A low emission zone (LEZ) was implemented in the Barcelona metropolitan area in 2020, concurrently with the onset of the COVID-19 pandemic. We used a synthetic control method to estimate the impacts of the LEZ on NO 2 , PM 2.5 and PM 10 concentrations in Barcelona, accounting for the effects of COVID-19 and other underlying trends, in addition to meteorology and desert dust outbreaks. Results showed that the LEZ, reduced annual mean NO 2 concentrations by 7.6 μg/m 3 (95% CI: 6.7, 8.5 μg/m 3 ) at traffic stations, and by 4.5 μg/m 3 (95% CI: 3.7, 5.3 μg/m 3 ) at urban background stations. The LEZ was less effective in reducing PM 2.5 and PM 10 concentrations. These results indicate that the LEZ in Barcelona was effective in reducing NO 2 concentrations, while the effects on PM were more uncertain, probably due to the major secondary PM origin. • Synthetic control was used to quantify the effects of LEZ on air quality • LEZ effectively decreased NO 2 concentrations in Barcelona • Estimated reductions on PM were small and more uncertain • The effectiveness of LEZ on air quality is likely pollutant-dependent
Santos et al. (Sun,) studied this question.