Fast response measurements of the dispersion of nanoparticles in a vehicle

The distributions of nanoparticles (below 300 nm in diameter) change rapidly after 7 emission from the tail pipe of a moving vehicle due to the influence of transformation processes. 8 Information on this time scale is important for modelling of nanoparticle dispersion but is 9 unknown because the sampling frequencies of available instruments are unable to capture these 10 rapid processes. In this study, a fast response differential mobility spectrometer (Cambustion 11 Instruments DMS500), originally designed to measure particle number distributions (PNDs) and 12 concentrations in engine exhaust emissions, was deployed to measure particles in the 5–1000 nm 13 size range at a sampling frequency of 10 Hz. This article presents results of two separate studies; 14 one, measurements along the roadside in a Cambridge (UK) street canyon and, two, 15 measurements at a fixed position (20 cm above road level), centrally, in the wake of a single 16 moving diesel–engined car. The aims of the first measurements were to test the suitability and 17 recommend optimum operating conditions of the DMS500 for ambient measurements. The aim 18 of the second study was to investigate the time scale over which competing influences of dilution 19 and transformation processes (nucleation, condensation and coagulation) affect the PNDs in the 20 wake of a moving car. Results suggested that the effect of transformation processes was nearly 21 complete within about 1 s after emission due to rapid dilution in the vehicle wake. Furthermore, 22 roadside measurements in a street canyon showed that the time for traffic emissions to reach the 23 roadside in calm wind conditions was about 45 6 s. These observations suggest the hypothesis 24 that the effects of transformation processes are generally complete by the time particles are 25 observed at roadside and the total particle numbers can then be assumed as conserved. A 26 corollary of this hypothesis is that complex transformation processes can be ignored when 27 modelling the behaviour of nanoparticles in street canyons once the very near–exhaust processes 28 are complete. 29