Abstract
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 4th 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.
Highlights
Definitions and sources of background ozoneOzone (O3) is a key secondary air pollutant associated with a number of health issues including asthma and premature death (Bell et al, 2004; Lippmann, 1993; Silva et al, 2013; Landrigan et al, 2018)
Anthropogenic emissions of nitrogen oxides (NOx) and some volatile organic compounds (VOCs) have decreased in the U.S over the past several decades, and peak O3 levels have declined in most areas of the U.S as a result (Cooper et al, 2012; Simon et al, 2015; Strode et al, 2015)
Correlations between observations and contributions at CHAT over the whole period are generally consistent with previous studies (US Environmental Protection Agency (EPA), 2013; Zhang et al, 2011; Emery et al, 2012) showing that: (1) USBO is a significant fraction of total O3 at the CHAT and RMNP sites; (2) the observed and predicted O3 are most strongly correlated with the local contribution; and (3) boundary conditions are anti-correlated with the local contribution
Summary
Ozone (O3) is a key secondary air pollutant associated with a number of health issues including asthma and premature death (Bell et al, 2004; Lippmann, 1993; Silva et al, 2013; Landrigan et al, 2018). In addition to USB O3, an alternative useful metric for evaluating modeled mole fractions is “baseline” O3, which is the distribution of O3 observations at a rural or remote site that has not been influenced by recent, local emissions (HTAP, 2010) We note that this definition differs from the one adopted by a National Research Council (NRC) report (NRC, 2010), which defined baseline as “the statistically defined lowest abundances of O3 in the air flowing into a country.”. It is important to keep in mind that baseline O3 is not the same as USB (or NAB) O3, but both can be characterized by a seasonal mean, MDA8, 3-year ODV, and other statistical metrics Because states develop their SIPs by evaluating O3 response to emissions controls on the highest modeled O3 days, an especially useful metric is the estimate of USB and NCOS O3 on those days. What do observations and models tell us about the spatial and temporal pattern, variability, trends, and episodic peaks in baseline and background O3 across the continental U.S.?
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