Atmospheric particles in central London during July 1996 have been apportioned to sources by two complimentary techniques:(1) morphological characteristics, determined using high-power light microscopy; (2) chemical composition determined by energy dispersive spectroscopy. During the study period, the majority of particles were found to be either biological in origin, or from vehicle-related combustion processes. Trends in the latter group of particles are explored further. Numbers of particles per cubic metre were determined at an hourly resolution over a period of six days, and were found to be significantly correlated with other traffic pollutants such as benzene and 1,3-butadiene, although there is no relationship with mass of PM10 as measured by a nearby UK Govment monitoring station. We suggest in this paper that numbers of particles from car exhausts are more representative of traffic pollution than current estimates which use monitoring by mass.