Fine-Scale Variations in Aerosol Transport within a Street Canyon – a Pilot Field Study I.D. Longley, M.W. Gallagher, M. Flynn, J.R. Dorsey, P.I. Williams Physics Department, University Of Manchester Institute of Science and Technology, Manchester, UK Campaign mean mass size distribution Ultra-fine mode number concentrations Fine mode number concentrations and fluxes Coarse mode concentrations and fluxes One- and ten-minute number concentrations of ultra-fine aerosol (<100nm) concentrations at street level are very sensitive to air flow direction. When flow is from the road towards the pavement, concentrations at this side of the canyon are much higher, due mostly to increased numbers of less- diluted particles in the size range 10-80nm. This can lead to brief increases in concentration by an order of magnitude above the mean. After wind direction, traffic flow rate and wind speed are the most important determinants of concentration. However, in perpendicular flow, wind speed in-canyon is poorly related to roof- level wind speed. Fine mode (  m) number concentrations follow a diurnal cycle related to anthropogenic activity, particularly traffic flow. Emission fluxes also have a similar diurnal cycle, but more closely linked to sensible heat flux. The same conclusions were drawn above the urban canopy in the SASUA project in Edinburgh, see above (Dorsey et al, 2002). The coarse mode behaviour could be split into three periods: 1.Decay following wind- driven re-suspension event, 2.Moderate wind diurnal cycle 3.Low wind diurnal cycle Decay rate was dependent upon size and wind speed. Particles in the size range 4-8  m dominated during moderate winds. In high winds re-suspension of extra particles, especially in the range 8-10  m, enhanced PM 10. Turbulence Longitudinal (u) turbulence roughly double lateral (v) and vertical (w). At low wind speeds local sources (including traffic and) dominate. Turbulence also enhanced in perpendicular flow. The component modes of PM 10 varied independently due to differing transport characteristics. A two-week campaign in an asymmetric street canyon in Manchester with busy traffic (~ vehicles.day -1 ) in October Measurements of ultra-fine (SMPS), fine (ASASP-X) and coarse (FSSP) size-segregated particle concentrations. Fine and coarse mode emission fluxes determined by eddy correlation. Above: canyon site with definition of wind directions References: Dorsey, J.R.; Nemitz, E.; Gallagher, M.W.; Fowler, D; Williams, P.I; Bower, K.N.; Beswick, K.M., Direct Measurements and Parameterisation of Aerosol Flux, Concentration and Emission Velocity Above a City. Atmos. Environ. 36, Longley, I.D.; Gallagher, M.W.; Dorsey, J.R.; Flynn, M.; Allan, J.D.; Alfarra, M.R.; D. Inglis, D.. A case-study of aerosol (4.6nm<D p <10  m) number and mass size distribution measurements in a busy street canyon in Manchester, U.K. Atmos. Environ. [Accepted, 2002]. Vertical turbulence could be described by the parameterisation  w 2 = (  U) 2 +  wt 2 where  wt 2 = AT + B and T = traffic flow rate In low winds, reduced deposition/dispersion in the size range 4-6  m had the biggest effect on above- normal night-time coarse mass concentrations. Mass emission fluxes (left) followed a diurnal cycle, with an early-afternoon peak. Traffic peaks had more influence on larger sizes. Above: N 0.1 I.e. number concentration (4.6nm < D p < 100nm)