A FLUID MODELING STUDY OF CONCENTRATION DISTRIBUTIONS AT URBAN INTERSECTIONS

Concentration measurements at urban intersections are poorly understoo d, yet are very important because of the air pollution implications on human health and regulatory decision-making. In this study, we have m odelled concentrations at an urban intersection and the nearby environ ment using an atmospheric boundary-layer wind tunnel. A regular array of uniform, low-rise rectangular urban blocks was studied using quanti tative gas tracer techniques. In the model, avenues were about 50% wid er than streets, while the ratio of building height to avenue width wa s 0.75. Pedestrian-level concentrations varied widely throughout the i ntersection and also with ambient wind direction. Emissions from sourc es in the street contribute more to intersection concentrations than d o equivalent emissions in the avenue for nearly all wind directions. S urface concentrations are greatest for ambient winds parallel to the s treet (theta = 90-degrees), and roughly equal for the other wind direc tions studied (30-degrees, 60-degrees and 90-degrees).