Modelling the impacts of emission changes on O₃ sensitivity, atmospheric oxidation capacity and pollution transport over the Catalonia region

Abstract. Tropospheric ozone (O₃) is an important surface pollutant in urban areas, and it has complex formation mechanisms that depend on the atmospheric chemistry and meteorological factors. The severe reductions observed in anthropogenic emissions during the COVID-19 pandemic can further our understanding of the photochemical mechanisms leading to O₃ formation and provide guidance for policies aimed at reducing air pollution. In this study, we use the air quality model WRF-Chem coupled with the urban canopy model BEP-BEM to investigate changes in the ozone chemistry over the Metropolitan Area of Barcelona (AMB) and its atmospheric plume moving northwards, which is responsible for the highest number of hourly O₃ exceedances in Spain. The trajectories of the air masses from the AMB to the Pyrenees are studied with the Lagrangian particle dispersion model FLEXPART-WRF. The aim is to investigate the response of ozone chemistry to changes in the precursor emissions. The results show that with the reduction in emissions: 1) the ozone chemistry tends to enter the nitrogen oxide (NOx)-limited or transition regimes; however, highly polluted urban areas are still in the Volatile Organic Compounds (VOC)-limited regime, 2) the reduced O₃ production is overwhelmed by reduced nitric oxide (NO) titration, resulting in a net increase in the O₃ concentration (up to 20 %) in the evening, 3) the increase in the maximum O₃ level (up to 6 %) during the lockdown could be attributable to an enhancement in the atmospheric oxidation capacity (AOC), 4) the daily maximum levels of ozone and odd oxygen species (O_x) generally decreased (4 %) in May with the reduced AOC, indicating an improvement in the air quality, and, 5) ozone precursor concentration changes in the AMB contribute to the pollution plume moving along the S–N valley to the Pyrenees. Our results indicate that O₃ abatement strategies cannot rely only on NO_x emission control but must include a significant reduction in anthropogenic sources of VOCs (e.g., for power plants and heavy industry). In addition, our results show that mitigation strategies intended to reduce O₃ should be designed according to the local meteorology, air transport, particular ozone regimes and AOC of the urban area.