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Geoscientific Model Development An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/gmd-2019-78
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/gmd-2019-78
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Methods for assessment of models 04 Apr 2019

Methods for assessment of models | 04 Apr 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Geoscientific Model Development (GMD).

Systematic bias in evaluating chemical transport models with maximum daily 8-hour average (MDA8) surface ozone for air quality applications

Katherine R. Travis1 and Daniel J. Jacob2,3 Katherine R. Travis and Daniel J. Jacob
  • 1Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA
  • 2School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
  • 3Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA

Abstract. Chemical transport models typically evaluate their simulation of surface ozone with observations of the maximum daily 8-hour average (MDA8) concentration, which is the standard air quality policy metric. This requires successful simulation of the surface ozone diurnal cycle including nighttime depletion, but models are generally biased high at night because of difficulty in resolving the stratified conditions near the surface. We quantify the problem with the GEOS-Chem model for the Southeast US during the NASA SEAC4RS aircraft campaign in August–September 2013. The model is unbiased relative to the daytime mixed layer aircraft observations but has a +5 ppb bias relative to MDA8 surface ozone observations. The model also does not capture observed occurrences of < 20 ppb MDA8 surface ozone on rainy days. Restricting the evaluation to afternoon hours and dry days removes the bias. Better understanding of surface layer stratification and ozone depletion under nighttime and rainy conditions is needed. Resolving the timing of the day-night transition in atmospheric stability and its correlation with plant stomata closure is critical.

Katherine R. Travis and Daniel J. Jacob
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Status: final response (author comments only)
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Katherine R. Travis and Daniel J. Jacob
Katherine R. Travis and Daniel J. Jacob
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Short summary
Models of ozone air pollution are often evaluated with the policy metric set by the Environmental Protection Agency of the maximum daily 8-hour average ozone concentration. These models may be used in policy settings to set and evaluate air quality regulations. However, most models have difficulty simulating ozone at night and therefore do not properly represent how ozone changes over the course of an entire day which results in biases in the 8-hour metric.
Models of ozone air pollution are often evaluated with the policy metric set by the...
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