Observational-based assessment of contributions to maximum ozone concentrations in the western United States

ABSTRACT

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.

Acknowledgment

I.C. Faloona’s effort was supported by the USDA National Institute of Food and Agriculture, (Hatch project CA-D-LAW-2481-H, “Understanding Background Atmospheric Composition, Regional Emissions, and Transport Patterns Across California”). David Parrish also works as an independent consultant (David.D.Parrish, LLC); he has had contracts with several US state and federal agencies, including two that supported earlier analyses of US background ozone. There are no real or perceived financial conflicts of interests for the authors. The authors are particularly grateful to the Ozone and Water Vapor Group of NOAA’s Global Monitoring Laboratory who have maintained the ozone sonde program at Trinidad Head for more than two decades, and to Detlev Helmig of Boulder A.I.R. LLC, who provided a critical reading of the manuscript.

Data availability statement

All data used in this analysis are available from the data archive of the US EPA, the California Air Resources Board, and the Global Monitoring Laboratory of NOAA; links to these sites are included in Section 2.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this paper can be accessed on the publisher’s website.

Additional information

Funding

This work was supported by the USDA National Institute of Food and Agriculture [Hatch project CA-D-LAW-2481-H].

Notes on contributors

David D. Parrish

David D. Parrish is an atmospheric chemist who now focuses on tropospheric ozone analyses. He has worked in atmospheric research in Boulder Colorado for more than 40 years, and currently is an independent scientist and consultant at David.D.Parrish, LLC.

Ian C. Faloona

Ian C. Faloona is a professor of atmospheric science at the University of California Davis, and a Bio-micrometeorologist with the Agricultural Experiment Station. He studied physical chemistry at the University of California Santa Cruz, spent 5 years as an air quality consultant with SECOR, Inc. in Fort Collins, Colorado, and then earned a Ph.D. in Meteorology at the Pennsylvania State University. His research interests include the airborne investigation of vertical mixing and near-field pollutant dispersion, observational emission estimates, planetary boundary layer dynamics, biogeochemical cycling, and atmosphere/ocean photochemistry.

Richard G. Derwent

Richard G. Derwent is an independent scientist and consultant on air pollution and atmospheric chemistry with rdscientific, Newbury, United Kingdom.