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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 29 Apr 2019

Research article | 29 Apr 2019

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

European NOx emissions in WRF-Chem derived from OMI: impacts on summertime surface ozone

Auke J. Visser1, K. Folkert Boersma1,2, Laurens N. Ganzeveld1, and Maarten C. Krol1,3 Auke J. Visser et al.
  • 1Wageningen University, Meteorology and Air Quality Section, Wageningen, the Netherlands
  • 2Royal Netherlands Meteorological Institute, R&D Satellite Observations, de Bilt, the Netherlands
  • 3Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, the Netherlands

Abstract. Ozone (O3) is a secondary air pollutant that negatively affects human and ecosystem health. Ozone simulations with regional air quality models suffer from unexplained biases over Europe, and uncertainties in the emissions of ozone precursor group nitrogen oxides (NOx = NO + NO2) contribute to these biases. The goal of this study is to use NO2 column observations from the OMI satellite sensor to infer top-down NOx emissions in the regional meteorology-chemistry model WRF-Chem, and to evaluate the impact on simulated surface O3 with in situ observations. We first perform a simulation for July 2015 over Europe and evaluate its performance against in situ observations from the AirBase network. The spatial distribution of mean ozone concentrations is reproduced satisfactorily. However, the simulated maximum daily 8-hour ozone concentration (MDA8 O3) is underestimated (mean bias error (MBE) = −14.2 μg m−3), and its spread is too low. We subsequently derive satellite-constrained surface NOx emissions using a mass balance approach based on the relative difference between OMI and WRF-Chem NO2 columns. The method accounts for feedbacks through OH, NO2's dominant daytime oxidant. Our optimized European NOx emissions amount to 0.50 Tg N (for July 2015), 0.18 Tg N higher than the bottom-up emissions (which lacked agricultural soil NOx emissions). Much of the increases occur across Europe, in regions where agricultural soil NOx emissions dominate. Our best estimate of soil NOx emissions in July 2015 is 0.1 Tg N, much higher than the bottom-up 0.02 Tg N natural soil NOx emissions from the MEGAN model. A simulation with satellite-updated NOx emissions reduces the systematic bias between WRF-Chem and OMI NO2 (slope = 0.98, r2 = 0.84), and reduces the low bias against independent surface NO2 measurements by 1.1 μg m−3 (−56 %). Following these NOx emission changes, daytime ozone is strongly affected, since NOx emission changes particularly affect daytime ozone formation. Monthly averaged simulated daytime ozone increases by 6.0 μg m−3, and increases of >10 μg m−3 are seen in regions with large emission increases. With respect to the initial simulation, MDA8 O3 has an improved spatial distribution, expressed by an increase in r2 from 0.40 to 0.53, and a reduced mean bias (−7.4 μg m−3, −48 %). Overall, our results highlight the dependence of surface ozone on its precursor NOx and demonstrate that simulations of surface ozone benefit from constraining surface NOx emissions by satellite NO2 column observations.

Auke J. Visser et al.
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Status: final response (author comments only)
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Auke J. Visser et al.
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Short summary
Health and ecosystem impacts of O3 generally occur when O3 concentrations are highest, but most air quality models underestimate peak O3. We derived European NOx emissions based on satellite NO2 column data and evaluated the impact on model-simulated NO2 and ozone. We show that a simulation with satellite-derived NOx emissions leads to better agreement with independent in situ observations of surface NO2 and O3, which helps to reduce the model underestimations of peak ozone concentrations.
Health and ecosystem impacts of O3 generally occur when O3 concentrations are highest, but most...