With an increasing percentage of the population living in cities, urban air quality has become an important factor in global health. Recent studies have clearly demonstrated the link between particulate matter (PM) exposure and adverse health outcomes. Within the urban environment, road traffic, particularly diesel vehicles, is one of the most significant emission sources of particles. Once particles are emitted, they can be modified by a number of physical and chemical processes. Many uncertainties exist regarding the semi-volatile organic component of the particles, which may evaporate and subsequently oxidise to form less volatile compounds which recondense onto solid particles, giving an increased particle mass.