Observed Ratios of Rooftop to Surface Concentrations in Built-Up City Centers

This paper focuses on some straightforward analyses of the observed ratio of rooftop to surface concentrations at distances less than a few hundred meters downwind of near-surface releases in built-up urban centers. These results are important when public health must be protected in populated urban areas with deliberate or accidental releases of toxic chemicals, or with significant traffic emissions. Observations of tracer concentrations, C (μg/m3), made at rooftop and near the surface from the Oklahoma City Joint Urban 2003 (JU2003) field experiment and from two Manhattan field experiments (Madison Square Garden 2005 or MSG05 and Midtown 2005 or MID05) are included in the analysis. SF6 tracer gas was released continuously at a mass emission rate Q (g/s) from point sources near street level during JU2003, and six different tracer gases were released during MSG05 and MID05. Concentrations, C, were observed by many samplers at street level and on building tops, including several tall skyscrapers (up to heights of 250 m). At distances of less than about 100 m from the source, the ratio of rooftop (heights of 100 to 250 m) to surface concentrations is usually in the range from 0.01 to 0.05, where the large amount of vertical spread is due to the large recirculating eddies adjacent to the tall buildings and other obstacle-generated eddies. By distances of about 500 m, the ratio approaches unity. The near-field observations are consistent with an effective initial vertical standard deviation, σz, of about 40 m. Comparisons of the observed ratios with predicted ratios by several state-of-the-art urban dispersion models reveal a general tendency towards underprediction, indicating that rates of vertical dispersion in built-up city centers are generally being underestimated by models.