In this experimental study, the ion collector was connected directly to the ionizer outlet via a very short connecting pipe. The connecting pipe is made of a stainless steel, 3.5 mm in an inner diameter and a length of 10 mm. The airflow was regulated and
controlled by a massflow meter and controller (Dwyer model GFC-1111) with a vacuum pump located at the end of the experimental equipment train. The massflows were typically in the range be-tween 1 and 5 L/min. A silica gel drying chamber was used to remove any remaining water from an air sample. Dried air samples werefiltered through a Pall HEPA capsulefilter (model 12144) and were then drawn into the ionizer. An adjustable commercial DC high voltage power supply (Leybold Didactic model 521721) was used to maintain the corona voltage difference in the ionizer