Long-term ozone trends at rural ozone monitoring sites across the United States, 1990-2010

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. Accordingly 83%, 66% and 20% of summertime eastern US sites experienced statistically significant ozone decreases in the 95th, 50th and 5th percentiles, respectively. During spring 43% of the eastern sites have statistically significant ozone decreases for the 95th percentile with no sites showing a significant increase. At the 5th percentile there is little overall change in the eastern US. In contrast, only 17% and 8% of summertime western US sites have statistically significant ozone decreases in the 95th and 50th percentiles, respectively. During spring no western site has a significant decrease, while 50% have a significant median increase. This dichotomy in US ozone trends is discussed in terms of changing anthropogenic and biomass burning emissions. Consideration is 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 are examined in relation to regional temperature trends.