Air quality assessment during the low emission zone implementation in Madrid (Spain)

Abstract

Low emission zones (LEZ) represent one of the strategies to reduce, or limit, emissions of air pollutants from road traffic within specific spatial areas in cities. This study aims to assess the effectiveness of Madrid LEZ by quantifying the reduction in ambient and background pollution, the latter representative of the pollution to which the population is chronically exposed. This latter fraction was estimated after clustering the analysed time series with Hidden Markov Models. For that purpose, ten monitoring stations were selected, and the behaviour of five key pollutant concentrations was studied through phases of the LEZ implementation in Madrid. The average reduction in the NO2, PM10 and PM2.5 ambient pollution concentration was 12.1 μg·m−3, 5.2 μg·m−3 and 3.4 μg·m−3, respectively. Regarding background pollution, these reductions were 9.6 μg·m−3 for NO2, 0.5 μg·m−3 for PM10, and 1.2 μg·m−3 for PM2.5. Diff-in-Diff analysis compares the changes in outcomes over time between a population enrolled in a program (sites within LEZ zone - the treatment group-) and a population that is not (sites outside the LEZ zone - the control group). This analysis showed that the reduction of NO2 background pollution is less affected by LEZ implementation and that meteorological factors could have slightly influenced the decrease in NO2 concentration. Road traffic contributions were estimated to be reduced by 12.8 μg·m−3 for NO2 and 5.3 μg·m−3 for PM10. The number of times NO2 and PM10 concentrations exceeded the limit values established by the Directive 2008/50/EC was also reduced, but not sufficiently for achieving legal compliance. Even though our results show that LEZ implementation in Madrid has been effective and represented an improvement in the air quality of the city, it still can be considered insufficient. Further, restrictive measures remain to be targeted for its impact on urban health to be more pronounced.

Se trata de la versión preprint del artículo. Se puede consultar la versión final en: https://doi.org/10.1016/j.uclim.2024.101995