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

► Time series of particle number counts measured at 3 sites from 2005 to 2010 were analysed. ► The time series show a marked reduction in concentration occurring in late 2007 which did not affect other pollutants. ► Changes in particle size distributions occurred at the same time. ► The reduction is attributed primarily to the introduction of sulphur free (<10 ppm) diesel fuel.