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

Tropospheric ozone (O-3) is an important surface pollutant in urban areas, and it has complex formation mechanisms that depend on the atmospheric chemistry and on meteorological factors. The severe reductions observed in anthropogenic emissions during the COVID-19 pandemic can further our understanding of the photochemical mechanisms leading to O-3 formation and provide guidance for policies aimed at reducing air pollution. In this study, we use the Weather Research and Forecasting model with Chemistry (WRF-Chem) coupled with the urban canopy building effect parameterization and building energy 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 O-3 exceedances in Spain. The trajectories of the air masses from the AMB to the Pyrenees are studied with the Lagrangian FLEXible PARTicle dispersion model with WRF (FLEXPART-WRF). The aim is to investigate the response of ozone chemistry to reduction in precursor emissions (NOx - nitrogen oxides; VOCs - volatile organic compounds). The results show that, with the reduction in emissions, (1) the ozone chemistry tends to enter the NOx-limited or transition regimes, but highly polluted urban areas are still in the VOC-limited regime; (2) the reduced O-3 production is overwhelmed by reduced nitric oxide (NO) titration, resulting in a net increase in the O-3 concentration (up to 20 %) in the evening; (3) the increase in the maximum O-3 level (up to 6 %) during the highest emission-reduction period could be attributed to an enhancement in the atmospheric oxidants hydroxyl and nitrate radical (OH and NO3) given their strong link with O-3 loss or production chemistry; (4) the daily maximum levels of ozone and odd oxygen species (O-x) generally decreased (4 %) in May - a period with intense radiation which favours ozone production - with the reduced atmospheric OH and NO3 oxidants, indicating an improvement in the air quality; and (5) ozone precursor concentration changes in the urban plume of Barcelona contribute significantly to the level of pollution along the 150 km south-to-north valley in the Pyrenees. Our results indicate that O-3 abatement strategies cannot rely only on NOx emission control but must include a significant reduction in anthropogenic sources of VOCs. In addition, our results show that mitigation strategies intended to reduce O-3 should be designed according to the local meteorology, air transport, particular ozone regimes, and oxidation capacity of the atmosphere of the urban area.