Long-term baseline ozone changes in the Western US: A synthesis of analyses

Quantification of the magnitude and long-term changes in ozone concentrations transported into the U.S. is important for effective air quality policy development. We synthesize multiple published trend analyses of western U.S. baseline ozone, and show that all results are consistent with an overall, non-linear change - a rapid increase (similar to 5 ppb/decade) during the 1980s that slowed in the 1990s, maximized in the mid-2000s, and was followed by a slow decrease (similar to 1 ppb/decade) thereafter. This non-linear change accounts for similar to 2/3 of the variance in 28 published linear trend analyses; we attribute the other 1/3 of the variance to unquantified autocorrelation in the analyzed data sets that result primarily from meteorologically driven interannual ozone variability. Recent systematic changes in baseline ozone on the U.S. West Coast have been relatively small - the standard deviation of the 2-year means over the 1990-2017 period is 1.5 ppb. International efforts to reduce anthropogenic precursor emissions from all northern mid-latitude sources could possibly reduce baseline ozone concentrations, thereby improving U.S. ozone air quality.

Automated summary

Research shows that baseline ozone concentrations in the western United States have exhibited non-linear changes over the past few decades, with a rapid increase in the 1980s, slowing in the 1990s, peaking in the mid-2000s, and then slowly decreasing. The remaining variance is mainly attributed to unquantified data autocorrelation and meteorologically-driven interannual ozone variability.