Devices for obtaining size distributions, such as Sequential/Scanning Mobility Particle Sizers (SMPS) often require a period of several minutes to obtain a distribution. During this time, if the particle concentration changes, the distribution is distorted and can be inaccurate. Ideally, high temporal resolution is required for the study of emissions from individual vehicles at roadside sites. Such systems have recently become commercially available. However, in order to prolong the use of low time-resolution systems, it is beneficial to determine under what circumstances they can reliably be used. This was a topic of studies by Yao et al. (2006a, b) who concluded that even at 6 s resolution, significant size distribution distortion could occur at roadsides. Clearly the characteristic timescale of variability in concentration, as well as concentration itself, is a key factor in determining the suitability and accuracy of SMPS operation. This study explores the possible artifacts created under circumstances of a single, sudden and short-lived increase in particle concentration, such as is often observed at roadsides due to passing vehicles.