10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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In-Situ Characterization of Aerosol Nanoparticles at Close-To-Ambient Conditions by Small Angle X-Ray Scattering (SAXS)
PAULUS S. BAUER, Heinz Amenitsch, Bernhard Baumgartner, Christian Rentenberger, Paul M. Winkler, Universitaet Wien & VDSP, Vienna, Austria
Abstract Number: 329 Working Group: Instrumentation
Abstract The characterization of airborne particles is one of the main tasks in aerosol science. Especially the analysis of nanoparticles below 100nm requires particular attention, since they exhibit highly dynamic behavior that may impact cloud properties and hence global climate (IPCC, 2013). To determine the size and number concentration of aerosol nanoparticles Differential Mobility Particle Sizer (DMPS) and Condensation Particle Counter (CPC) are mostly used. A common drawback of these techniques is that they remove the aerosol particles from their original environment. Thereby, nanoparticles can get modified (e.g. by evaporation) or get lost (e.g. by wall collisions inside the instrument), which may affect the measured size distribution and concentration (Wang, J. et al. (2002). J. Aerosol Sci. 33, 6, 843).
An in-situ measurement technique can overcome these shortcomings of the conventional aerosol instruments. Small-angle X-ray scattering (SAXS), commonly used in material science or in biochemical process analysis, can fill this gap. It is capable of measuring in-situ particle size distribution in the nanometer range (Sztucki, M. et al. (2007), J. Appl. Phys. 101, 114304. Laksmono, H. et al. (2011), Phys. Chem. Chem. Phys. 13, 5855). These studies were operated with extremely high nanoparticle concentrations of ~1012/cc and carrier gas pressures ~2 kPa to acquire proper SAXS signals. Conventional aerosol instrumentation on the other hand typically requires measurement conditions close to ambient pressure and concentrations not exceeding 106/cc.
Here we report synchrotron-based SAXS experiments on nanoparticle characterization. To provide a representative environment for aerosols a flow tube was operated at ambient pressure and concentrations of about 106/cc. This leads to a very low volume fraction of about 10-10 (see Beaucage, G. et al. (2004), Nat. Mater. 3, 370). A critical aspect for SAXS experiments with aerosols and this low volume fraction is the background scattering signal originating from the carrier gas. The air background usually is of the same order of magnitude as the signal from the nanoparticles. This issue was lately resolved by replacing air with helium as carrier gas. The usage of helium provides the opportunity of operating the flow tube under ambient conditions (temperature, pressure), and permits the parallel sampling by modified state-of-the-art aerosol instruments like CPC and DMPS system. To complete the picture, electron microscopy images were taken from aerosol samples.