Publication Date:
2017-09-01
Description:
Increasing free tropospheric ozone (O 3 ), combined with the high elevation and often deep boundary layers at western US 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 (95 th percentile) at US EPA CASTNET sites has decreased during summer, as a result of decreasing US emissions. In contrast, an increase in springtime 5 th 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. GEOS-Chem results show that the surface O 3 attributable to transport from the upper troposphere and stratosphere were 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 US surface O 3 .
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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