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

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 O3levels to protect public health and welfare, but unraveling the origins of surface O3is complicated by the presence of contributions from multiple sources including background sources like stratospheric transport, wildfires, 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 O3over the continental U.S., and evaluate how it influences attainment of the air quality standards. We conclude that spring and summer seasonal mean U.S. background O3(USB O3), or O3formed 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 4thhighest MDA8s exceeding 60 ppb, at some locations. At lower elevation sites, e.g., along the West and East Coasts, seasonal mean MDA8 USB O3is in the range of 20–40 ppb, with generally smaller contributions on the highest O3days. The uncertainty in U.S. background O3is around ±10 ppb for seasonal mean values and higher for individual days. Noncontrollable O3sources, such as stratospheric intrusions or precursors from wildfires, can make significant contributions to O3on some days, but it is challenging to quantify accurately these contributions. We recommend enhanced routine observations, focused field 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 O3in the U.S.

Exploring 2016–2017 surface ozone pollution over China: source contributions and meteorological influences

“…Ozone enhancements from soil NO x emissions become notable when there are pulsing soil emissions after rainfall (e.g., ∼ 10 ppbv in mid-August in Xi'an). They can be either positive or negative, reflecting the monthly or even daily shifts of the ozone chemical production regime, as also reported from satellite observations (Jin and Holloway, 2015) and several field campaigns (SOILNO, blue), and the stratosphere (STRAT, purple). The estimation of ozone source attributions is described in the text.…”

Intercomparison of lidar, aircraft, and surface ozone measurements in the San Joaquin Valley during the California Baseline Ozone Transport Study (CABOTS)

“…The primary measurements (see Fig. 1a) included electrochemical cell (ECC) ozonesondes (Johnson et al, 2002) (Trousdell et al, 2016) and the NASA Alpha Jet Atmospheric eXperiment (AJAX) (Yates et al, 2015), and ozone and backscatter lidar measurements by the truck-based NOAA ESRL TOPAZ lidar system (Asher et al, 2018) in the Santa Lucia Mountains west of Visalia, as well as using the extensive networks of regulatory surface monitors maintained by the California Air Resources Board and the San Joaquin Valley Air Pollution Control District (SJVAPCD). The Bodega Bay and Half Moon Bay sites were located on the coast to sample the Pacific inflow, and the VMA was chosen for the TOPAZ operations because of its central location in the SJV, the availability of the runway and airspace for low approaches and aircraft profiles, and the presence of the co-located SJVAPCD wind profiler and radio acoustic sounding system (RASS) (Bao et al, 2008).…”

OH chemistry of non-methane organic gases (NMOGs) emitted from laboratory and ambient biomass burning smoke: evaluating the influence of furans and oxygenated aromatics on ozone and secondary NMOG formation

“…Once emitted, biomass burning NMOGs may undergo photochemical reactions to form ozone and secondary organic aerosol (SOA) (Hobbs et al, 2003;Yokelson et al, 2009;Akagi et al, 2013). Wildfire smoke is believed to significantly contribute to summertime ozone levels in fire-prone regions, such as the western United States (Jaffe et al, 2008(Jaffe et al, , 2013(Jaffe et al, , 2018. An assessment of historical ozone data from 1989 to 2004 has shown that daily mean ozone increases by 2 ppb for every 1 million acres of area burned (Jaffe et al, 2008).…”

“…Field observations have shown that ozone enhancement ratios ( O 3 / CO) may increase (e.g., 0.7 ppb ppb −1 , Andreae et al, 1988;Mauzerall et al, 1998), decrease (e.g., −0.07 ppb ppb −1 , Alvarado et al, 2010), or remain unchanged downwind of wildfires (Jaffe and Wigder, 2012). The extent of ozone production depends on multiple factors, including NMOG/NO x ratios, downwind meteorology, and incident solar radiation (Akagi et al, 2013;Jaffe et al, 2018). Ozone production may also be slowed through peroxyacetyl nitrate formation (PAN), which is affected, in part, by NMOG functionality and total NO x and NMOG emissions (Alvarado et al, 2010;Liu et al, 2016;Müller et al, 2016;Jaffe et al, 2018).…”