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A new experimental approach to study the hygroscopic and the optical properties of aerosols: application to ammonium sulfate particles

Author(s): C. Denjean | P. Formenti | B. Picquet-Varrault | Y. Katrib | E. Pangui | P. Zapf | J. F. Doussin

Journal: Atmospheric Measurement Techniques Discussions
ISSN 1867-8610

Volume: 6;
Issue: 4;
Start page: 6935;
Date: 2013;
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A new methodology for the determination of size distribution and optical properties of aerosols as a function of the relative humidity (RH) in simulation chamber is described. The hygroscopic properties of aerosols can be investigated by exposing aerosols to varying RH ranging from 0 to 100% for approximately 1 h. This method is also proved useful in both providing information on the RH dependence of size and scattering coefficient (σscat) of the overall size distribution. The complex refractive index (CRI) of aerosols can be retrieved at λ = 525 nm, as well as in the visible spectrum. Ammonium sulfate particles that have well known optical and hygroscopic properties were used for the measurements. The particle's growth was compared to values obtained for one selected size of particles (150 nm) with a H-TDMA and the effect of the residence time for particles humidification was investigated. The CRI of ammonium sulfate particles obtained from the two methods (1.54–1.57) compared favorably to each other and are also in reasonable agreement with the literature values. When the humidification is performed in the chamber for a few minutes, a continuous increase of the ammonium sulfate particle's size and σscat was observed from RH values as low as 30% RH. Comparison of the measured and modeled values based on Köhler and Mie theories shows that layers of water are adsorbed on ammonium sulfate particles below the deliquescence point. In contradiction, the particle's growth reported with H-TDMAs showed a prompt deliquescence of ammonium sulfate particles with no continuous growth in size at low RH. The findings highlight the need to allow sufficient time for particle-water vapor equilibrium in investigating the aerosols hygroscopic properties. H-TDMA instruments induce limited residence time for humidification and seem to be insufficient for water adsorption on ammonium sulfate particles.
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