Ozone pollution: a persistent challenge in large cities like Mexico City, Los Angeles and Beijing.

Over the past decade or so, many large cities around the world have made little to no progress in lowering ground-level ozone concentrations, despite significant reductions in key precursor pollutants directly emitted into the atmosphere. Ozone comes from complex chemical reactions in the air that make it difficult to control. Current control measures implemented in some cities have apparently reached their limit. While stricter emission regulations, improvements in technology and cleaner fuels have prevented a return to previous ozone levels, they have not reduced them further. They have triggered changes in the mixture of precursor species (i.e., nitrogen oxides and volatile organic compounds) due to differences in the extent to which their emissions have been reduced, emerging emission sources and the increasing relevance of emissions previously overlooked, such as those related to cooking and the use of household cleaning and personal care products. Similarly, as the contribution of typical emission sources (e.g., combustion of fossil fuels) has decreased, biogenic contributions have become more important, as well as the influence of regional and transboundary pollution. These changes have also responded to increasing urbanisation in the face of a changing climate that favours ozone production. There is no recipe that all cities can follow to tackle ambient ozone; however, it is necessary to review why ozone concentrations have not decreased in some cities and what they are doing about it in order to use it as a reference to update, improve and develop control measures tailored to local conditions, as actions implemented in one city may be ineffective or impractical in another. In such a context, this article examines the cases of three metropolitan areas: the Mexico City Metropolitan Area, the Los Angeles Basin and the Beijing-Tianjin-Hebei (