	Introduction

Exposure to ozone (often referred to as "smog"), regardless of where that ozone originates, has been linked to negative human health effects, including respiratory ailments and premature death. The Clean Air Act (CAA) directs the U.S. Environmental Protection Agency (EPA) to establish National Ambient Air Quality Standards (NAAQS) to protect public health (primary standards) and welfare (secondary standards). However, uncertainty between exposure and health response increases at very low ozone concentrations—that is, below 20 parts per billion (ppb). State air quality regulators develop SIPs to attain NAAQS for ambient air in their states, and they have jurisdiction only over the sources of emissions within their borders. The levels of pollution flowing into nonattainment regions, generally referred to as "background pollution," may be making it more difficult for some areas of the United States to meet attainment. The CAA also mandates EPA to review the NAAQS every five years and revise them as appropriate. Members of Congress may have an interest in better understanding background ozone from natural and international sources, particularly as EPA reviews the 2015 ozone standard. Contributions from sources of background ozone may become important as states with nonattainment areas develop SIPs that attempt to quantify these contributions and consider ways to address them. To assist Congress in understanding these issues, this report defines background ozone , focusing on natural and international sources, and describes what is currently known about these sources. Current research suggests that natural sources and sources outside the United States may contribute to total ozone in those areas at certain times. This report deals exclusively with ozone measured at ground level and its adverse health and material effects. A third term, North American background , is also defined in this report for added clarity. Natural Background (NB)

Natural background ozone is what the average concentration of ground-level ozone would be without any human influence. Ozone forms naturally in the lower levels of the atmosphere due to natural emissions of precursors from sources including lightning, vegetation, wildfires, and methane. The "Challenges in Estimating Background Ozone" section discusses specific challenges and uncertainties associated with the data and the modeling projects. 2. 3. Nonetheless, it will over time react and contribute to background ozone. North American Background (NAB)

North American background (NAB) is the estimated concentration of ozone that excludes the effects of all human-caused emissions in North America. NAB includes all NB sources as well as human-caused sources of emissions from countries outside of North America. U.S. Background (USB)

U.S. background (USB) includes all contributions from NB and NAB plus all human-caused emissions from Mexico and Canada. Challenges in Estimating Background Ozone

The contribution of background ozone to total local ozone concentrations varies from location to location, day to day, and even hour to hour. However, measurement data of the pollutants being modeled are also limited, which increases the challenge associated with evaluating the performance of the model in capturing ozone formation and movement. The lack of a decreasing ozone trend at many monitors in the West could be due to a number of causes: increasing seasonal average temperatures, an increase in incidents of fire since 1986, emissions from oil and gas development, increasing contributions from international transport of air pollution, and increasing global methane concentrations. While emission inventories from Asia are highly uncertain—and outdated in some modeling cases—several sources of data suggest that Asian emissions, and potentially their impact on U.S. air quality, have peaked. Stakeholders suggest challenges with meeting the NAAQS, in part due to the difficulty of reducing ozone in areas with potentially large contributions from background sources and in part due to lack of data availability to conduct demonstrations of those background contributions. Some states, especially states in the western United States, have asserted that natural and non-U.S. sources of ozone and precursors have increased the ozone concentrations in their states. Therefore, Congress may consider funding implications of the following recommendations, made by stakeholders in the scientific and regulatory communities, that are intended to improve the understanding of contributions from background ozone:
International engagement and/or cooperation at the federal level and through research collaborations that may improve understanding of non-U.S. contributions to U.S. air quality and may increase cooperation for pollution reduction goals. EPA's review of the ozone NAAQS is underway and set to be completed in 2020, with background ozone contributions suggested as a topic to be addressed.
Exposure to ozone, a common air pollutant, has been linked to early death, plant and crop damage, and damage to property. The U.S. Environmental Protection Agency (EPA) sets National Ambient Air Quality Standards (NAAQS) for ground-level ozone to protect human health and welfare with, by law, a "margin of safety." States that contain areas with ozone concentrations above these standards must develop plans to reduce emissions and improve air quality. However, states have direct control only over emission sources located within their borders.
The Clean Air Act (CAA) requires EPA to re-evaluate the NAAQS every five years to include the latest science and technological advancements. Studies reporting the human health impacts of ozone increasingly suggest that ozone exposure may not be completely safe at any level. With the potential for a NAAQS re-evaluation leading to science-based recommendations for a tighter standard, some stakeholders have expressed increasing concern that future—and even current—ozone standards could be difficult to meet due to the contribution of "background ozone," which arises from a variety of sources described in this report. In some areas of the United States, background ozone may be approaching 70 parts per billion (ppb) on some days, the current level of the NAAQS.
Some Members of Congress have expressed interest in adverse health effects that occur at or below the current standard, challenges some nonattainment areas may have in meeting current standards, and particularly the responsibilities for meeting the health standard, given interstate and international transport. EPA's review of the ozone NAAQS is underway and set to be completed in 2020, with background ozone contributions suggested as a topic to be addressed. Congress may have an interest in better understanding scientific capabilities, needs, and efforts to improve understanding of contributions from background sources, as well as options for regulatory responses.
Defining Background Ozone
Three terms are used for different types of background ozone, and distinguishing among them can be important for regulatory purposes.
1. Natural background. Ozone concentrations that would be present without any human influence or contribution from anywhere on the globe. Natural background includes contributions from wildfires, vegetation, lightning, ozone in the stratosphere, and global methane concentrations. Contributions to background ozone from wildfires and methane have been increasing over the past several decades. 2. North American background. Ozone concentrations absent human-caused emissions from North America. North American background includes all sources in natural background plus ozone from international sources outside North America. Studies suggest that Asian emissions may be contributing to ozone in the United States, especially in Western states, but that those contributions may be beginning to decrease. 3. United States background. Ozone concentrations absent human-caused emissions from the United States. U.S. background includes all sources in North American background plus ozone formed from emission sources in Mexico and Canada.
Challenges in Estimating Background Ozone
The CAA provides alternative regulatory options for areas that successfully demonstrate significant influence from some specific sources of natural background ozone on ozone exceedences. However, such demonstrations may be difficult to conduct and reliably assess, given data and analytical challenges:
Emissions inventories. Current understanding of the amount, location, and type of pollutant emissions from many types of sources is insufficient. Therefore inventories typically provide estimations, which may not be precise enough for apportioning contributions. Weather data. Meteorological data (i.e., wind speed, wind direction, temperature, cloud cover, humidity, etc.) are not currently measured at a fine enough spatial scale to adequately represent relevant weather processes. Ambient air quality measurements. Data on pollutant concentrations are limited, which increases the challenge of understanding ozone formation and movement. Fine spatial and temporal measurements are needed both horizontally across the surface and vertically to higher levels of the atmosphere. Source contribution variability. Background ozone source contributions change by year, season, day, and hour and from location to location. This makes it difficult to project future contributions, including when contributions will be relevant to attainment status.
This report provides information on sources of background ozone, presents key challenges in addressing these sources, and discusses potential options to overcome these challenges.