An Assessment of Ground Level and Free Tropospheric Ozone Over California and Nevada Journal of Geophysical Research: Atmospheres

Increasing free tropospheric ozone (O-3), combined with the high elevation and often deep boundary layers at western U.S. surface stations, poses challenges in attaining the more stringent 70 ppb O-3 National Ambient Air Quality Standard. As such, use of observational data to identify sources and mechanisms that contribute to surface O-3 is increasingly important. This work analyzes surface and vertical O-3 observations over California and Nevada from 1995 to 2015. Over this period, the number of high O-3 events (95th percentile) at the U.S. Environmental Protection Agency Clean Air Status and Trends Network (CASTNET) sites has decreased during summer, as a result of decreasing U.S. emissions. In contrast, an increase in springtime 5th percentile O-3 indicates a general increase of baseline O-3. During 2012 there was a peak in exceedances and in the average spring-summer O-3 mixing ratios at CASTNET sites. Goddard Earth Observing System-Chem results show that the surface O-3 attributable to transport from the upper troposphere and stratosphere was increased in 2013 compared to 2012, highlighting the importance of measurements aloft. Vertical O-3 measurements from aircraft, ozonesondes, and lidar show distinct seasonal trends, with a high percentage of elevated O-3 laminae (O-3 > 70 ppb, 3-8 km) during spring and summer. Analysis of the timing of high O-3 surface events and correlation between surface and vertical O-3 data is used to discuss varying sources of western U.S. surface O-3.