defining a "background" O₃ 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 O₃ 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 O₃ mixing ratio profile clusters exhibit thin laminae of high O₃ above Trinidad Head. A third, consisting of background (~20 - 40 ppbv) O₃, occurs in ~10% of profiles. The high O₃ 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 O₃ sources (reanalyses, trajectories, remotely-sensed carbon monoxide), but distinguishing chemical and stratospheric influences of the elevated O₃ is difficult. There is marked and long-lasting impact of the elevated tropospheric O₃ on high-altitude surface O₃ monitors at Lassen Volcanic and Yosemite National Parks, and Truckee, CA. Days corresponding to the high O₃ clusters exhibit hourly surface O₃ anomalies of +5 - 10 ppbv compared to a climatology; the anomalies can last up to four days. The profile and surface O₃ links demonstrate the importance of regular ozonesonde profiling at Trinidad Head.