First multi-site assessment of tropospheric baseline ozone along the U.S. west coast

Monitoring of the full tropospheric (sea level to tropopause) baseline ozone flowing into the western United States from the North Pacific Ocean has been limited to the weekly ozonesondes launched from Trinidad Head in northern California since 1997. Profiles from other locations along the coast have been too sparse to indicate whether or not Trinidad Head is representative of baseline ozone at different latitudes. To explore this question, the IONS-2010 ozonesonde network was implemented during a six-week period (May 10 - June 19, 2010) of the CalNex experiment. Near-daily ozonesonde profiles from 4 sites spanning 870 km of the California coast show that in the lowest km of the atmosphere ozone arriving at Trinidad Head is similar to the rest of the California coast. But above this level the sites differ at many altitudes, with a prominent ozone enhancement that slopes from the mid-troposphere above northern California down to the lower free troposphere above southern California. FLEXPART retroplumes from every profile indicate the regions of the coastal troposphere that experience relatively strong influence from the stratosphere, the remote marine boundary layer or the industrialized regions of Asia. FLEXPART forward plumes reveal the surface regions of North America impacted by the greatest mixing ratios of baseline ozone. Finally, comparison of the coastal profiles to ozone measurements made from the NOAA WP-3D aircraft and two inland ozonesonde sites shows that within California, the short-term (< 5 days) influence of western North America on tropospheric ozone extends from the surface to approximately 3 km above sea level.