Average versus high surface ozone levels over the continental U.S.A.: Model bias, background influences, and interannual variability
U.S. background ozone (O3) includes O3 produced from anthropogenic O3 precursors emitted outside of the U.S.A., from global methane, and from any natural sources. Using a suite of sensitivity simulations in the GEOS-Chem global chemistry-transport model, we estimate the influence from individual background versus U.S. anthropogenic sources on total surface O3 over ten continental U.S. regions from 2004–2012. Evaluation with observations reveals model biases of +0–19ppb in seasonal mean maximum daily 8-hour average (MDA8) O3, highest in summer over the eastern U.S.A. Simulated high-O3 events cluster too late in the season. We link these model biases to regional O3 production (e.g., U.S. anthropogenic, biogenic volatile organic compounds (BVOC), and soil NOx, emissions), or coincident missing sinks. On the ten highest observed O3 days during summer (O3_top10obs_JJA), U.S. anthropogenic emissions enhance O3 by 5–11ppb and by less than 2ppb in the eastern versus western U.S.A. The O3 enhancement from BVOC emissions during summer is 1–7ppb higher on O3_top10obs_JJA days than on average days, while intercontinental pollution is up to 2ppb higher on average vs. on O3_top10obs_JJA days. In the model, regional sources of O3 precursor emissions drive interannual variability in the highest observed O3 levels. During the summers of 2004–2012, monthly regional mean U.S. background O3 MDA8 levels vary by 10–20ppb. Simulated summertime total surface O3 levels on O3_top10obs_JJA days decline by 3ppb (averaged over all regions) from 2004–2006 to 2010–2012 in both the observations and the model, reflecting rising U.S. background (+2ppb) and declining U.S. anthropogenic O3 emissions (−6ppb). The model attributes interannual variability in U.S. background O3 on O3_top10obs days to natural sources, not international pollution transport. We find that a three-year averaging period is not long enough to eliminate interannual variability in background O3.
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See the data and code underlying this article at https://doi.org/10.7916/D8Z90W3X.