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

Concentrations alone do not provide answer to the question about how emissions are changing over the surface, in part due to simultaneous changes in transport, emissions, dynamics, photochemistry, and chemical feedback. Therefore simulating a chemical transport model benefiting from a multi-specie inversion framework using well-characterized observations should differentiate those influences enabling to closely focus on the emission part. This 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 individual months of March, April, and May 2020 (lockdown) and 2019 (baseline). We observe a noticeable decline in the magnitude of NOx emissions in March (14-31%) in several major cities such as Paris, Madrid, and Milan expanding further to Rome, Brussels, Frankfurt, London, Warsaw, Belgrade, Kyiv, and Moscow (34-51%) in April. A large variability associated with changes in NOx emissions is indicative of varying dates and the degree of restrictions enacted to prevent the spread of the virus. For instance, NOx emissions remain in 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 number of TROPOMI samples 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 dominantly influenced 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, Milan, London, and Rome. This striking result is only achievable by jointly incorporating HCHO and NO2 observations into the inversion context. 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: +7.85±11.27%, +3.76 ppbv). This dataset is not publicly accessible because: The data belongs to first author. It can be accessed through the following means: please contact the first author (ahsouri@cfa.harvard.edu). Format: netCDF, observation data, satellite data. 

This dataset is associated with the following publication:
Souri, A.H., K. Chance, J. Bak, C.R. Nowlan, G. Gonzalez Abad, Y. Jung, C. Wong, J. Mao, and X. Liu. Unraveling pathways of elevated ozone induced by the 2020 lockdown in Europe by an observationally constrained regional model using TROPOMI.   Atmospheric Chemistry and Physics. Copernicus Publications, Katlenburg-Lindau,  GERMANY, 21: 18227–18245, (2021).

Tags: lockdown, nox, ozone, voc
Organization: U.S. Environmental Protection Agency