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

and Physics cess Atmospheric Measurement Techniques Open Access Biogeosciences Open Access Atmos. Meas. Tech., 6, 349–357, 2013 www.atmos-meas-tech.net/6/349/2013/ doi:10.5194/amt-6-349-2013 © Author(s) 2013. CC Attribution 3.0 License. of the Past S. Sjogren, G. P. Frank, M. I. A. Berghof, and B. G. Martinsson Open Access Continuous stand-alone controllable aerosol/cloud droplet dryer for atmospheric sampling Climate Division of Nuclear Physics, Lund University, Lund, Sweden Received: 30 May 2012 – Published in Atmos. Meas. Tech. Discuss.: 8 August 2012 Revised: 18 January 2013 – Accepted: 22 January 2013 – Published: 13 February 2013 Geoscientific Instrumentation Atmospheric aerosol is important for climate. Large efforts Methodsand and are done to systematically investigate monitor its propData vary Systems erties. Many aerosol properties with relative humid1 Introduction Open Access ity (RH). For measurements, and especially for data quality concerns for comparisons between stations/networks, a dry aerosol is important. Aerosol dryers and the drying proGeoscientific cess have been described previously (e.g. Martinsson et al., Modelregenerating Development 1992). An automatically dryer has recently been presented by Tuch et al. (2009); see references therein for additional drying methods. That dryer used a three-way valve alternating between twoHydrology silica gel dryers. Transmission for and that system was well characterized in the diameter size range System from 3 to 800 nm. The 50Earth % transmission for larger particles was calculated at 6 µm diameter.Sciences In our application we want to extend the aerosol measurement size range to drying of larger particles and cloud/fog droplets up to 25 µm diameter, which require a straight vertical path, in order to reduce impaction and sedimentation losses. This excludes the use Science of a standard valve or Ocean tee. The dryer should also work continuously. Therefore the design presented in this paper was used. In addition, the drying airflow rate can be varied to maintain a set RH (lower than ambient) in the sample flow, should that be required (in the future an additional humidifying setup can also be envisaged to maintain constant say Solid Earth 25 % RH). A closed loop for the drying air was designed for minimum variability due to flow changes and gas transfer into the aerosol sample flow. Open Access Open Access Open Access Open Access Abstract. 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. Earth System Dynamics Open Access Correspondence to: S. Sjogren () Published by Copernicus Publications on behalf of the European Geosciences Union. Open Access The Cryosphere M S. Sjogren et al.: Aerosol dryer Discussion Paper 350 | Discussion Paper | Discussion Paper | Atmos. Meas. Tech., 6, 349–357, 2013 www.atmos-meas-tech.net/6/349/2013/ | hole is for the exit flow of the inner drying airflow. Further details of the inlet, situated above the dryer, is found in Frank et al.(2004). 2 Materials and methods mize droplet losses due to impaction. Such losses prevent us from using any ordinary type of valve before the drying proBelow the criteria for designing a dryer are detailed, as well cess, and we have thus chosen a straight vertical path from as the method to measure the particle transmission and the the inlet through the dryer. Further details of the inlet sitwater vapour removal of the dryer. uated 20 above the dryer are found in Frank et al. (2004). The dryer considered consists of two concentric, cylindrical stain2.1 Design criteria less steel mesh metal nets (wire diameter 0.35 mm, open area 2.25 mm2 ) (see Fig. 1), with the sample flow in the middle The main principle of the dryer is to dry sample air flowbetween these. On the other sides of the metal nets are two ing vertically downwards in counterflow with dry airflows, cylindrical membranes (inner and outer membrane tube). The these flows being separated by membranes. Two concentric, plastic fiber mats (see below) supporting these membranes cylindrical membranes were used, with the sample flow in face towards the metal nets and the aerosol flow. Two sepathe middle, annularly, between the dry airflows (see Fig. 1). rate closed loop dry airflows circulate on each of the other The membrane is needed due to different flow velocities and sides of the membrane tubes, achieving the transfer of huflow directions of sample flow and drying flow, which would midity from the sample flow due to the humidity gradient otherwise result in mixing between the two. The membrane (pumps used are model 6025se/12vdc, Thomas, USA). The allows water vapour exchange by diffusion, but no particle metal nets reduce electrostatic charging of the membranes transfer. The two dry airflows used are arranged in closed and are fixed and grounded to the stainless steel main housloops, in order to prevent leakage of air between sample ing of the dryer, and also act to stabilise the membranes meflow and either closed loop flow through the membrane. The chanically (Ogren et al., 1985). closed loops should be tight and of good quality, to avoid As a design criterion a sufficient transfer rate of water leaks developing over time. vapour is required to maintain the dried sample flow below a The actual dryer described here is used for an instruuser-specified maximum RH. This specified RH and the minment measuring cloud droplet size distributions, the drop (...truncated)