Unraveling Pathways of Elevated Ozone Induced by the 2020 Lockdown in Europe by an Observationally Constrained Regional Model: Non-Linear Joint Inversion of NO x and VOC Emissions using TROPOMI

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.