Continuous stand-alone controllable aerosol/cloud droplet dryer for atmospheric sampling

We describe a general-purpose dryer designed for continuous sampling of atmospheric aerosol, where a specified relative humidity (RH) of the sample flow (lower than the atmospheric humidity) is required. It is often prescribed to measure the properties of dried aerosol, for instance for monitoring networks. The specific purpose of our dryer is to dry cloud droplets (maximum diameter approximately 25 μm, highly charged, up to 5 × 102 charges). One criterion is to minimise losses from the droplet size distribution entering the dryer as well as on the residual dry particle size distribution exiting the dryer. This is achieved by using a straight vertical downwards path from the aerosol inlet mounted above the dryer, and removing humidity to a dry, closed loop airflow on the other side of a semi-permeable GORE-TEX membrane (total area 0.134 m2). The water vapour transfer coefficient, k, was measured to be 4.6 × 10-7 kg m 2 s 1% RH 1 in the laboratory (temperature 294 K) and is used for design purposes. A net water vapour transfer rate of up to 1.2 × 10-6 kg s 1 was achieved in the field. This corresponds to drying a 5.7 L min 1 (0.35 m3 h 1) aerosol sample flow from 100% RH to 27% RH at 293 K (with a drying air total flow of 8.7 L min 1). The system was used outdoors from 9 May until 20 October 2010, on the mountain Brocken (51.80° N, 10.67° E, 1142 m a.s.l.) in the Harz region in central Germany. Sample air relative humidity of less than 30% was obtained 72% of the time period. The total availability of the measurement system was >94% during these five months.