Seminar

Abstract

This analysis provides an up-to-date assessment of long term (1990-2010) rural ozone trends using all available data in the western and eastern USA. Rather than focus solely on average ozone values or violations of the National Ambient Air Quality Standard, the study considers the full range of ozone values, reporting trends for the 5th, 50th and 95th ozone percentiles. Domestic ozone precursor emissions decreased strongly during the study period. In response, and in agreement with many recent modeling studies, rural ozone has decreased across much of the eastern US, especially at the 95th percentile and during summer. Modeling studies also conclude that decreasing domestic ozone precursor emissions in the western US should also result in ozone decreases. However, ozone decreases in the rural western US are limited, with most sites showing steady ozone levels or increasing ozone, especially in spring. This dichotomy in US ozone trends will be discussed in terms of changing anthropogenic and biomass burning emissions. Consideration will be given to the concept that increasing baseline ozone flowing into the western US is counteracting ozone reductions due to domestic emission reductions. An update to the springtime free tropospheric ozone trend above western North America shows that ozone has increased significantly from 1995-2011 at the rate of 0.41 ± 0.27 ppbv yr^-1. Finally, the ozone changes will be examined in relation to regional temperature trends.