A linear impactor for fog droplet sampling

A new linear impactor for fog droplet sampling is described. This accelerates the sampling surface, by means of a pneumatic system, instead of accelerating the air containing droplets (as for aspiration impactors). The advantage of this instrument, compared with aspiration impactors, is that the sample does not undergo any modification (break, evaporation etc.), and the extremely short sampling time (about 0.03 s) results in a low coating factor for the sampling surface, avoiding droplet overlapping; moreover, because the sampling time is short, the size distributions may be compared with simultaneous optical measurements. The use of extensible housing tunnels enables the sampling volume to be varied, giving a large number of collected droplets, even with small ambient droplet concentrations. Some comparisons are presented between impactor data and measurements from other instruments.