Traffic management strategies for emissions reduction: recent experience in London

Air pollution strategies in London over the last 12 years have centered upon the congestion charging scheme, and at the same time, the fitting of particle traps to London buses, the low emissions zone (LEZ), and the Mayor’s Air Quality Strategy (MAQS). The 2003 congestion charging scheme achieved much of the scheme’s aims, but the demand to travel and the need for road space eroded the initial benefits. While fitting particle traps on buses was predicted to reduce particulate matter (PM) exhaust emissions, the introduction of phases 1 and 2 of the LEZ and MAQS strategies were both predicted to have modest emission impacts. Reliance on new Euro-standard vehicles to reduce emissions, and as a way of designing LEZs, has been problematic, with oxides of nitrogen (NOx) and nitrogen dioxide (NO2) emissions from diesel vehicles reducing less than predicted. Consequently, the UK has not met annual NO2 European Union (EU) limit values, necessitating a time extension application. A mismatch between PM10 ambient trends and emissions has also been reported, with the long-term performance of PM particle filters remaining an important question. Assessing London’s traffic management schemes has relied upon emission inventories and dispersion models, and to date, there has been no confirmation of the effects of the schemes using ambient data, a challenging and important area of research. However, measurements of ambient NOx, NO2, ozone, PM species, and roadside vehicle emissions have all contributed to the improvement of road traffic emission inventories in London, and it remains important to undertake ambient monitoring to assess future schemes. Looking forward, the real-world emissions performance of Euro 6/VI vehicles, selective catalytic reduction, and the ultra-low emissions zone in London will play a critical role in meeting EU limit values for ambient NO2, and in light of the increasing health evidence of urban air pollution, policy makers should aim to reduce PM concentrations toward health-based World Health Organization guideline values.