What’s A Factor Of Ten Worth? Particle Deposition Rates Re-Measured
May 21, 2004
Early laboratory studies of the rates at which airborne particles of inhalable size are removed by direct transfer to the surface indicated deposition velocities of the order of 0.1 cm/s. These low values were in agreement with expectations based on wind tunnel studies and numerical models. However, they were in disagreement with field measurements, especially those measurements made over tall vegetation – from cornfields to forests. Measurements made over pine forests indicated values more like 2 cm/s during daytime, for example. The numerical modeling community was split in its view of these higher values. Many modelers claimed that the higher numbers indicated a failure of the experimental methods. Now, new methods for measuring deposition velocities have been developed by ARL researchers at Oak Ridge, Tennessee, and the results are starting to accrue. There is direct confirmation of the higher values. Adding this new information to the results of field studies elsewehere in the world (especially in Europe) leads to the unavoidable conclusion that the classical devotion to a value of 0.1 cm/s (or maybe as high as 0.2 cm/s) is most definitely misleading. The experimental data indicate values an order of magnitide higher, much in agreement with the early results that caused dismay in the first place.
There is obviously more that needs to be understood. For the present, it is enough to recognize that some classical models describing particle retention rates in the air are likely to be wrong, especially when the surface below is forested.
Contact information: Bruce B. Hicks
Phone: (301) 713-0684