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Volume 18, issue 16 | Copyright

Special issue: Global and regional assessment of intercontinental transport...

Atmos. Chem. Phys., 18, 12123-12140, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 22 Aug 2018

Research article | 22 Aug 2018

Average versus high surface ozone levels over the continental USA: model bias, background influences, and interannual variability

Jean J. Guo1, Arlene M. Fiore1, Lee T. Murray2,3,a, Daniel A. Jaffe4, Jordan L. Schnell5,b, Charles T. Moore6, and George P. Milly2 Jean J. Guo et al.
  • 1Department of Earth and Environmental Sciences and Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, USA
  • 2Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, USA
  • 3NASA Goddard Institute for Space Studies, New York, NY, USA
  • 4University of Washington, School of STEM, Bothell, WA and Department of Atmospheric Science, Seattle, WA, USA
  • 5NOAA Geophysical Fluid Dynamics Laboratory, Atmospheric and Oceanic Sciences, Princeton University, Princeton, NJ, USA
  • 6WESTAR and WRAP, Fort Collins, CO, USA
  • anow at: Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY, USA
  • bnow at: Department of Earth and Planetary Sciences, Northwestern University, Chicago, IL, USA

Abstract. US background ozone (O3) includes O3 produced from anthropogenic O3 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 O3 over 10 continental US regions from 2004 to 2012. Evaluation with observations reveals model biases of +0–19ppb in seasonal mean maximum daily 8h average (MDA8) O3, highest in summer over the eastern USA. Simulated high-O3 events cluster too late in the season. We link these model biases to excessive regional O3 production (e.g., US anthropogenic, biogenic volatile organic compounds (BVOCs), and soil NOx, emissions), or coincident missing sinks. On the 10 highest observed O3 days during summer (O3_top10obs_JJA), US anthropogenic emissions enhance O3 by 5–11ppb and by less than 2ppb in the eastern versus western USA. 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 versus on O3_top10obs_JJA days. During the summers of 2004–2012, monthly regional mean US background O3 MDA8 levels vary by up to 15ppb from year to year. Observed and simulated summertime total surface O3 levels on O3_top10obs_JJA days decline by 3ppb (averaged over all regions) from 2004–2006 to 2010–2012, reflecting rising US background (+2ppb) and declining US anthropogenic O3 emissions (−6ppb) in the model. The model attributes interannual variability in US background O3 on O3_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 O3 on the highest observed O3 days.

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We use the GEOS-Chem model to estimate the influence from anthropogenic and background sources to ozone over the USA. Novel findings include the point that year-to-year background variability on the 10 highest observed ozone days is driven mainly by natural sources and not international or intercontinental pollution transport. High positive model biases during summer are associated with regional ozone production. The EPA 3-year average metric falls short of its aim to remove natural variability.
We use the GEOS-Chem model to estimate the influence from anthropogenic and background sources...