Atmospheric Measurement Techniques www.atmos-meas-tech.net 1867-1381 1867-8548 3 1 2010 10.5194/amt-3-39-2010 http://www.atmos-meas-tech.net/3/39/2010/ http://www.atmos-meas-tech.net/3/39/2010/amt-3-39-2010.html http://www.atmos-meas-tech.net/3/39/2010/amt-3-39-2010.pdf 39 50 2010-01-21 Measurement of relative humidity dependent light scattering of aerosols R. Fierz-Schmidhauser P. Zieger G. Wehrle A. Jefferson J. A. Ogren U. Baltensperger E. Weingartner ernest.weingartner@psi.ch Laboratory of Atmospheric Chemistry, Paul Scherrer Institut, Villigen, Switzerland CIRES, University of Colorado, Boulder, 80305, Colorado, USA National Oceanic and Atmospheric Administration (NOAA), Boulder, 80305, Colorado, USA Relative humidity (RH) influences the water content of aerosol particles and therefore has an important impact on the particles' ability to scatter visible light. The RH dependence of the particle light scattering coefficient (σsp is therefore an important measure for climate forcing calculations. We built a humidification system for a nephelometer which allows the measurement of σsp at a defined RH in the range of 40–90%. This RH conditioner consists of a humidifier followed by a dryer, which enables us to measure the hysteresis behavior of deliquescent aerosol particles. In this paper we present the set-up of a new humidified nephelometer, a detailed characterization with well defined laboratory generated aerosols, and a first application in the field by comparing our instrument to another humidified nephelometer. Monodisperse ammonium sulfate and sodium chloride particles were measured at four different dry particle sizes. Agreement between measurement and prediction based on Mie theory was found for both σsp and f(RH)=σsp(RH)/σsp(dry) within the range of uncertainty. 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