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This study explores the possibilities of using a Climet CI-8060 optical particle counter as a fog and cloud droplet counter in ambient conditions. Intercomparisons with an AERAS cascade impactor (modified for fog measurements) show that the performance of the instrument is excellent and yields accurate droplet size distributions in the range of 0.54 to 25 μm in one-minute time resolution. The CI-8060 was applied successfully in a field experiment investigating in-cloud scavenging of air pollutants. It is shown that the cloud droplet size distribution is a key parameter for the depletion of pollution by wet deposition. Being aware of the fact that commercial instruments built particularly for the purpose of cloud droplet sizing exist, the CI-8060 has some advantages: it is inexpensive, and it is rugged and easy to use in the field.

Field experiments with low-pressure cascade impactors (Type AERAS LPI 3010.06) have been carried out to determine whether coating of the impaction surfaces is necessary for the correct determination of aerosol mass size distributions in rural and suburban areas. In general, an impactor with coated surfaces collects up to 100% more giant particles ( d a > 1 μ m), and more than 20% more total mass, than an uncoated reference impactor. Also, the size distribution is in much better agreement with optical particle counter data, and the total mass is closer to values determined with a high-volume filter sampler, than in the case of the uncoated impactor.

Below-cloud scavenging coefficients for particles in the diameter range 0.28 < da < 15 μm could be determined as a function of the aerosol diameter da in a field experiment. The agreement with state-of-the-art theory is best for particles of about 2 μm diameter. Particles below this size seem to be much more efficiently removed than theory predicts - the experimental values are 10 to 100 times larger for aerosols of da≈0.5 μm. Giant particles of more than 2 μm diameter seem to be less efficiently scavenged than predicted. For 5 μm particles the experimental scavenging coefficients are about 2 to 5 times lower than the theoretical values. Reasons for these deviations are discussed in detail. The dependence on rainfall rate was found to be less pronounced than predicted by theory. The overall aerosol concentration decrease by below-cloud scavenging was found to be 6% (by number) or 21% (by volume), for a mean rainfall rate of 0.85 mm/hr and a mean duration of 170 minutes.