DRY DEPOSITION OF PARTICLES TO OCEAN SURFACES

Dry deposition of atmospheric particles mainly depends on wind speed a nd particle diameter. The dry deposition velocity, v(d), is found to v ary by a factor of 100-1,000 with diameter in a likely diameter range, adding uncertainty to deposition estimates, because the diameter dist ribution for many particulate species is not well known. Experimental studies of v(d) over both water and solid surfaces largely confirm the above cited huge variations with diameter. However, a number of studi es over natural waters report both much less diameter variation and ov erall larger deposition velocities. The consensus has developed that i f those latter estimates are correct, it is likely to be associated wi th the spray and bubble activities characterizing natural water surfac es for moderate to high winds. The influence of these phenomenons on d ry deposition is fairly easily modelled. However, it has proven diffic ult to verify the models due to the difficulties of measuring the rela tively small deposition rates under spray condition over the natural w aters. Here are presented data on this subject obtained in a wind/wate r tunnel, where the bubble and spray process were modelled by use of 2 ,600 submerged aquarium frits. We summarize the experimental set up. E arlier experiments showed that the tunnel spray and bubble process des cribed the natural processes satisfactorily, in the sense that the bub ble and spray production spectra could be related to the white cap cov er in a way similar to the expressions over the ocean. The results of the experiments reported here are that the bubbling and spray processe s have no significant influence on the deposition rate of the particle s. This somewhat surprising result is discussed both with respect to t he consequences for the practical methods to estimate deposition of pa rticulate species and in relation to earlier experimental results show ing larger values and less diameter dependence for v(d).