Five years after Barcelona introduced its low-emission zone (LEZ), researchers have found that the measures have reduced nitrogen dioxide levels by nearly 8 µg/m³.
The study used a sophisticated synthetic control method to isolate the LEZ’s impact from two major confounders: the COVID-19 pandemic and everyday weather variability. By comparing air quality at stations inside the 95 km² restricted zone with control stations far outside it, the team could estimate what would have happened without the LEZ.
Vanessa N. dos Santos, predoctoral researcher at ISGlobal and UPF and first author of the study, explained: ‘We used an innovative technique, the statistical SC-PI method, which allows us to construct a simulated version of the area without the LEZ, in order to compare what would have happened if the measure had not been implemented. This ‘synthetic zone’ serves as a reference to estimate what levels of NO₂, PM10 and PM2.5 would have been recorded without the measure and to compare them with the actual values.’
The results show that between 2020 and 2022, the LEZ reduced annual mean nitrogen dioxide (NO₂) concentrations by 7.6 µg/m³ at traffic-heavy monitoring sites, and by 4.5 µg/m³ at urban background stations. That represents a roughly 16% cut from pre-LEZ levels.
However, the policy’s effect on particulate matter was less dramatic. Reductions in PM₁₀ and PM₂.₅ were around 1 µg/m³ or less, and in some model specifications, they were not statistically significant. The researchers suggest this is because traffic contributes less to PM concentrations than to NO₂, with secondary particle formation from agriculture, heating and industry playing a larger role.
The fact that the LEZ was introduced around the time of the pandemic lockdowns made evaluation difficult, but by using control areas, the study accounted for the COVID-related drop in traffic that also affected non-LEZ zones.
Even with the reductions achieved, Barcelona’s NO₂ levels remain above both the updated EU directive and WHO guidelines. The authors conclude that while LEZs are effective against nitrogen dioxide, tackling particulate matter will require additional measures targeting regional agricultural emissions, shipping, and industrial sources.
Xavier Basagaña, ISGlobal researcher and coordinator of the study. said: ‘Our results suggest that the LEZ in the Barcelona metropolitan area is an effective tool to mitigate air pollution, although its effects depend on the type of pollutant. Although it clearly reduced NO₂, its impact on particulate matter was limited. To reach the levels recommended by WHO in 2021, additional measures will probably be required, such as further reducing traffic or acting on other emission sources, including agricultural ones.’
The full research can be read here.
Photo: Florian Wehde
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