12 Mar 2021

12 Mar 2021

Review status: this preprint is currently under review for the journal ACP.

Unraveling Pathways of Elevated Ozone Induced by the 2020 Lockdown in Europe by an Observationally Constrained Regional Model: Non-Linear Joint Inversion of NOx and VOC Emissions using TROPOMI

Amir H. Souri1, Kelly Chance1, Juseon Bak2, Caroline R. Nowlan1, Gonzalo González Abad1, Yeonjin Jung1, David C. Wong3, Jingqiu Mao4,5, and Xiong Liu1 Amir H. Souri et al.
  • 1Atomic and Molecular Physics (AMP) Division, Harvard–Smithsonian Center for Astrophysics, Cambridge, MA, USA
  • 2Institute of Environmental Studies, Pusan National University, Busan, South Korea
  • 3U.S. Environmental Protection Agency, Center for Environmental Measurement & Modeling, Research Triangle Park, NC, USA
  • 4Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA
  • 5Department of Chemistry and Biochemistry, University of Alaska Fairbanks, Fairbanks, AK, USA

Abstract. Questions about how emissions are changing during the COVID-19 lockdown periods cannot be answered by observations of atmospheric trace gas concentrations alone, in part due to simultaneous changes in atmospheric transport, emissions, dynamics, photochemistry, and chemical feedback. A chemical transport model simulation benefiting from a multi-species inversion framework using well-characterized observations should differentiate those influences enabling to closely examine changes in emissions. This approach has another advantage in that we can, to a certain extent, disentangle the chemical and physical processes involved in the formation of ozone. Accordingly, we jointly constrain NOx and VOC emissions using well-characterized TROPOMI HCHO and NO2 columns during the months of March, April, and May 2020 (lockdown) and 2019 (baseline). We observe a noticeable decline in the magnitude of NOx emissions in March 2020 (14–31 %) in several major cities including Paris, London, Madrid, and Milan expanding further to Rome, Brussels, Frankfurt, Warsaw, Belgrade, Kyiv, and Moscow (34–51 %) in April. The large variability of changes in NOx emissions is indicative of different dates and the degree of restrictions enacted to prevent the spread of the virus. For instance, NOx emissions remain at somewhat similar values or even higher in northern Germany and Moscow in March 2020 compared to the baseline. Comparisons against surface monitoring stations indicate that the model estimate of the NO2 reduction is underestimated, a picture that correlates with the TROPOMI frequency impacted by cloudiness. During the month of April, when ample TROPOMI samples are present, the surface NO2 reductions occurring in polluted areas are described fairly well by the model (model: −21 ± 17 %, observation: −29 ± 21 %). Changes in VOC emissions are dominated by eastern European biomass burning activities and biogenic isoprene emissions. In March, however, TROPOMI HCHO sets an upper limit for HCHO changes such that the chemical feedback of NOx on HCHO constrained by TROPOMI NO2 reveals a non-negligible decline in anthropogenic VOC emissions in Paris (−9 %), Milan (−29 %), London (−5 %), and Rome (−5 %). Results support an increase in surface ozone during the lockdown. In April, the constrained model features a reasonable agreement with maximum daily 8 h average (MDA8) ozone changes observed at the surface (r = 0.43), specifically over central Europe where ozone enhancements prevail (model: +3.73 ± 3.94 %, +1.79 ppbv, observation: +7.35 ± 11.27 %, +3.76 ppbv). Results of integrated process rates of MDA8 surface ozone over central Europe in the same month suggest that physical processes (dry deposition, advection and diffusion) decrease ozone on average by −4.83 ppbv, while ozone production rates dampened by largely negative JNO2[NO2]-kNO+O3[NO][O3] become less negative, leading ozone to increase by +5.89 ppbv. Experiments involving fixed anthropogenic emissions suggest that meteorology (mainly as air temperature and photolysis) contributes to 42 % enhancement in MDA8 surface ozone over the same region with the remaining part (58 %) coming from changes in anthropogenic emissions. Results illustrate the capability of satellite data of major ozone precursors to help atmospheric models capture the essential character of ozone changes induced by abrupt emission anomalies.

Amir H. Souri et al.

Status: open (until 07 May 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-121', Anonymous Referee #3, 02 Apr 2021 reply
  • RC2: 'Comment on acp-2021-121', Anonymous Referee #1, 06 Apr 2021 reply
  • RC3: 'Comment on acp-2021-121', Anonymous Referee #2, 15 Apr 2021 reply

Amir H. Souri et al.

Data sets

Inversion Estimates Amir H. Souri, Kelly Chance, Juseon Bak, Caroline Nowlan, Gonzalo González Abad, Yeonjin Jung, David Wong, Jingqiu Mao, and Xiong Liu

Amir H. Souri et al.


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Short summary
The global pandemic is believed to have an impulsive impact on emissions of air pollutants such as nitrogen dioxide (NO2) and formaldehyde (HCHO). This study rigorously quantifies the changes in the amount of NOx and VOC emissions via state-of-the-art inverse modeling technique using satellite observations during the lockdown 2020 with respect to a baseline over Europe, which in turn, it permits unraveling atmospheric processes being responsible for ozone formation.