How do meteorological conditions impact the effectiveness of various traffic measures on NOx concentrations in a real hot-spot?

Recently, air quality has become a major concern for policy makers around the world, which has led to the implementation of mitigation measures. In particular, in urban areas most measures affect the road transport sector, as this is one of the main contributors to air pollution in those areas. Recent studies have pointed out the need to determine the importance of external factors such as the meteorological conditions on the net effect on air quality of mitigation strategies. Due to the strong spatial variability of urban air pollution, high spatial resolution modelling is necessary. In this work, the impacts on emissions and nitrogen oxides (NO_x) concentrations of several mitigation strategies on a real air pollution hot spot in southern Madrid (Spain) are simulated at microscale under different meteorological conditions. The results show that the meteorological conditions affect local NO_x concentrations, and its net changes can be comparable to those due to emission reductions. In particular, meteorological conditions in 2019 induced higher NO_x concentrations than in 2016, despite the local emissions were reduced by 50 % from 2016 to 2019. On the other hand, the impact of the implementation of a Low Emissions Zone (LEZ) on NO_x concentrations is small and consistent with values found in other LEZs around Europe. However, this impact varies up to 70 % depending on the meteorological conditions. The impacts of a mitigation strategy are largely influenced by the meteorological conditions, and therefore the achievement of the target reduction of concentrations pursued by these measures will depend on the meteorological conditions.