Atmospheric oxidants such as ozone (O₃), hydroxyl and nitrate radicals (OH and NO₃) determine the ability of the urban atmosphere to process organic and inorganic pollutants, which have an impact on air quality, environmental health and climate. Madrid city has experienced an increase of 30–40% in ambient air O₃ levels, along with a decrease of 20–40% in NO₂, from 2007 to 2014. Using air pollution observations and a high-resolution air quality model, we find a large concentration increase of up to 70% and 90% in OH and NO₃, respectively, in downtown Madrid (domain-wide average increase of 10% and 32% for OH and NO₃, respectively). The results also show an 11% reduction in the nitric acid concentrations, leading to a remarkable denoxification of this urban atmosphere with implications for lower PM_2.5 levels and nitrogen input into ecosystems. This study suggests that projected worldwide NO_x emission reductions, following air quality standards, will lead to important changes in the oxidizing capacity of the atmosphere in and around large cities.