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| Titel |
A new experimental approach to study the hygroscopic and optical properties of aerosols: application to ammonium sulfate particles |
| VerfasserIn |
C. Denjean, P. Formenti, B. Picquet-Varrault, Y. Katrib, E. Pangui, P. Zapf, J. F. Doussin |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 7, no. 1 ; Nr. 7, no. 1 (2014-01-23), S.183-197 |
| Datensatznummer |
250115566
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| Publikation (Nr.) |
 copernicus.org/amt-7-183-2014.pdf |
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| Zusammenfassung |
A new methodology for the determination of the changes due to hygroscopic
growth with relative humidity of the number size distribution and optical
properties of polydispersed aerosols is described. This method uses the
simulation chamber CESAM where the hygroscopic properties of polydispersed
aerosol particles can be investigated in situ by exposing them to RH ranging from 0
to 100% for approximately 1 h. In situ humidification is used to provide
simultaneous information on the RH-dependence of the particle size and the
corresponding scattering coefficient (σscat), and that for the
entire size distribution. Optical closure studies, based on integrated
nephelometer and aethalometer measurements, Mie scattering calculations and
measured particle size distributions, can therefore be performed to yield
derived parameters such as the complex refractive index (CRI) at λ
= 525 nm. The CRI can also be retrieved in the visible spectrum by
combining differential mobility analyzer (DMA) and white light aerosol
spectrometer (Palas Welas®) measurements.
We have applied this methodology to ammonium sulfate particles, which have
well known optical and hygroscopic properties. The CRI obtained from the two
methods (1.54–1.57) compared favourably to each other and are also in
reasonable agreement with the literature values. 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. When the humidification was 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 modelled 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. These findings
highlight the need to allow sufficient time for particle-water vapour
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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