LegiStorm

Summary:

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. 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. 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. 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.