Fine-Scale Variations in Aerosol Transport within a Street Canyon – a Pilot Field Study

1. 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 • A two-week campaign in an asymmetric street canyon in Manchester with busy traffic (~20 000 vehicles.day-1) in October 2001. • 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 Campaign mean mass size distribution The component modes of PM10 varied independently due to differing transport characteristics. Ultra-fine mode number concentrations Fine mode number concentrations and fluxes Coarse mode concentrations and fluxes • The coarse mode behaviour could be split into three periods: • Decay following wind-driven re-suspension event, • Moderate wind diurnal cycle • Low wind diurnal cycle Fine mode (0.1-0.5mm) number concentrations follow a diurnal cycle related to anthropogenic activity, particularly traffic flow. • 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. • 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). • Decay rate was dependent upon size and wind speed. • Particles in the size range 4-8mm dominated during moderate winds. • In high winds re-suspension of extra particles, especially in the range 8-10mm, enhanced PM10. • 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. • In low winds, reduced deposition/dispersion in the size range 4-6mm had the biggest effect on above-normal night-time coarse mass concentrations. Above: N0.1 I.e. number concentration (4.6nm < Dp < 100nm) Turbulence Vertical turbulence could be described by the parameterisation sw2 = (aU)2 + swt2 where swt2 = AT + B and T = traffic flow rate • 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. • Mass emission fluxes (left) followed a diurnal cycle, with an early-afternoon peak. Traffic peaks had more influence on larger sizes. References: Dorsey, J.R.; Nemitz, E.; Gallagher, M.W.; Fowler, D; Williams, P.I; Bower, K.N.; Beswick, K.M., 2002. Direct Measurements and Parameterisation of Aerosol Flux, Concentration and Emission Velocity Above a City. Atmos. Environ. 36, 791-800. 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<Dp<10mm) number and mass size distribution measurements in a busy street canyon in Manchester, U.K. Atmos. Environ. [Accepted, 2002].