Accuracy of various diffusion and stability schemes over Washington, D.C.


Roland R. Draxler


Atmospheric Environment, Vol. 21, No. 3, pp. 491-499, 1987


Abstract - One year of continuous 8-h samples at three locations in and around Washington, D.C. in conjunction with tracer releases at three sites 14-40 km from the samplers were used to evaluate the accuracy of various stability and dispersion techniques.  The combination of the Pasquill-Gifford dispersion curves and a stability category determined from the wind-direction fluctuation showed the least biased calculations. The precision of all methods was similar.  When measured and calculated concentrations were paired in space and time, the scatter ranged over two orders of magnitude. The calculations were sensitive to the meteorological data, such that when the spatial density of meteorological sites was increased, the calculations improved.  Corrections to account for enhanced dispersion over an urban area were only appropriate when the calculations were based on a rural meteorological site.  Turner's stability classification method did not provided enough unstable cases, especially during the daytime, and hence resulted in over-calculation. Briggs' urban dispersion curves resulted in an under calculation by a factor of five. Greatest differences between dispersion methods were observed for night-time samples.


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