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 (~5 ppb/decade) during the 1980s that slowed in the 1990s, maximized in the mid-2000s, and was followed by a slow decrease (~1 ppb/decade) thereafter. This non-linear change accounts for ~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.Implications: Ozone is an air pollutant with significant human and ecological health impacts. Air masses transported into the western U.S. from over the Pacific Ocean carry ozone concentrations that are, on average, a large fraction of the U.S. health standard. The US EPA policy assessment conducted for the recent review of the ozone National Ambient Air Quality Standard (NAAQS) found that 2016 regional average MDA8 ozone concentrations in the western US maximized in summer at ~52 ppb and that ~40 ppb of that maximum was contributed by ozone of natural and transported anthropogenic contributions. Thus, quantifying these trans-boundary background ozone concentrations has been identified as an important issue for a complete understanding of US air quality. Published analyses of temporal trends of these transported ozone concentrations vary widely, from early reports of increases to more recent reports of decreases. We show that the long-term ozone changes are nonlinear, with substantial concentration increases (as large as ~5 ppb/decade) before the mid-2000s when a maximum is reached, followed by a small decrease of ~1 ppb/decade thereafter. Superimposed on the overall changes is significant interannual variability that makes accurate determination of systematic trends over decade-scale time periods uncertain. The end of the previously increasing trends, and the recent decrease in transported ozone concentrations, is a good news for U.S. air quality, as it eases the difficulty of achieving the ozone air quality standard.

Archived Ozone Design Values (ODVs) provide smoothed temporal records of maximum ozone concentrations impacting monitoring sites throughout the US. Utilizing time series of ODVs recorded at sites along the US West Coast, we separately estimate ODV contributions from US background ozone and from production driven by US anthropogenic precursor emissions. Sondes launched from Trinidad Head in northern California measure the vertical distribution of baseline ozone transported ashore from the Pacific; this profile is reflected in the increase of the US background ODV contribution with monitoring site elevation in both rural and urban areas. The ODVs that would result from US background ozone alone are small at coastal, sea level locations (average ~45 ppb), but increase with altitude; above 1 km US background ODVs can exceed 60 ppb. US background ozone contributions now constitute the majority of the maximum ODVs throughout the US west coast region, including the Los Angeles urban area, which records the country's highest ODVs. US anthropogenic emissions presently cause enhancements of 35 to 55 ppb to the maximum ODVs in the Los Angeles area; thus, local emission controls can further reduce ozone even though the background contribution is larger. In other US west coast urban areas ODV enhancements from US anthropogenic emissions are much smaller than the US background ODV contribution. The past decrease in US anthropogenic ODV enhancements from emission controls is larger than generally realized - a factor of more than 6 from 1980 to 2020, while US background ODV contributions varied to only a small extent over those four decades. Wildfire impacts on ODVs are significant in urban areas of the Pacific Northwest, but not over the vast northern US rural region. There is an indication that agricultural emissions of nitrogen oxides in California's Salinas Valley increase downwind maximum ODVs by 5-10 ppb.Implications: In 2020 the ozone design values (ODVs) resulting from transported background ozone alone are now larger than the ODV enhancements from US anthropogenic precursor emissions, even in the Los Angeles urban area, where the nation's highest ODVs are recorded. The US anthropogenic ODV enhancements have been reduced by more than a factor of 6 from 1980 to 2020. The maximum US background ODV contributions have varied somewhat, but in each of the US west coast urban areas it was 60 ppb or larger in 2000. These contributions are so large that reducing maximum urban ODVs to the 70 ppb required by the 2015 ozone NAAQS is very difficult. There remains relatively little room for further reducing ODVs through domestic emission controls alone. From this perspective, degraded US ozone air quality in the western US is primarily due to the US background ozone contribution, with the US anthropogenic enhancement making a significant, but smaller contribution. Notably, the US background ODV has slowly decreased (~1 ppb decade^-1; Parrish, Derwent, and Faloona 2021) since the mid-2000s; cooperative, international emission control efforts aimed at continuing or even accelerating this background ozone decrease may be an effective approach to further ODV reductions, since the US background ODV is largely due to a hemisphere-wide, transported reservoir of ozone with contributions from all northern midlatitude continents. Given the major contribution of background ozone to observed ODVs, future reviews of the ozone NAAQS will be better informed if observational-based estimates of background ODV contributions are considered, in addition to model-derived estimates upon which past reviews have solely relied.

A simple compartmental model is used to simulate the average (climatic) annual and seasonal distribution of ozone in the Northern Hemisphere midlatitude troposphere. The model (reminiscent of earlier examples) consists of a circular set of segments - 4 marine and 5 continental - which span the globe. Each segment consists of two well-mixed compartments, one representing the free troposphere overlying one representing the boundary layer. A minimal set of parameters with values taken from generally accepted and measured behavior is used to describe the ozone sources and sinks within the segments and inter-compartmental flow and mixing. The model accurately simulates measured seasonal cycles of ozone throughout the midlatitude troposphere. By virtue of relatively rapid circular zonal flow the model reproduces the observed nearly uniform free troposphere that behaves as a reservoir, responding to the combined boundary layer and stratospheric inputs. The distinctive seasonal cycles of the free troposphere and the marine boundary layer are well reproduced by the strong seasonal dependence of the loss mechanism initiated by ozone photolysis that yields O1D. The critical role of the marine boundary layer in the global ozone balance and the constraints that it places on the net continental production are clearly revealed. Sensitivity analysis identifies which of the basic set of process parameters most require better understanding. Minimalist models such as this, used in conjunction with detailed CTM model simulations, can help provide a comprehensive understanding of the climatic behavior and trends in tropospheric ozone.