Measuring and modelling the airborne particulate matter mass concentrationfield in the street environment: model overview and evaluation

This paper discusses the outline structure and preliminary evaluation of an emission-dispersion model for predicting the temporal and spatial distribu tion of vehicle-derived airborne particulate matter mass concentration in s treet canyons. The model is called Street Level Air Quality (SLAQ). SLAQ is semi-empirical, in that it uses not only results from field and wind tunne l experiments but also theory and models derived from multiple runs of nume rical routines in order to simulate the basic physical processes within the street canyon. A combination of a plume model, for the direct contribution of vehicle exhaust, and a box model for the recirculating part of the poll utants in the street, is used to predict concentration for receptors within the canyon. Emission rates of vehicle-derived particulate. matter are calc ulated within SLAQ, which serve as input to the dispersion module. Exhaust emission rates are scaled element by element along the street for each of t he lanes according to the direction of traffic flow to account for modal op eration of vehicles near signalised intersections. This refinement allows S LAQ to account for non-uniformity in along-canyon emission rates and to mod el a street that has several intersections along its length. Thermal turbul ence due to environmental surface sensible heat and vehicle-generated heat is accounted for in the model. Other features of SLAQ include correction fo r the urban heat island effect, dry deposition, wet deposition, particle se ttling and estimation of wind direction standard deviation, when this latte r data is not available. SLAQ has been evaluated in a street in Loughboroug h, Leicestershire, United Kingdom and a correlation coefficient of 0.8 betw een the modelled and measured concentrations has been obtained. (C) 1999 El sevier Science B.V. All rights reserved.