Ground – fixed and on – board measurements of nanoparticles in the wake of a moving vehicle 1

Abstract 6 An integrated experimental methodology has been applied to measure number and size distributions 7 of particles in the 5–560 nm size range in the wake of a diesel car running at different speeds. 8 Measurements were made at both ground–fixed (0.10 and 0.25 m above the ground level) and on– 9 board (in 12 different sampling locations behind the moving car) measurement configurations using 10 a fast response differential mobility spectrometer (Cambustion DMS50) with a sampling frequency 11 up to 10 Hz. Results from both the experimental campaigns were analysed to understand the 12 dynamics, dispersion and transport of nanoparticle emissions in the wake of a moving vehicle. 13 Temporal changes in results were divided into three main stages (pre–evolution, evolution and post– 14 evolution) after the release of exhaust emissions from the tailpipe. Evolution stage is of most 15 interest where all the changes to particle number and size distribution occurred. Up to four 16 evolution sub–stages were observed, each showing distinct evolution patterns of particle size 17 distributions, depending on the particular experimental run. In agreement with previous studies, 18 dilution was found to be the dominant process throughout all the evolution stages. The first 19 evolution sub–stage was common to all the measurements, and consisted of an initial particle 20 number concentrations and distributions change due to rapid (less than 1 s) nucleation followed by a 21 rapid increase of accumulation mode particle number concentrations. After this first sub–stage the 22 presence of vehicle wake with recirculating particles and the possible influence of other 23 transformation processes lead to complex interactions. Results from the two experimental data sets 24 clearly confirm the presence of two separate groups of particles: (i) new particles, which are freshly 25 emitted and come directly from the tailpipe and (ii) relatively aged particles, which are entrained 26 within the recirculation vortices of the vehicle wake and reside there for a longer time. The two 27 groups have different characteristics and interact with each other. This interaction has often been 28 overlooked in past studies about local scale dispersion of nanoparticle from moving vehicles. 29