A large reduction in airborne particle number concentrations at the time of the introduction of “sulphur free” diesel and the London Low Emission Zone

Hourly measurements of airborne particle number concentration (a metric dominated by ultrafine particles) at two sites in London and a site in Birmingham show that over a period of a few months in late 2007 concentrations were reduced by between 30 and 59%. At the London kerbside site particle number emissions from vehicles were reduced by 65% compared to emissions of NOx. These reductions occurred at the same time as the introduction of “sulphur free” diesel fuel and the London Low Emission Zone for heavy goods vehicles, and in view of the reduction in Birmingham and the limited change in vehicle technologies at that time, is probably primarily due to the change in fuel sulphur. There is an absence of any similar reduction in the concentration of the regulated air quality mass metrics which may provide an opportunity for further evaluating the effects of ultrafine particle concentrations on human health.