Observed tracer concentrations in downtown Oklahoma City and Manhattan – variations with downwind distance and ratios of rooftop to surface concentrations

This paper focuses on some straightforward similarity formulas that can be used to describe urban tracer gas concentration observations, such as the initial near-field concentration maximum, the ratio of rooftop to surface concentrations in the near field, and the simple power law for the decrease of maximum concentration with distance. Observations from the two Manhattan field experiments (Madison Square Garden 2005 or MSG05 and Midtown 2005 or MID05) and from the Oklahoma City Joint Urban 2003 (JU2003) field experiment are included in the analysis. Six different tracer gases were released at a rate, Q (g/s), from continuous point sources near street level during MSG05 and MID05. SF6 tracer gas was released during JU2003. Concentrations, C (g/m^3), were observed by many samplers at street level and on building tops, including several tall skyscrapers. It is found that the maximum near-field concentrations can be fit by a Gaussian plume model with assumed initial cloud standard deviation, σo, of 10 m in the same street as the release, and 40 m if the tracer cloud passes around a few buildings. At distances of less than about 100 m from the source, the ratio of rooftop (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. At downwind distances, x, out to about 1000 m (or about ten times the mean building height H), the maximum concentration, C, varies with distance, x, according to the power law CuH^2/Q = 10/(x/H)^2. The wind speed, u (m/s), represents the average wind in the lower part of the urban canopy.