CONCENTRATION FIELDS AT URBAN INTERSECTIONS - FLUID MODELING STUDIES

Concentration measurements at urban intersections are poorly understood yet are very important because of the air pollution implications on human health and regulatory decision-making. In this study, we have modeled concentrations at an urban intersection and the nearby environment using an atmospheric boundary-layer wind tunnel. A regular array of uniform, low-rise rectangular urban blocks was studied using both flow visualization and quantitative gas tracer techniques. In the model, avenues were about 50% wider than streets, while the ratio of building height-to-street width was 0.75. Pedestrian-level concentrations varied widely throughout the intersection and also with ambient wind direction. Emissions from sources in the street contribute more to intersection concentrations than do equivalent emissions in the avenue for nearly all wind directions. Surface concentrations are greatest for ambient winds parallel to the street (theta = 90-degrees), and roughly equal for the other wind directions studied (30,60 and 90-degrees). The corresponding concentration fields aloft indicate that vertical dispersion is an important determinant of the surface concentration and its variability.