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Volume 16, issue 21
Atmos. Chem. Phys., 16, 13561-13577, 2016
https://doi.org/10.5194/acp-16-13561-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmos. Chem. Phys., 16, 13561-13577, 2016
https://doi.org/10.5194/acp-16-13561-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 01 Nov 2016

Research article | 01 Nov 2016

Why do models overestimate surface ozone in the Southeast United States?

Katherine R. Travis1, Daniel J. Jacob1,2, Jenny A. Fisher3,4, Patrick S. Kim2, Eloise A. Marais1, Lei Zhu1, Karen Yu1, Christopher C. Miller1, Robert M. Yantosca1, Melissa P. Sulprizio1, Anne M. Thompson5, Paul O. Wennberg6,7, John D. Crounse6, Jason M. St. Clair6, Ronald C. Cohen8, Joshua L. Laughner8, Jack E. Dibb9, Samuel R. Hall10, Kirk Ullmann10, Glenn M. Wolfe11,12, Illana B. Pollack13, Jeff Peischl14,15, Jonathan A. Neuman14,15, and Xianliang Zhou16,17 Katherine R. Travis et al.
  • 1Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
  • 2Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA
  • 3Centre for Atmospheric Chemistry, School of Chemistry, University of Wollongong, Wollongong, NSW, Australia
  • 4School of Earth and Environmental Sciences, University of Wollongong, Wollongong, NSW, Australia
  • 5NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
  • 6Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
  • 7Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA, USA
  • 8Department of Chemistry, University of California, Berkeley, CA, USA
  • 9Earth System Research Center, University of New Hampshire, Durham, NH, USA
  • 10Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO, USA
  • 11Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA
  • 12Joint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore, MD, USA
  • 13Department of Atmospheric Science, Colorado State University, Colorado, USA
  • 14University of Colorado, Cooperative Institute for Research in Environmental Sciences, Boulder, CO, USA
  • 15NOAA Earth System Research Lab, Boulder, CO, USA
  • 16Department of Environmental Health Sciences, State University of New York, Albany, New York 12201, USA
  • 17Wadsworth Center, New York State Department of Health, Albany, New York, USA

Abstract. Ozone pollution in the Southeast US involves complex chemistry driven by emissions of anthropogenic nitrogen oxide radicals (NOx  ≡  NO + NO2) and biogenic isoprene. Model estimates of surface ozone concentrations tend to be biased high in the region and this is of concern for designing effective emission control strategies to meet air quality standards. We use detailed chemical observations from the SEAC4RS aircraft campaign in August and September 2013, interpreted with the GEOS-Chem chemical transport model at 0.25°  ×  0.3125° horizontal resolution, to better understand the factors controlling surface ozone in the Southeast US. We find that the National Emission Inventory (NEI) for NOx from the US Environmental Protection Agency (EPA) is too high. This finding is based on SEAC4RS observations of NOx and its oxidation products, surface network observations of nitrate wet deposition fluxes, and OMI satellite observations of tropospheric NO2 columns. Our results indicate that NEI NOx emissions from mobile and industrial sources must be reduced by 30–60 %, dependent on the assumption of the contribution by soil NOx emissions. Upper-tropospheric NO2 from lightning makes a large contribution to satellite observations of tropospheric NO2 that must be accounted for when using these data to estimate surface NOx emissions. We find that only half of isoprene oxidation proceeds by the high-NOx pathway to produce ozone; this fraction is only moderately sensitive to changes in NOx emissions because isoprene and NOx emissions are spatially segregated. GEOS-Chem with reduced NOx emissions provides an unbiased simulation of ozone observations from the aircraft and reproduces the observed ozone production efficiency in the boundary layer as derived from a regression of ozone and NOx oxidation products. However, the model is still biased high by 6 ± 14 ppb relative to observed surface ozone in the Southeast US. Ozonesondes launched during midday hours show a 7 ppb ozone decrease from 1.5 km to the surface that GEOS-Chem does not capture. This bias may reflect a combination of excessive vertical mixing and net ozone production in the model boundary layer.

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Short summary
Ground-level ozone pollution in the Southeast US involves complex chemistry driven by anthropogenic emissions of nitrogen oxides (NOx) and biogenic emissions of isoprene. We find that US NOx emissions are overestimated nationally by as much as 50 % and that reducing model emissions by this amount results in good agreement with SEAC4RS aircraft measurements in August and September 2013. Observations of nitrate wet deposition fluxes and satellite NO2 columns further support this result.
Ground-level ozone pollution in the Southeast US involves complex chemistry driven by...
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