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| Titel |
The white-light humidified optical particle spectrometer (WHOPS) – a novel airborne system to characterize aerosol hygroscopicity |
| VerfasserIn |
B. Rosati, G. Wehrle, M. Gysel, P. Zieger, U. Baltensperger, E. Weingärtner |
| 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 ; 8, no. 2 ; Nr. 8, no. 2 (2015-02-25), S.921-939 |
| Datensatznummer |
250116144
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| Publikation (Nr.) |
 copernicus.org/amt-8-921-2015.pdf |
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| Zusammenfassung |
Aerosol particles experience hygroscopic growth at enhanced relative humidity
(RH), which leads to changes in their optical properties. We developed the
white-light humidified optical particle spectrometer (WHOPS), a new
instrument to investigate the particles' hygroscopic growth. Here we present
a detailed technical description and characterization of the WHOPS in
laboratory and field experiments. The WHOPS consists of a differential
mobility analyzer, a humidifier/bypass and a white-light aerosol
spectrometer (WELAS) connected in series to provide fast measurements of particle
hygroscopicity at subsaturated RH and optical properties on airborne
platforms. The WELAS employs a white-light source to minimize ambiguities in
the optical particle sizing. In contrast to other hygroscopicity instruments,
the WHOPS retrieves information of relatively large particles (i.e., diameter
D > 280 nm), therefore investigating the more optically relevant size
ranges.
The effective index of refraction of the dry particles is retrieved
from the optical diameter measured for size-selected aerosol samples
with a well-defined dry mobility diameter. The data analysis
approach for the optical sizing and retrieval of the index of
refraction was extensively tested in laboratory experiments with
polystyrene latex size standards and ammonium sulfate particles of
different diameters. The hygroscopic growth factor (GF) distribution
and aerosol mixing state is inferred from the optical size
distribution measured for the size-selected and humidified aerosol
sample. Laboratory experiments with pure ammonium sulfate particles
revealed good agreement with Köhler theory (mean bias of
~3% and maximal deviation of 8% for GFs at RH =
95%).
During first airborne measurements in the Netherlands, GFs (mean
value of the GF distribution) at RH = 95% between 1.79
and 2.43 with a median of 2.02 were observed for particles with a dry diameter of
500 nm. This corresponds to hygroscopicity parameters
(κ) between 0.25 and 0.75 with a median of 0.38. The GF
distributions indicate externally mixed particles covering the whole
range of GFs between ~1.0 and 3.0. On average, ~74%
of the 500 nm particles had GFs > 1.5,
~15% had GF < 1.1 and
the remaining ~1% showed values of 1.1 < GF < 1.5. The more hygroscopic mode sometimes peaked at
GF > 2, indicating influence of sea-salt particles, consistent
with previous ground-based particle hygroscopicity measurements in
this area. The mean dry effective index of refraction for 500 nm
particles was found to be rather constant with a value of 1.42
±
0.04 (mean ± 1SD). |
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