Fog Impactors – Sampling devices for ground based cloud studies
Isokinetic Cloud Probing System (ICPS)
The Isokinetic Cloud Probing System (ICPS) is a single-stage cloud water impactor. It operates according to the principles of inertial impaction of atmospheric droplets on a plane surface by means of a vertical slit impaction stage. The 50% cut-off diameter of the impactor is calculated for droplet diameters between 5 and 6 µm at flow rates from 150 to 200 m3 h-1 following the design rules of Marple and Willeke (1976)*. The selected cut-off diameter can be varied within certain ranges by changing the flow rate.
During operation the cloudy air enters the device through the adjustable inlet and passes through the rectangular nozzle. Those droplets with diameters greater than the cut-off size are impacted on the impaction plate. The sampled cloud water on the impaction plate is forced by the airflow to the outer edges and flows downwards within the slits on each side of the impaction plates and subsequently into the replaceable vials.
To obtain isokinetic sampling conditions, the proper alignment of the instrument to the actual wind direction and wind speed is required. Therefore the impactor is fixed on a stainless steel duct allowing the upper part of the duct, along with the collector and the wind vane, to rotate according to the actual wind direction. To provide the greatest possible freedom for rotation, the air is sucked through the inner part of the steel duct, equipped with a gas proof ball-bearing.
To adapt the impactor inlet velocities to the actual mean wind speeds, the width of the inlet can be adjusted by means of the adjustable inlet.
The whole instrument, including impaction plates, is made from acrylic glass, which allows visual control of the instrument’s performance during operation. The impaction plate can easily be removed from the impactor, which facilitates cleaning without disassembling the whole instrument.
For special applications the instrument can be modified with respect to the cut-off diameter or the used materials. For more information please contact us.
Two-Stage Fogwater Impactor (TFI3)
The Two-Stage Fogwater Impactor is a double-stage cloud water impactor. It consists of three vertical slit impaction stages, one (stage 1) to collect the larger droplets, followed by two identical stages in parallel (stage 2). These collect those droplets which passed the first stage, but which are above a well defined cut-off diameter.
The cut-off diameters of the first stage are between 10 and 12µm, and of the second stages between 5 and 6 µm, calculated for flow rates from 150 to 200 m3 h-1. The cut-off diameters of the two stages can be varied within certain ranges by changing the flow rate.
The figure below shows a sketch of the impactor: the air enters the device through the adjustable inlet (1) and passes through the nozzle of the first stage (2). Those droplets with diameters greater than the cut-off size of the first stage are impacted on the first impaction plate (3). Now the air stream splits into two equal parts and is accelerated on each side by the nozzles (4) of the second impaction stages (5).
The sampled cloud water on each impaction plate is forced by the airflow to the outer edges and runs downwards within the slits on each side of the impaction plates into the replaceable vials.To adapt the impactor inlet velocities to the actual mean wind speeds, the width of the inlet can be adjusted.
As the ICPS, the whole instrument, including impaction plates, is made from acrylic glass, which allows visual control of the instrument’s performance during operation. Each impaction plate can easily be removed from the impactor, which facilitates the cleaning process after each cloud period without disassembling the whole instrument.
*Reference: Marple, V.A., K. Willeke (1976): Inertial impactors: Theory, design and use, in: Benjamin Y. H. Liu (ed.), Fine particles – aerosol generation, measurement, sampling, and analysis, Academic Press, 411-445.
