Pronounced decrease of ambient particle number emissions from diesel traffic in Denmark after reduction of the sulphur content in diesel fuel

Measurements of particle number concentration and size distributions in a C openhagen street canyon in January-March 1999, and one year later in 2000, have revealed that the number of particles in the ultrafine size range belo w 100 nm decreased significantly in the period between the two campaigns. T he decrease was especially large in the size range below 30 nm. By two comm on factors, we found high co-variance between ultrafine particles, nitrogen oxides, and carbon monoxide. The co-variation can be used to assess the co ntributions and size distributions of particles from the two main sources, petrol traffic and diesel traffic. Here, we show that the drop in particle concentrations can be consistently explained by a 56% fall in the average p article emission from the diesel vehicles in the street. In the same period , we found no change in the particle emission from petrol cars. The change is probably due to the reduction of the sulphur content in diesel fuel from approximately 0.05% to less than 0.005%, implemented in all parts of Denma rk in July 1999. (C) 2001 Elsevier Science Ltd. All rights reserved.