Dry deposition of particles to ocean surfaces

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Abstract
Dry deposition of atmospheric particles mainly depends on wind speed and particle diameter. The dry deposition velocity, Vd, is found to vary by a factor of 100-1,000 with diameter in a likely diameter range, adding uncertainty to deposition estimates, because the diameter distribution for many particulate species is not well known. Experimental studies of vd over both water and solid surfaces largely confirm the above cited huge variations with diameter. However, a number of studies over natural waters report both much less diameter variation and overall larger deposition velocities. The consensus has developed that if those latter estimates are correct, it is likely to be associated with the spray and bubble activities characterizing natural water surfaces for moderate to high winds. The influence of these phenomenons on dry deposition is fairly easily modelled. However, it has proven difficult to verify the models due to the difficulties of measuring the relatively small deposition rates under spray condition over the natural waters. Here are presented data on this subject obtained in a wind/water tunnel, where the bubble and spray process were modelled by use of 2,600 submerged aquarium frits. We summarize the experimental set up. Earlier experiments showed that the tunnel spray and bubble process described the natural processes satisfactorily, in the sense that the bubble and spray production spectra could be related to the white cap cover in a way similar to the expressions over the ocean. The results of the experiments reported here are that the bubbling and spray processes have no significant influence on the deposition rate of the particles. This somewhat surprising result is discussed both with respect to the consequences for the practical methods to estimate deposition of particulate species and in relation to earlier experimental results showing larger values and less diameter dependence for vd.