Oxidation capacity changes in the atmosphere of large urban areas in Europe: Modelling and experimental campaigns in...

Air pollution is a major concern for human health and the environment. Consequently, environmental standards have become stricter to improve air quality. Thanks to this, the ambient levels of O₃ precursors such as VOCs and NO_X have decreased. However, O₃ levels in Europe, especially during winter, have increased, potentially impacting on atmospheric oxidation capacity and the associated chemistry of tropospheric oxidants. In this work, we focus on recent changes in the oxidation capacity of urban atmospheres. The study is conducted with the results of the CMAQ modelling system with a regional resolution with 12 × 12 km² across the entire European continent for the winter (January) and summer (July) of 2007 and 2015. The 2015 meteorological data is used for both years to emphasise emission changes during the studied period. We scrutinise the changes in ambient concentration levels of the main tropospheric oxidants (O₃ and HO_X radicals) in five representative cities, Valencia, Madrid, Milan, Berlin, and The Hague. The enhanced O₃ formation in winter seems to be due to the low VOC/NO_X ratio, while the opposite trend in summer may be related to a relatively high ratio. Additionally, photooxidation experiments are carried out in the EUPHORE chambers to study the effect of changes in NO_X concentration and NO/NO₂ ratio on the variation of the given oxidants at constant VOCs concentrations. For the baseline experiments, two scenarios are selected based on the model results of 2015: two representative winter and summer days of low and high pollution in Berlin and Madrid, respectively. The role of VOC/NO_X and NO/NO₂ ratios on atmospheric reactivity is discussed. As a result, it is first suggested that further decreases in ambient NO_X levels are required to reduce ambient O₃ levels. Moreover, additional factors should be considered when designing local-specific emission abatement strategies.