Evaluation of industrial emissions of tropospheric ozone precursors in Spain (2017–2022)

Tropospheric ozone (O₃) is a secondary atmospheric pollutant formed through complex photochemical reactions of precursors, mainly involving nitrogen oxides (NOx) and volatile organic compounds (VOC), which poses critical challenges to air quality, public health, and environmental sustainability. This study provides an in-depth analysis of industrial emissions of O₃ precursors in Spain, focusing on their spatial distribution, sectoral contributions, temporal trends, and the effectiveness of realistic mitigation strategies. By analysing the emission inventories of the precursors and collecting information from visits to 50 industrial facilities in the four critical O₃ air basins in Spain, the research identifies the main emission sources, assesses the application of Best Available Techniques (BAT) and examines the potential impacts of decarbonisation initiatives. Findings revealed that a small number of facilities contribute largely to total emissions of precursors, with significant regional and sectoral differences in monitoring practices and emission abatements. NOx emissions, largely channelled, have seen significant reductions mainly due to the high penetration of renewable energies in electricity generation in Spain, while VOC emissions remain challenging due to their diffuse nature and reliance on solvent-intensive processes. Decarbonisation efforts are shown to have a dual effect, with potential reductions in NOx but limited influence on VOC emissions. The recommendations include strengthening regulatory harmonisation, improving emission monitoring methodologies, especially for multi-source and non-channelled emissions, as well as promoting innovation in precursor reduction technologies. The conclusions and recommendations will be integrated into the Spanish Tropospheric Ozone Mitigation Plan (STOMP) and may provide valuable insights for other mitigation plans.