A concentration correction scheme for Lagrangian particle model and its application in street canyon air dispersion modelling

Pollutant dispersion in street canyons with various configurations was simu lated by discharging a large number of particles into the computation domai n after developing a time-dependent wind field. Trajectory of the released particles was predicted using a Lagrangian particle model developed in an e arlier study. A concentration correction scheme, based on the concept of "v isibility", was adopted for the Lagrangian particle model to correct the ca lculated pollutant concentration field in street canyons. The corrected con centrations compared favourably with those from wind tunnel experiments and a linear relationship between the computed concentrations and wind tunnel data were found. The developed model was then applied to four simulations t o test for the suitability of the correction scheme and to study pollutant distribution in street canyons with different configurations. For those cas es with obstacles presence in the computation domain, the correction scheme gives more reasonable results compared with the one without using it. Diff erent flow regimes are observed in the street canyons, which depend on buil ding configurations. A counterclockwise rotating vortex may appear in a two -building case with wind flow from left to right, causing lower pollutant c oncentration at the leeward side of upstream building and higher concentrat ion at the windward side of downstream building. On the other hand, a stabl e clockwise rotating vortex is formed in the street canyon with multiple id entical buildings, resulting in poor natural ventilation in the street cany on. Moreover, particles emitted in the downstream canyon formed by building s with large height-to-width ratios will be transported to upstream canyons . (C) 2001 Elsevier Science Ltd. All rights reserved.