Measuring and modelling the airborne particulate matter mass concentration field 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 distribution of vehicle-derived airborne particulate matter mass concentration in street 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 tunnel experiments but also theory and models derived from multiple runs of numerical 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 pollutants in the street, is used to predict concentration for receptors within the canyon. Emission rates of vehicle-derived particulate matter are calculated within SLAQ, which serve as input to the dispersion module. Exhaust emission rates are scaled element by element along the street for each of the lanes according to the direction of traffic flow to account for modal operation of vehicles near signalised intersections. This refinement allows SLAQ to account for non-uniformity in along-canyon emission rates and to model a street that has several intersections along its length. Thermal turbulence due to environmental surface sensible heat and vehicle-generated heat is accounted for in the model. Other features of SLAQ include correction for the urban heat island effect, dry deposition, wet deposition, particle settling and estimation of wind direction standard deviation, when this latter data is not available. SLAQ has been evaluated in a street in Loughborough, Leicestershire, United Kingdom and a correlation coefficient of 0.8 between the modelled and measured concentrations has been obtained.