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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 13 Nov 2018

Research article | 13 Nov 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Estimating background contributions and U.S. anthropogenic enhancements to maximum ozone concentrations in the northern U.S.

David D. Parrish1,2,3 David D. Parrish
  • 1Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, USA
  • 2NOAA/ESRL Chemical Sciences Division, Boulder, Colorado, USA
  • 3David.D.Parrish, LLC, Boulder, Colorado, USA

Abstract. U.S. ambient ozone concentrations have two components: U.S. background ozone and enhancements produced from the country’s anthropogenic precursor emissions; only the enhancements effectively respond to national emission controls. We investigate the temporal evolution and spatial variability of the largest ozone concentrations, i.e., those that define the ozone design value (ODV) upon which the National Ambient Air Quality Standard (NAAQS) is based, within the northern tier of U.S. states. We focus on two regions: rural western states, with only small anthropogenic precursor emissions, and the urbanized northeastern states, which include the New York City urban area, the nation's most populated. The U.S. background ODV (i.e., the ODV remaining if U.S. anthropogenic precursor emissions were reduced to zero) is estimated to vary from 54 to 63 ppb in the rural western states, and to be smaller and nearly constant (45.8 ± 1.7 ppb) throughout the northeastern states. These U.S. background ODVs correspond to 65 to 90 % of the 2015 NAAQS of 70 ppb. Over the past two to three decades U.S. emission control efforts have decreased the anthropogenic ODV enhancements at an approximately exponential rate with an e-folding time constant of ~ 22 years. These ODV enhancements are small in the rural western states (2.4 ± 1.2 ppb in 2000), with much larger state maximum ODV enhancements (~ 35–64 ppb in 2000) in the northeastern states. The U.S. background ODV contribution is significantly larger than the present-day ODV enhancements due to photochemical production from U.S. anthropogenic precursor emissions in the urban as well as the rural regions investigated. Forward projections of past trends suggest that average maximum ODVs in northeastern U.S. will drop below the NAAQS of 70 ppb by about 2021, assuming that the exponential decrease of the ODV enhancements can be maintained and the U.S. background ODV remains constant. This estimate is much more optimistic than in the Los Angeles urban area, where a similar approach estimates ~ 2050 for the maximum ODV to reach 70 ppb (Parrish et al., 2017). The primary reason for this large difference is the significantly higher U.S. ODV background (62.0 ± 2.0 ppb) estimated for the Los Angeles urban area. The approach used in this work has some unquantified uncertainties that are discussed. Models can also estimate U.S. background ODVs; some of those results are shown to correlate with the observational estimates derived here (r2 values for different models are ~ 0.31 to 0.85), but are on average systematically lower by 4 to 12 ppb. Further model improvement is required until their output can accurately reproduce the time series and variability of observed ODVs, and the uncertainties in the two approaches can be reduced through additional comparisons.

David D. Parrish
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Status: final response (author comments only)
Status: final response (author comments only)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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David D. Parrish
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Publications Copernicus
Short summary
Background ozone transported into cities contributes greatly to urban concentrations. Based upon projections of past trends, the largest ozone concentrations upon which the 70 ppb National Ambient Air Quality Standard is based will reach that standard by ~ 2021 in the New York City area, but much later (~ 2050) in the Los Angeles region. The much smaller background contribution in New York City (45.8 ± 1.7 ppb) than in Los Angeles (62.0 ± 2.0 ppb) is the primary reason for this large difference.
Background ozone transported into cities contributes greatly to urban concentrations. Based upon...