Design and performance of an automatic regenerating adsorption aerosol dryer for continuous operation at monitoring sites

Atmos. Meas. Tech., 2, 417–422, 2009 www.atmos-meas-tech.net/2/417/2009/ © Author(s) 2009. This work is distributed under the Creative Commons Attribution 3.0 License. Atmospheric Measurement Techniques Design and performance of an automatic regenerating adsorption aerosol dryer for continuous operation at monitoring sites T. M. Tuch, A. Haudek, T. Müller, A. Nowak, H. Wex, and A. Wiedensohler Leibniz Institute for Tropospheric Research, Leipzig, Germany Received: 9 April 2009 – Published in Atmos. Meas. Tech. Discuss.: 23 April 2009 Revised: 22 July 2009 – Accepted: 23 July 2009 – Published: 30 July 2009 Abstract. Sizes of aerosol particles depend on the relative humidity of their carrier gas. Most monitoring networks require therefore that the aerosol is dried to a relative humidity below 50% r.H. to ensure comparability of measurements at different sites. Commercially available aerosol dryers are often not suitable for this purpose at remote monitoring sites. Adsorption dryers need to be regenerated frequently and maintenance-free single column Nafion dryers are not designed for high aerosol flow rates. We therefore developed an automatic regenerating adsorption aerosol dryer with a design flow rate of 1 m3 /h. Particle transmission efficiency of this dryer has been determined during a 3 week experiment. The lower 50% cut-off was found to be smaller than 3 nm at the design flow rate of the instrument. Measured transmission efficiencies are in good agreement with theoretical calculations. One dryer has been successfully deployed in the Amazon river basin. We present data from this monitoring site for the first 6 months of measurements (February 2008–August 2008). Apart from one unscheduled service, this dryer did not require any maintenance during this time period. The average relative humidity of the dried aerosol was 27.1+/−7.5% r.H. compared to an average ambient relative humidity of nearly 80% and temperatures around 30◦ C. This initial deployment demonstrated that these dryers are well suitable for continuous operation at remote monitoring sites under adverse ambient conditions. Correspondence to: T. M. Tuch () 1 Introduction Diameters of aerosol particles, and thereby their physical and optical properties, depend on the relative humidity of the carrier gas (e.g. Mozurkewich, 1986; Ten Brink et al., 2000; Wex et al., 2006). To investigate aerosol effects on climate it would be desirable to measure physical and optical properties at ambient relative humidity. This would either require avoiding any temperature (and thereby relative humidity) change during the transport from ambient air to the measurement volume or the reconditioning of the aerosol to ambient relative humidity prior to the measurement. The latter approach has been implemented in research grade instruments such as the Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) but most commercially available instruments used at monitoring sites are not designed to preserve atmospheric humidity conditions (e.g. due to internal heating by electronic components). In many instruments, the reduction of relative humidity is even desired to avoid condensation of water vapour on internal surfaces. Consequently, in the past, comparison of aerosol parameters from different measurement sites with different humidity conditions has been difficult. With the establishment of major measurement networks like the Global Atmosphere Watch (GAW) program of the World Meteorological Organization (WMO) or the European Supersites for Atmospheric Aerosol Research (EUSAAR) and many others this problem needed to be solved. Because significant aerosol growth typically starts at relative humidity levels greater than about 50%, a common approach is to measure the aerosol below this threshold level (Baltensperger et al., 2003). The philosophy is therefore to conduct the measurements at low relative humidity conditions to be comparable between different sampling sites. Especially under warm and humid conditions active drying of the aerosol is necessary to achieve this goal. Published by Copernicus Publications on behalf of the European Geosciences Union. 418 T. M. Tuch et al.: Design and performance of an automatic regenerating adsorption aerosol dryer only suitable for aerosol flow rates up to 2 l/min. They should not be used at higher flow rates to avoid turbulent deposition of particles in the dryer. Commercially available bundles of 50–100 single Nafion tubes, capable of handling higher flow rates, are designed to dry gases. Used for aerosol drying, these dryers suffer from impaction losses on the faceplate of the bundles, and a pressure drop of several 10th of hPa depending on the flow rate. Gore-Tex dryers with larger inner diameters can be built avoiding this restriction of commercially available Nafion dryers. Gore-Tex dryers need a constant dry air supply of at least twice the aerosol flow to maintain a relative humidity of the aerosol of less than 50%. Unlike chemical absorbents Gore-Tex dryers do not have a buffering capacity. Such a buffering capacity would, however, be desirable for aerosol dryers to smoothen fast fluctuations of the relative humidity of the ambient aerosol. Diffusion dryers using chemical adsorbents such as Silica gel are designed specifically designed for the minimization of aerosol losses at high flow rates. In these dryers, the chemical adsorbent needs to be exchanged and regenerated on a regular basis. Often this is not feasible at remote monitoring sites that are not permanently manned. We therefore developed an automatically regenerating chemical adsorption dryer for long-term measurements of aerosol properties. Here, we present the design and performance of this automated aerosol diffusion dryer. 2 Fig. 1. Schematic view of the aerosol dryer. Currently three methods are used to condition the aerosol to the required relative humidity. A first approach is to heat aerosol, although the temperature must be kept low to minimize evaporation, e.g. up to 20% of ammonium nitrate evaporates from the aerosol within 5 s at a temperature of 42 degrees Celsius (Bergin et al., 1997). Losses at 50 degrees Celsius were observed comparing filter based measurements with TEOM derived mass concentrations (Cyrys et al., 2001, Charron et al., 2004). This method of aerosol drying is therefore primarily suitable for cold and dry regions, where mild heating at 30◦ C is sufficient to condition the aerosol to the required relative humidity. Presently, semi-permeable tubes (Nafion, Wilmington, DE or Gore-Tex, W. L. Gore and Associates, Newark, DE) are used for aerosol drying. Because of the small diameter of commercially available single Nafion tubes these dryers are Atmos. Meas. Tech., 2, 417–422, 2009 Design and operation of the automated aerosol diffusion dryer The aerosol dryer is housed in a separate shelter which can be deployed on the roof of a measurement laboratory. This shelter may either be ai (...truncated)