AQC supports the Clean Cities Campaign by quantifying the impact of low- and zero-emission zones in Europe

The European Commission is reviewing the Ambient Air Quality Directive, but its impact assessment does not account for the potential impacts of further LEZs and ZEZs. There is a concern that opportunities for air quality improvements might be missed. Clean Cities Campaign thus commissioned AQC to collate and evaluate evidence of the effects of LEZs and ZEZs on air quality in European cities.

The impacts of LEZs and ZEZs are difficult to assess. This is mainly due to the problems of constructing a robust counter-factual, and in isolating the effect of the Zone from the wider changes to the vehicle fleet and driver/operator behaviours (as well as wider societal changes).  Many previous studies have shown LEZs to have minimal effects, but this often related to disappointing real-world performance of early ‘Euro’ emissions standards.  Other published studies have shown much larger effects, but have often failed to adequately control for other factors affecting air quality, making their conclusions tenuous.

AQCs review draws on published literature, a search of selected national and EU databases, and the expert knowledge of the authors.  Its main findings are:

• Well-designed, carefully implemented and stringently enforced LEZs can make a significant contribution to improving air quality in urban areas. NO₂ concentration reductions of around 40% are not only theoretically possible but have been demonstrated in practice. Reductions of around 20% have already been achieved and may be more achievable in a wider set of circumstances. ZEZs could have a greater effect, but this depends on when they are introduced, the relative change in the fleet, and the proportion of the fleet they cover.
• In terms of Particulate Matter (PM₁₀ and PM_2.5), the potential for reduction by LEZs and ZEZs is smaller. This is related to both the fact that the proportion of locally measured PM concentrations associated with exhaust emissions is much smaller than for NO₂ and also that a larger proportion of the diesel fleet will already have Diesel Particulate Filters fitted. However, in some circumstances, such as in countries with older vehicle fleets (and especially, e.g. municipal buses), LEZs may still have a significant positive impact on reducing PM emissions. In addition, there is an ongoing debate on the importance of reducing not only the particle mass (PM) but also particle numbers (PN), which have been found to still be emitted in large quantities by Euro 6 petrol and diesel vehicles.
• LEZs can positively improve air quality in areas outside the Zone and prior to full implementation (pre-compliance). Significant preparatory work, in terms of technical design, financial support and social outreach, are beneficial for the Zone to be both implemented successfully, and to achieve the forecast benefits.
• The impact of a LEZ inevitably reduces over time, as older vehicles are naturally replaced. Emissions standards within an LEZ may, however, be tightened in the future.

Read AQC’s Review Here

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