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Discussion papers
https://doi.org/10.5194/acp-2019-168
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-168
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 25 Feb 2019

Research article | 25 Feb 2019

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

Using satellite observations of tropospheric NO2 columns to infer long-term trends in US NOx emissions: the importance of accounting for the free tropospheric NO2 background

Rachel F. Silvern1, Daniel J. Jacob1,2, Loretta J. Mickley2, Melissa P. Sulprizio2, Katherine R. Travis3, Eloise A. Marais4, Ronald C. Cohen5,6, Joshua L. Laughner5,a, Sungyeon Choi7, Joanna Joiner7,8, and Lok N. Lamsal8,9 Rachel F. Silvern et al.
  • 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA
  • 2School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
  • 3Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA
  • 4Department of Physics and Astronomy, University of Leicester, Leicester, UK
  • 5Department of Chemistry, University of California, Berkeley, CA, USA
  • 6Department of Earth and Planetary Science, University of California, Berkeley, CA, USA
  • 7Science Systems and Applications Inc., Lanham, MD, USA
  • 8NASA Goddard Space Flight Center, Greenbelt, MD, USA
  • 9Goddard Earth Sciences Technology and Research, Universities Space Research Association, Columbia, Maryland, USA
  • anow at: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA

Abstract. The National Emission Inventory (NEI) of the US Environmental Protection Agency (EPA) reports a steady decrease of US NOx emissions over the 2005–2017 period at a rate of 0.1 Mt a−1 (53 % decrease over the period), reflecting sustained efforts to improve air quality. Tropospheric NO2 columns observed by the satellite-based Ozone Monitoring Instrument (OMI) over the US show a steady decrease until 2009 but a flattening afterward, which has been attributed to a flattening of NOx emissions in contradiction with the NEI. We show here that the steady 2005–2017 decrease of NOx emissions reported by the NEI is in fact consistent with observed network trends of surface NO2 and ozone concentrations. The OMI NO2 trend is instead similar to that observed for nitrate wet deposition fluxes, where post-2009 flattening is due to an increasing relative contribution of non-anthropogenic background (mainly lightning and soils) and not to a flattening of anthropogenic emissions. This is confirmed by contrasting OMI NO2 trends in urban winter, where the background is low and OMI NO2 shows a steady 2005–2017 decrease consistent with the NEI, and rural summer, where the background is high and OMI NO2 shows no significant 2005–2017 trend. A GEOS-Chem model simulation driven by NEI emission trends for the 2005–2017 period reproduces these different trends except for the post-2009 flattening of OMI NO2, which we attribute to a model underestimate of free tropospheric NO2. Better understanding is needed of the factors controlling free tropospheric NO2 in order to relate satellite observations of tropospheric NO2 columns to the underlying NOx emissions and their trends. Focusing on urban winter conditions in the satellite data minimizes the effect of this free tropospheric background.

Rachel F. Silvern et al.
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Status: final response (author comments only)
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Rachel F. Silvern et al.
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Short summary
The US EPA reports a steady decrease in nitrogen oxide (NOx) emissions from fuel combustion over the 2005–2017 period, while satellite observations show a leveling off after 2009 suggesting emission reductions and related air quality gains have halted. We show the sustained decrease in NOx emissions is in fact consistent with observed trends in surface NO2 and ozone concentrations, and that the flattening of the satellite trend reflects a growing influence from the non-anthropogenic background.
The US EPA reports a steady decrease in nitrogen oxide (NOx) emissions from fuel combustion over...
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