Assessing the Impact of Wildland Fire Smoke on Exceptional Events and Air Quality Standards Across the U.S.

Wildland fire events are driving a decline in air quality, predominately in the western U.S., but the entire country has felt the effects in 2023 due to Canadian wildfires. Pollutants from wildfire events such as fine particulate matter (PM_2.5) and ozone can cause both acute and long-term health impacts for those exposed. The National Ambient Air Quality Standards (NAAQS) are set to provide public health and welfare protection, including protecting sensitive groups, guard against decreased visibility, and avoid damage to animals, crops, vegetation, and buildings. When the NAAQS are exceeded, air agencies are required to develop implementation plans designed to bring the region into attainment by limiting emissions or production of the exceeding pollutant. With the increase in wildland fires in recent years, smoke emissions are significantly enhancing air pollution concentrations, leading to an unprecedented number of days impacted by wildfire smoke in many locations. Meanwhile, the U.S. Environmental Protection Agency (EPA) has recently proposed tightening the primary NAAQS standard for PM_2.5, and the Clean Air Scientific Advisory Committee (CASAC) has recommended tightening of the ozone NAAQS as well. For air agencies that are near attainment but for wildland fire influence, proving that uncontrollable wildfire smoke caused an exceedance of the NAAQS through an exceptional event demonstration allows agencies to exclude air quality observation data on impacted days from attainment determinations. Overall, creating and reviewing exceptional event demonstrations is laborious for both the air agencies and EPA regional offices, and as a result, the full scale of wildland fire smoke impacts on air quality and NAAQS attainment under current and future standards is not fully understood. Given the health and economic impacts of exceptional events, better documentation of these impacts is needed to support decision making for the protection of air quality. In this study, we report a nationwide screening methodology for PM_2.5 and ozone enhancements due to wildland fire smoke emissions. The screening approach quantifies the impact of wildland fire emissions on air quality monitoring stations across the conterminous United States. To implement the screening methodology, we combine widely used tools in exceptional event demonstrations, including the National Oceanic and Atmospheric Administration (NOAA) Hazard Mapping System (HMS) smoke maps, monitor observations of PM_2.5 and ozone, smoke-specific PM_2.5 model results from statistical and dispersion modeling, and Generalized Additive Modeling (GAM) using meteorological variables. We have identified a list of probable exceptional events for the design value period of 2020-2022 at all PM_2.5 and ozone monitors throughout the U.S. by combining these datasets. In addition to nationwide screening, we conducted an in-depth assessment of locations with high numbers of probable exceptional events and areas with lower-than-expected probable exceptional events as case studies. For these locations, we developed full analyses of selected smoke days to understand the strengths and weaknesses of the screening in the context of well-documented events. For example, in areas with high numbers of probable exceptional events, we compared the screened probable exceptional event dates with NOAA’s High-Resolution Rapid Refresh (HRRR) smoke product and other wildland fire emitted pollutants such as carbon monoxide, nitrogen oxides, and volatile organic compounds. For areas with lower-than-expected probable exceptional events, we evaluated dates that were likely smoke-impacted and determined why they were not identified as probable exceptional events. These case studies demonstrate the nature of wildland fire exceptional events and provide context to the screening analysis that can be used to iteratively improve the process of the full national assessment. Using the results of the refined national screening, we assessed the overall and monitor-level impacts of wildfire smoke on air quality with respect to current and proposed future health-based NAAQS for PM_2.5 and ozone. These results provided directly applicable policy-relevant information on possible NAAQS attainment or nonattainment for many areas across the U.S. Overall, the results of this study are broadly applicable and provide (1) evidence of the scale of the wildfire smoke contribution to air pollution nationally over a recent period, (2) an assessment of regulatory implications of smoke impacts with respect to current and future air quality regulations, and (3) a screening methodology that can be used in the future to identify probable wildfire smoke exceptional events.