Guo, Jean J.; Fiore, Arlene M.; Murray, Lee T.; Jaffe, Daniel A.; Schnell, Jordan L.; Moore, Charles T.; Milly, George P.

US background ozone (inline-formulaO3) includes inline-formulaO3 produced from anthropogenic inline-formulaO3 precursors emitted outside of the USA, 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 sources versus US anthropogenic sources on total surface inline-formulaO3 over 10 continental US regions from 2004 to 2012. Evaluation with observations reveals model biases of inline-formula+0–19 ppb in seasonal mean maximum daily 8 h average (MDA8) inline-formulaO3, highest in summer over the eastern USA. Simulated high-inline-formulaO3 events cluster too late in the season. We link these model biases to excessive regional inline-formulaO3 production (e.g., US anthropogenic, biogenic volatile organic compounds (BVOCs), and soil inline-formulaNOx, emissions), or coincident missing sinks. On the 10 highest observed inline-formulaO3 days during summer (inline-formulaO3_top10obs_JJA), US anthropogenic emissions enhance inline-formulaO3 by 5–11 ppb and by less than 2 ppb in the eastern versus western USA. The inline-formulaO3 enhancement from BVOC emissions during summer is 1–7 ppb higher on inline-formulaO3_top10obs_JJA days than on average days, while intercontinental pollution is up to 2 ppb higher on average versus on inline-formulaO3_top10obs_JJA days. During the summers of 2004–2012, monthly regional mean US background inline-formulaO3 MDA8 levels vary by up to 15 ppb from year to year. Observed and simulated summertime total surface inline-formulaO3 levels on inline-formulaO3_top10obs_JJA days decline by 3 ppb (averaged over all regions) from 2004–2006 to 2010–2012, reflecting rising US background (inline-formula+2 ppb) and declining US anthropogenic inline-formulaO3 emissions (inline-formula−6 ppb) in the model. The model attributes interannual variability in US background inline-formulaO3 on inline-formulaO3_top10obs days to natural sources, not international pollution transport. We find that a 3-year averaging period is not long enough to eliminate interannual variability in background inline-formulaO3 on the highest observed inline-formulaO3 days.

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Guo, Jean J. / Fiore, Arlene M. / Murray, Lee T. / et al: Average versus high surface ozone levels over the continental USA: model bias, background influences, and interannual variability. 2018. Copernicus Publications.

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