Pseudo - simultaneous measurements for the vertical variation of coarse , fine and ultrafine 2 particles in an urban street canyon 3 Authors 4

Pseudo–simultaneous measurements for vertical variation of coarse, fine and ultra fine particles in an urban street canyon. Atmospheric Environment 42, 4304–4319. Abstract 1 The vertical variation of particle number distributions and concentrations in a street 2 canyon is the result of the competing influences of meteorology, traffic and transformation 3 processes overall and for various particle size ranges. A recently developed instrument, the 4 'fast response differential mobility spectrometer DMS500', measured particle number 5 distributions (PNDs) in the 5-2738 nm range, pseudo-simultaneously, at four different heights 6 (z/H = 0.09, 0.19, 0.40 and 0.64) on the leeward side of an 11.6 m deep street canyon which 7 had a height to width ratio of near unity. Measurements were made in Cambridge, UK 8 between the 20 th and 21 st March 2007. 9 The PNDs were bimodal with the same shape at each height, and with similar values 10 of both the peak and geometric mean particle diameters in each mode. This suggested that 11 transformation processes were not important. Coagulation and condensation time scales were 12 comparable and large and these processes should have had a negligible effect on the PNDs. 13 The particle number concentrations (PNCs) changed significantly with height from a 14 maximum at z/H = 0.19 and decreasing towards both the lowest (z/H = 0.09) and highest (z/H 15 = 0.64) sampling points. The decrease in PNCs with height in the upper part of the canyon 16 was attributed to the removal of particles as a result of mass exchange between street canyon 17 and the wind above while the reduction in the PNC towards street level was thought to be due 18 to dilution and dry deposition. 19 Over 99% of the PNCs were found in 10-300 nm range whereas the particle mass 20 concentrations were almost equally distributed between the 10-1000 nm and 1000-2738 nm 21 size range at each height. The PNCs in the 10-30 nm and the 30-300 nm size range were 22 linearly correlated with the traffic volume but poorly correlated with the rooftop wind speed. 1. Introduction 1 Regulations to control the emission of atmospheric particulate matter (PM) are based 2 on limits for PM 10 (D p ≤10µm) and PM 2.5 (D p ≤2.5µm), using particle mass concentrations, 3 not particle number concentrations. Recent toxicological studies indicate greater toxicity for 4 ultrafine particles (D p ≤100 nm) than coarser …