Surface ozone is a recognized health hazard. In the US, domestic human-caused emissions of ozone precursors have declined since 1990. One would expect that the concentrations of ozone across the country would have decreased as well. However, in the western US, springtime ozone has increased significantly from 1995 to 2011. The increase is likely due to increasing emissions in Asia and their long-range transport to the western US.
In a newly published paper, Pao Baylon et al. examined baseline ozone, measurements of tropospheric ozone with a negligible influence from local emissions, at Mt. Bachelor Observatory (MBO), a high-elevation site in central Oregon. In Spring 2012, they observed an increase in ozone at MBO and at other sites in the western US compared to previous years. They showed that this increase was due to enhanced upper troposphere/lower stratosphere (UT/LS) events. In addition, they found that, in Spring 2012, several sites measured ozone concentrations above the National Ambient Air Quality Standard (NAAQS) that were influenced by ozone transported from the UT/LS.
Because of the influence of ozone transported downwind from the UT/LS, these surface sites experienced an increase in the number of days when their maximum daily 8-hour average (MDA8) ozone mixing ratios exceeded the standard. Under current standards, high-ozone events such as UT/LS episodes and Asian long-range transport events could affect the attainment status of a monitoring site if these episodes are not identified as exceptional events, which the EPA defines as an uncontrollable event that affected air quality. Understanding the nature and the year-to-year variability of exceptional events is therefore critical for effective implementation of the US NAAQS.
Click here to read the full Environmental Science & Technology paper.
Caption: Ozone (O3) from the upper troposphere/lower stratosphere (UT/LS) is measured at Mt. Bachelor Observatory, a mountaintop site. Some of this ozone is transported downwind to surface sites. The ozone measured at these surface sites includes ozone resulting from local emissions and transported ozone.