Scientific assessment of background ozone over the U.S.: Implications for air quality management

Ozone (O₃) is a key air pollutant that is produced from precursor emissions and has adverse impacts on human health and ecosystems. In the U.S., the Clean Air Act (CAA) regulates O₃ levels to protect public health and welfare, but unraveling the origins of surface O₃ is complicated by the presence of contributions from multiple sources including background sources like stratospheric transport, wildfies, biogenic precursors, and international anthropogenic pollution, in addition to U.S. anthropogenic sources. In this report, we consider more than 100 published studies and assess current knowledge on the spatial and temporal distribution, trends, and sources of background O₃ over the continental U.S., and evaluate how it inflattainment of the air quality standards. We conclude that spring and summer seasonal mean U.S. background O₃ (USB O₃), or O₃ formed from natural sources plus anthropogenic sources in countries outside the U.S., is greatest at high elevation locations in the western U.S., with monthly mean maximum daily 8-hour average (MDA8) mole fractions approaching 50 parts per billion (ppb) and annual 4^th highest MDA8s exceeding 60 ppb, at some locations. At lower elevation sites, e.g., along the West and East Coasts, seasonal mean MDA8 USB O₃ is in the range of 20-40 ppb, with generally smaller contributions on the highest O₃ days. The uncertainty in U.S. background O₃ is around ±10 ppb for seasonal mean values and higher for individual days. Noncontrollable O₃ sources, such as stratospheric intrusions or precursors from wildfires, can make significant contributions to O₃ on some days, but it is challenging to quantify accurately these contributions. We recommend enhanced routine observations, focused fi studies, process-oriented modeling studies, and greater emphasis on the complex photochemistry in smoke plumes as key steps to reduce the uncertainty associated with background O₃ in the U.S.