The white-light humidified optical particle spectrometer (WHOPS) - a novel airborne system to characterize aerosol hygroscopicity

dc.contributor.authorRosati, Bernadette
dc.contributor.authorWehrle, Günther
dc.contributor.authorGysel, Martin
dc.contributor.authorZieger, Paul
dc.contributor.authorBaltensperger, Urs
dc.contributor.authorWeingartner, Ernest
dc.date.accessioned2024-02-08T09:05:28Z
dc.date.available2024-02-08T09:05:28Z
dc.date.issued2015
dc.description.abstractAerosol 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).
dc.identifier.doi10.5194/amt-8-921-2015
dc.identifier.issn1867-8548
dc.identifier.issn1867-1381
dc.identifier.urihttps://irf.fhnw.ch/handle/11654/44286
dc.identifier.urihttps://doi.org/10.26041/fhnw-8096
dc.issue2
dc.language.isoen
dc.publisherCopernicus
dc.relation.ispartofAtmospheric Measurement Techniques
dc.rights.urihttps://creativecommons.org/licenses/by/4.0/
dc.spatialGöttingen
dc.subject.ddc620 - Ingenieurwissenschaften und Maschinenbau
dc.titleThe white-light humidified optical particle spectrometer (WHOPS) - a novel airborne system to characterize aerosol hygroscopicity
dc.type01A - Beitrag in wissenschaftlicher Zeitschrift
dc.volume8
dspace.entity.typePublication
fhnw.InventedHereYes
fhnw.ReviewTypeAnonymous ex ante peer review of a complete publication
fhnw.affiliation.hochschuleHochschule für Technikde_CH
fhnw.affiliation.institutlnstitut für Sensorik und Elektronikde_CH
fhnw.openAccessCategoryGold
fhnw.pagination921–939
fhnw.publicationStatePublished
relation.isAuthorOfPublication54997bb8-cf4a-4120-b0c7-f8e731e8eea1
relation.isAuthorOfPublication05dd9a19-7a24-4325-805a-2d121483b168
relation.isAuthorOfPublication.latestForDiscovery54997bb8-cf4a-4120-b0c7-f8e731e8eea1
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