Development and calibration of a ground-based active collector for cloud- and fogwater

In spring 1985, field experiments were started to study the scavenging processes of atmospheric substances. Besides the chemical analysis of precipitation samples, these studies required simultaneous collection of cloud water for chemical analysis. In particular, a ground-based cloud water collector was needed, suitable for use on the top of a TV-tower. Existing designs of ground-based cloud or fogwater samplers be divided into two general classes: a) passive collectors, which utilize the ambient wind to impact the droplets on the collection surface; b) active collectors, which accelerate the droplets to a certain velocity as they approach the collection surface. A well-known passive collector is the one described by Falconer and Falconer. Teflon-strings are extended between two disks which are 1m apart. The disadvantage of this collector, for our experiments, was that the collector strings are always exposed to the ambient air, so that contamination by aerosol impact during dry period s cannot be excluded. Furthermore, because of the length of the strings, impacted droplets need a certain time to drain off, during which they remain exposed to the ambient air stream and continue to scavenge trace gases. Active collectors, such as the rotating arm collector described by Jacob et al., or jet impactors were not suitable for the use on top of the TV-tower because of safety or power requirements. (IFU)