Progress on Understanding U.S. Background Ozone

Ozone (O₃) is a key secondary air pollutant that increases respiratory stress and is associated with a number of health issues, up to and including premature mortality. Anthropogenic emissions of O₃ precursors have been dramatically reduced in the US over the past several decades and thus peak O₃ concentrations have declined in most areas. At the same time, new evidence continues to demonstrate health effects at lower concentrations and so the U.S. Environmental Protection Agency (EPA) tightened the National Ambient Air Quality Standards (NAAQS) for O₃in 2015. An area is in attainment of the standard if the 3-year average of the annual 4th highest maximum daily 8-hour average (MDA8), called the “Design Value (DV)”, is 70 ppb or below.

Observations from remote sites show that seasonal mean O3 concentrations are in the range of 30-50 ppbv, thus this “background” air is already a substantial fraction of the 70 ppb standard. On some days, observations and models show that surface O₃ is significantly enhanced by Non-Controllable O₃ Sources (NCOS), such as intrusions of upper troposphere/lower stratospheric (UTLS) air, long-range transport from international sources or from natural precursor emissions, such as wildfires.

Because of the importance of background O₃ we conducted an assessment of current knowledge and key gaps in our knowledge. For this review, we focused on U.S. Background O₃ (USBO), which is O₃ formed from natural sources in the U.S. plus natural and anthropogenic sources in countries outside the U.S. USBO varies on a daily and seasonal basis and is generally estimated from global chemical transport models, but these models must be informed with observations. A quantitative understanding of USBO is essential for setting the NAAQS and in state and local efforts to meet the clean air act. At the same time, the Clean Air Act (CAA) includes several mechanisms, including the exceptional events rule, the rural transport provision and section 179B to account for high O₃ due to uncontrollable sources. These rules are especially important for sites which experience frequent high O₃ days due to USBO.

Our review focused on six scientific questions:

1. What methods have been used to study background O₃ and what are the strengths, weaknesses and uncertainties of these methods?
2. What do observations and models tell us about the spatial and temporal pattern, variability, trends and episodic peaks in baseline/background O₃ across the continental US?
3. What do observations and models tell us about the sources of background O₃?
4. How does our understanding of USBO fit into our model of local air quality and how do uncertainties in USBO propagate into uncertainties in source attribution?
5. What strategies can be used to quantify daily to seasonal variations in NCOS and what are the strengths and weaknesses of each method?
6. What strategies are needed to improve our estimates of USBO and NCOS and what are our recommendations for future research in this area?

In this presentation, I will summarize our key conclusions and our recommendations for future research to improve our knowledge of background O₃.

The figure shows a conceptual model for O₃ sources in the U.S. Boxes shown in yellow represent domestic/controllable sources. Boxes in blue show USBO/non-controllable sources. The inset bar chart shows an example of how both domestic and USBO sources combine to produce elevated O₃ concentrations.