Further improvement in London's air quality demands more than the Ultra Low Emission Zone policy

Traffic emission is one of the most important sources of urban air pollution. Low emission zone (LEZ) is a flagship policy intervention to reduce urban emissions in many parts of the world. However, it is challenging to evaluate the effectiveness of such policies, with different methods resulting in several times in discrepancies. Here, we employed a causal framework to quantify the effects of Ultra-LEZ (ULEZ, 2019, central London) and its city-wide expansion (2023) on air quality in London. We found that the 2019 ULEZ led to 19.6% and 8.2% (~13.3 and 2.7 µg m^-3) reductions in nitrogen dioxide (NO₂) at traffic and urban background sites, respectively, in the three months after its implementation within central London, with positive spillover effects across Inner and Outer London. However, the ULEZ 2023 expansion showed no detectable impact on NO₂. Furthermore, we found no significant benefits of ULEZ on fine particle (PM_2.5) and PM_2.5, and NO₂ remained well above World Health Organization air quality guideline. Our results illustrate that ULEZ alone is insufficient to clean up the air in London and emissions from other sources such as domestic, commercial and industrial emissions and regional pollution should also be tackled.