One Year of Tracer Dispersion Measurements over Washington, D.C.


Roland R. Draxler


Atmospheric Environment, Vol. 21, No. 1, pp. 69-77, 1987


Abstract - Two perfluorocarbon tracers were release for 6 h from several locations about 20 km outside of Washington, D.C. at 36-h intervals for over 1 year. Continuous air samples were collected at 8-h intervals at one urban and two suburban sites and at monthly intervals at 93 sites all over the region. Over 50% of the 8-h samples showed no tracer concentration.  However, about 20% of the samples had significant amounts of tracer, so that about 600 values under a variety of meteorological conditions are available for analysis.  Although the Potomac River is only 100 m below the surrounding terrain, the tracer releases from within the river valley indicated that the trace flow was channeled along the river valley.  For tracer released away from the river valley, the tracer tended to pass aloft and not mix down into the river valley. Sequential concentration measurements frequently showed high values for extended periods after the tracer release terminated, decreasing exponentially with a half-life of about 3 h.  This suggests that the finite tracer plume may have a rather long upwind trailing edge toward the release point. The average rate of change in the vertical dispersion during the summer and winter was found to be proportional to the 06 power of distance.



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