January 30, 2008
Conference Paper

USE OF TRACER DATA FROM THE MADISON SQUARE GARDEN 2005 FIELD EXPERIMENT TO TEST A SIMPLE URBAN DISPERSION MODEL

Abstract

A simple urban dispersion model is tested that is based on the Gaussian plume model and the Briggs’ urban dispersion curves. A key aspect of the model is that an initial dispersion coefficient (sigma) of 40 m is assumed to apply in the x, y, and z directions in built-up downtown areas. This initial sigma accounts for mixing in the local street canyon and/or building wakes. At short distances (i.e., when the release is in the same street canyon as the receptor and there are no obstructions in between), the initial lateral sigma is assumed to be less, 10 m. Observations from tracer experiments during the Madison Square Garden 2005 (MSG05) field study are used for model testing. MSG05 took place in a 1 km by 1 km area in Manhattan surrounding Madison Square Garden. Six different perfluorocarbon tracer (PFT) gases were released concurrently from five different locations around MSG, and concentrations in the air were observed by 20 samplers near the surface and seven samplers on building tops. There were two separate continuous 60 minute tracer release periods on each day, beginning at 9 am and at 11:30 am. Releases took place on two separate days (March 10 and 14). The samplers provided 30 minute averaged PFT concentrations from 9 am through 2 pm. This analysis focuses on the maximum 60-minute averaged PFT gas concentration at each sampler location for each PFT for each release period. Stability was assumed to be nearly neutral, because of the moderate winds and the mechanical mixing generated by the buildings. Input wind direction was the average observed building-top wind direction (285° on March 10 and 315° on March 14). Input wind speed was the average street-level observed wind speed (1.5 m/s for both days). To be considered in the evaluation, both the observed and predicted concentration had to exceed the threshold. Concentrations normalized by source release rate, C/Q, were tested. For all PFTs, samplers, and release times, the median observed and predicted C/Q are within 40% of each other, and 43 % of the time the concentration predictions are within a factor of two of the observations. The scatter plots show that the typical error is about the same magnitude as the mean concentration. When only the surface observations are considered, the performance is better, with the median observed and predicted C/Qs within 10 % of each other. The overall 60 minute-averaged maximum C/Q is underpredicted by about 40 % for the surface samplers and is overpredicted by about 25 % for the building-top samplers.

Revised: August 7, 2009 | Published: January 30, 2008

Citation

Hanna S.R., E. Baja, J.E. Flaherty, and K.J. Allwine. 2008. USE OF TRACER DATA FROM THE MADISON SQUARE GARDEN 2005 FIELD EXPERIMENT TO TEST A SIMPLE URBAN DISPERSION MODEL. In 15th Joint Conference on the Applications of Air Pollution Meteorology with the Air & Waste Management Association, Paper No. 4.1. Boston, Massachusetts:American Meteorological Society. PNNL-SA-58750.