Quantification of the magnitude and long-term changes of ozone concentrations transported into the US is important for effective air quality policy development. We synthesize multiple published trend analyses of western US baseline ozone, and show that all results are consistent with an overall, non-linear change – rapid increase during the 1980s that slowed in the 1990s, maximized in the mid-2000s, and was followed by a slow decrease thereafter. This non-linear change accounts for ~2/3 of the variance in the published linear trend analyses; we attribute the other 1/3 to unquantified autocorrelation in the analyzed data sets. Recent systematic changes in baseline ozone at the US 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 US ozone air quality.