Street versus rooftop level concentrations of 1 fine particles in a Cambridge street canyon

3 Dispersion of particles, as evidenced by changes in their number distributions (PNDs) and 4 concentrations (PNCs), in urban street canyons, is still not well understood. This study compares 5 measurements by a fast–response particle spectrometer (DMS500) of the PNDs and the PNCs (5– 6 1000 nm, sampled at 1 Hz) at street and rooftop levels in a Cambridge (UK) street canyon, and 7 examines mixing, physical and chemical conversion processes, and the competing influences of 8 traffic volume and rooftop wind speed on the PNDs and the PNCs in various size ranges. PNCs at 9 street level were ≈ 6.5 times higher than at rooftop. Street–level PNCs followed the traffic volume 10 and decreased with increasing wind speed, showing a larger influence of wind speed on 30–300 11 nm particles than on 5–30 nm particles. Conversely, rooftop PNCs in the 5–30 nm size range 12 increased with wind speed, whereas those for particles between 30–300 nm did not vary with wind 13 speed. 14