Atmos. Meas. Tech., 2, 363–378, 2009 www.atmos-meas-tech.net/2/363/2009/ © Author(s) 2009. This work is distributed under the Creative Commons Attribution 3.0 License. Atmospheric Measurement Techniques Intercomparison study of six HTDMAs: results and recommendations J. Duplissy 1 , M. Gysel 1 , S. Sjogren 1 , N. Meyer 2,* , N. Good 3 , L. Kammermann 1 , V. Michaud 4 , R. Weigel 4 , S. Martins dos Santos 5 , C. Gruening 5 , P. Villani 4 , P. Laj 4 , K. Sellegri 4 , A. Metzger 1 , G. B. McFiggans 3 , G. Wehrle 1 , R. Richter 1 , J. Dommen 1 , Z. Ristovski 2 , U. Baltensperger 1 , and E. Weingartner 1 1 Laboratory of Atmospheric Chemistry, Paul Scherrer Institut, 5232, Villigen PSI, Switzerland 2 International Laboratory for Air Quality and Health, Queensland University of Technology, QLD 4000, Brisbane, Australia 3 Centre for Atmospheric Sciences, University of Manchester, M13 9PL, Manchester, UK 4 Laboratoire de M´ et´ eorologie Physique, Blaise Pascal Univ., 63000, Clermont-Ferrand, France 5 Climate Change Unit, Joint Research Center, 21027, Ispra, Italy * now at: Laboratory for Energy Analysis, Paul Scherrer Institut, 5232, Villigen PSI, Switzerland Received: 22 August 2008 – Published in Atmos. Meas. Tech. Discuss.: 30 October 2008 Revised: 3 June 2009 – Accepted: 17 June 2009 – Published: 24 July 2009 Abstract. We report on an intercomparison of six dif- ferent hygroscopicity tandem differential mobility analysers (HTDMAs). These HTDMAs are used worldwide in labora- tory experiments and field campaigns to measure the water uptake of aerosol particles and have never been intercom- pared. After an investigation of the different design of the instruments with their advantages and inconveniencies, the methods for calibration, validation and data analysis are pre- sented. Measurements of nebulised ammonium sulphate as well as of secondary organic aerosol generated from a smog chamber were performed. Agreement and discrepancies be- tween the instruments and to the theory are discussed, and final recommendations for a standard instrument are given, as a benchmark for laboratory or field experiments to ensure a high quality of HTDMA data. 1 Introduction Atmospheric aerosols are typically hygroscopic and absorb significant amounts of water at high relative humidity (RH). Hygroscopic growth with increasing RH influences the light scattering by particles, their potential to act as cloud con- densation nuclei, and their chemical reactivity. The RH de- pendence of light scattering is one of the parameters needed to estimate the direct climate forcing by aerosol particles. Correspondence to: E. Weingartner (ernest.weingartner@psi.ch) Thus, the dry aerosol particle size distribution and their size- dependent hygroscopic growth factors must be known to model the humidity dependence of the light scattering of an aerosol. Efforts are currently undertaken to include the ef- fects of hygroscopic growth of aerosol particles in global cli- mate models (GCM) in order to better predict the scatter- ing properties and size distribution of aerosols under vary- ing humidity conditions (Randall et al., 2007). The two most widely used techniques enabling measurement of the change in the amount of water absorbed to an aerosols parti- cle with varying RH are the single aerosol particle levitation technique using an electrodynamic balance (EDB; Tang and Munkelwitz, 1993) and the hygroscopicity tandem differen- tial mobility analyser technique (HTDMA; Liu et al., 1978; Swietlicki et al., 2008). The EDB technique, which measures the properties of individual super-micrometer aerosol parti- cles, is suitable for laboratory measurements. The HTDMA technique, which probes the hygroscopicity of all aerosol particles of a well-defined dry diameter at once, is suitable for field and laboratory measurements. HTDMA instruments cover the sub-micrometer diameter range, which contains the majority of the atmospheric aerosol particles. All HTDMAs existing worldwide are custom built instru- ments and very few intercomparison studies between differ- ent instruments have been reported. This gap has been filled with this study as part of the EC project EUSAAR (European Supersites for Atmospheric Aerosol Research; http://www. eusaar.net/). Two intercomparison workshops have been con- ducted involving in total six different HTDMAs from five re- search groups with the aim to harmonise the design and the Published by Copernicus Publications on behalf of the European Geosciences Union.