Abstract
•Roadside measurements of particle size distributions from 2010 to 2021.•Diesel particle filters introduced from 2011 steadily reduce black carbon concentrations.•Nucleation mode particles are little changed and total ultrafine particles show a small decline in concentration.•Ultrafine particle concentrations remain in the “high” category as defined by WHO.
Diesel engines are a major contributor to emissions of both Black Carbon (BC) and ultrafine particles. Analysis of data from the only roadside monitoring site in Europe with a continuous dataset for size-segregated particle number count (Marylebone Road, London) from 2010 to 2021 reveals that the growing number of vehicles fitted with a Diesel Oxidation Catalyst (DOC) and Diesel Particle Filter (DPF) has been very effective in controlling the emissions of solid particles and hence BC, but that there has been little change in the liquid mode (<30 nm) particles, and that concentrations of ultrafine particles (<100 nm) still well exceed the threshold for “high” concentrations (>104 cm−3 /24-hour mean) defined by WHO. BC declined by 81% between 2014 and 2021, but the ultrafine particle (<100 nm) count declined by only 26%. Consequently, in locations worldwide with heavy diesel traffic, concentrations of ultrafine particles are likely to remain “high” for the foreseeable future unless more effective abatement technologies are implemented.