The white-light humidified optical particle spectrometer (WHOPS) - a novel airborne system to characterize aerosol hygroscopicity
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Authors
Rosati, Bernadette
Wehrle, Günther
Zieger, Paul
Baltensperger, Urs
Author (Corporation)
Publication date
2015
Typ of student thesis
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Type
01A - Journal article
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Parent work
Atmospheric Measurement Techniques
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Volume
8
Issue / Number
2
Pages / Duration
921–939
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Publisher / Publishing institution
Copernicus
Place of publication / Event location
Göttingen
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Abstract
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).
Keywords
Subject (DDC)
620 - Ingenieurwissenschaften und Maschinenbau
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ISBN
ISSN
1867-8548
1867-1381
1867-1381
Language
English
Created during FHNW affiliation
Yes
Strategic action fields FHNW
Publication status
Published
Review
Peer review of the complete publication
Open access category
Gold
License
Citation
ROSATI, Bernadette, Günther WEHRLE, Martin GYSEL, Paul ZIEGER, Urs BALTENSPERGER und Ernest WEINGARTNER, 2015. The white-light humidified optical particle spectrometer (WHOPS) - a novel airborne system to characterize aerosol hygroscopicity. Atmospheric Measurement Techniques. 2015. Bd. 8, Nr. 2, S. 921–939. DOI 10.5194/amt-8-921-2015. Verfügbar unter: https://doi.org/10.26041/fhnw-8096