Challenges and Approaches for Developing Ultrafine Particle Emission Inventories for Motor Vehicle and Bus Fleets

Motor vehicles in urban areas are the main source of ultrafine particles (diameters < 0.1 μm). Ultrafine particles are generally measured in terms of particle number because they have little mass and are prolific in terms of their numbers. These sized particles are of particular interest because of their ability to enter deep into the human respiratory system and contribute to negative health effects. Currently ultrafine particles are neither regularly monitored nor regulated by ambient air quality standards. Motor vehicle and bus fleet inventories, epidemiological studies and studies of the chemical composition of ultrafine particles are urgently needed to inform scientific debate and guide development of air quality standards and regulation to control this important pollution source. This article discusses some of the many challenges associated with modelling and quantifying ultrafine particle concentrations and emission rates for developing inventories and microscale modelling of motor vehicles and buses, including the challenge of understanding and quantifying secondary particle formation. Recommendations are made concerning the application of particle emission factors in developing ultrafine particle inventories for motor vehicle fleets. The article presents a précis of the first published inventory of ultrafine particles (particle number) developed for the urban South-East Queensland motor vehicle and bus fleet in Australia, and comments on the applicability of the comprehensive set of average particle emission factors used in this inventory, for OPEN ACCESS Atmosphere 2011, 2 37 developing ultrafine particle (particle number) and particle mass inventories in other developed countries.