Abstract

High time resolution, size segregated studies of ultrafine particle number concentrations (PNC) were performed at two UK roadside locations using a DMS500 particle spectrometer including sampling with and without a thermodenuder. Intercomparisons between the DMS500 , an SMPS and a CPC , indicate that the SMPS can underestimate PNC due to the transient nature of excursions in concentrations and the longer scan time of the instrument. Short duration transient periods of high number concentrations made considerable contributions to hourly average PNC . The transient nature of changes in PNC potentially affects the type of exposure experienced in the roadside environment, as well as indoor outdoor exchange. The transient shifts were dominated by Aitken mode particles during high traffic periods, with the Aitken mode associated with soot based diesel emissions. The thermodenuder studies indicated the presence of highly volatile liquid droplets in the nucleation mode which were lost on heating. Nucleation mode particle numbers were shown to be highly dependant on atmospheric and traffic conditions as well as pre-existing particle surface area. The transient nature of the modal concentrations and its significance for human exposure in urban areas will be disucssed as well as issues related to suitable particle concentration metrics.