Response of the ozone chemistry to changes in emissions over the Catalonia region

Tropospheric ozone (O₃) is an important surface pollutant in urban areas with complex formation mechanisms that depend on the atmospheric chemistry composition and meteorological factors. The severe reduction in anthropogenic emissions during the COVID-19 pandemic can serve to further our understanding of the photochemical mechanisms that lead to O₃ formation to provide guidance for policy aiming to reduce 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 northward, which is responsible for the highest number of hourly O₃ exceedances in Spain. The aim is to investigate the response of the ozone chemistry to changes in precursor emissions. Results show that with the reduction in emissions: 1) the ozone chemistry formation tends to go to the NOx-limited or transition regimes, however urban areas over highly polluted areas are still in the VOC-limited regime, 2) the reduced O₃ production is overwhelmed by the less nitric oxide (NO) titration resulting in a net increase of O₃ concentration (up to 20 %) in the afternoon, 3) the increase in maximum O₃ (up to 6%) during the lockdown could be attributed to an enhancement in the atmospheric oxidation capacity, 4) ozone and odd oxygen (O_x) maximum levels generally decrease (up to 4 %) in the relaxation period with a reduced atmospheric oxidation capacity (AOC), indicating an improvement of the air quality, and, 5) changes in ozone concentrations in the AMB contribute to the pollution plume along the S–N valley to the Pyrenees. Our results indicates that a protocol with strict measures to control NO_x emissions, without cutting significantly anthropogenic sources of VOCs (e.g. for power plants and heavy industry) is essential for O₃ abatement plans. In addition, our results show that the design of a mitigation strategy to reduce O₃ cannot be related only on emissions reductions because ozone chemistry depends on several other factors (AOC, ozone regimes, local meteorology, transport).