Aerosol particles are released into the atmosphere in huge numbers by road vehicles. When inhaled these particles can cause respiratory health problems and so they are considered to be a pollutant. The urban ‘street canyon’, where city streets are lined on both sides by buildings of 3 or 4 floors or more, is the environment into which much of this aerosol is released and where many people are exposed to the pollutant. The presence of the buildings forces the wind to blow in turbulent eddies which partially traps airborne pollutants within the ‘canyon’, preventing their dispersion, and sometimes giving rise to high concentrations. However, very little is currently known about how much aerosol is actually released into the canyon air and how much is trapped, and what these values depend upon.

The objective of SCAR is to measure these turbulent eddies, along with concentrations of airborne pollutants within a street canyon, and to use this data to calculate the rate at which aerosol particles are ventilated from the canyon space. The influence of meteorological factors such as wind speed and direction, and solar radiation will be determined.

SCAR consists of several short measurement campaigns within the streets of central Manchester, conducted throughout 2001. Ultrasonic anemometers are used in conjunction with a variety of instruments (ASASP-X, SMPS, etc.) for measuring aerosols. These instruments allow size-segregated measurements to distinguish between the different sized aerosols in the range 10 nm to 3 mm. Other pollutants, such as oxides of nitrogen, are also measured. The eddy correlation technique is used to calculate fluxes of the pollutants.

In the near future it is intended that this work will be extended to other streets in order to generalise the results. It is hoped that this work will give rise to a general parameterisation for emission of aerosol from any urban street canyon that can then be scaled up across a whole urban area. This will allow an estimation of the nature of the entire urban plume as it exports aerosols out of the city, as well as prediction of air quality within the urban area itself.

Additional information about the SCAR project can be found here.