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CHANNELLED FLOW

Measurements Of Fine-scale Variation In Concentrations Of Ultrafine Aerosol In An Urban Street Canyon I.D. Longley, M.W. Gallagher, J.R. Dorsey, M. Flynn, P.I. Williams Physics Department, University Of Manchester Institute of Science and Technology, Manchester, UK.

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CHANNELLED FLOW

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  1. Measurements Of Fine-scale Variation In Concentrations Of Ultrafine Aerosol In An Urban Street Canyon I.D. Longley, M.W. Gallagher, J.R. Dorsey, M. Flynn, P.I. Williams Physics Department, University Of Manchester Institute of Science and Technology, Manchester, UK • Aerosol instrumentation (including SMPS & ASASP-X) was located on one side of a street canyon in central Manchester (UK) with busy one-way traffic. • Measurements were made 24-hours-a-day for 10 days. • A single SMPS scan lasted 1 minute. A summary of 10-minute averages is presented in this poster. • Ultrasonic anemometers were co-located. A ‘mobile’ roving anemometer was moved around the canyon to gain a snapshot of the flow patterns. • Analysis indicated that the principal influence on particle number concentrations was the flow pattern in the canyon, resulting from the general wind direction and speed over the city. Wind directions are specified with respect to the canyon axis Approach flow: perpendicular perpendicular parallel Flow within the canyon: This diagram (right) relates the wind direction measured at 3.5 m height within the canyon adjacent to the aerosol instruments, with a reference measurement made at a nearby rooftop. Cross-canyon flow (away from instruments) CHANNELLED FLOW CROSS-CANYON FLOW (towards instruments) RIGHT: Perpendicular approach flow also led to channelling around 50 % of the time. Otherwise perpendicular approach flow led to a complex flow with canyon-scale vortices in all planes. In most observed cases flow was directly from the road to the instruments, leading to raised concentrations detailed below. LEFT: Parallel (and sometimes perpendicular) approach flow led to channelling. Instrument location represented by red block. N0.1 was also related to wind speed measured within the canyon (left). Wind speed in the canyon was related to that above the canyon, except in perpendicular flow (fuller details will appear in Longley at al. Atmos. Environ. [in press]). Ultrafine number concentrations below 0.1 mm (N0.1) formed log-normal distributions in each regime (left), but with concentrations 3 – 4 times greater in cross-canyon flow Particle number concentrations in the size range 0.1 – 0.5 mm (N0.1-0.5) were independent of in-canyon flow regime, and thus not freshly emitted or formed. N0.1-0.5 followed a diurnal pattern linked to the cycle in anthropogenic emission and ventilation flux (left). The extra particle numbers associated with cross-canyon flow were mostly in the size range 10 – 80 nm (left). For further details: Longley et al., 2003, Atmos. Environ. 37, 1563-1571.

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