Measurement of western US baseline ozone from the surface to the tropopause and assessment of downwind impact regions

Since 1997, baseline ozone monitoring from the surface to the tropopause along the U. S. west coast has been limited to the weekly ozonesondes from Trinidad Head, California. To explore baseline ozone at other latitudes, an ozonesonde network was implemented during spring 2010, including four launch sites along the California coast. Modeling indicated that North American pollution plumes impacted the California coast primarily below 3 km, but had no measurable impact on the average coastal ozone profiles. Vertical and latitudinal variation in free tropospheric baseline ozone appears to be partly explained by polluted and stratospheric air masses that descend isentropically along the west coast. Above 3 km, the dominant sources of ozone precursors were China and international shipping, while international shipping was the greatest source below 2 km. Approximately 8-10% of the baseline ozone that enters California in the 0-6 km range impacts the surface of the USA, but very little reaches the eastern USA. Within California, the major impact of baseline ozone above 2 km is on the high elevation terrain of eastern California. Baseline ozone below 2 km has its strongest impact on the low elevation sites throughout the state. To quantify ozone production within California we compared inland ozone measurements to baseline measurements. For average daytime conditions, we found no enhancements of lower tropospheric ozone in the northern Central Valley, but enhancements of 12-23% were found in the southern Central Valley. Enhancements above Joshua Tree were greater, 33-41%, while the greatest enhancements occurred over the LA Basin, 32-63%.