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| Titel |
Characterisation of organic contaminants in the CLOUD chamber at CERN |
| VerfasserIn |
R. Schnitzhofer, A. Metzger, M. Breitenlechner, W. Jud, M. Heinritzi, L.-P. De Menezes, J. Duplissy, R. Guida, S. Haider, J. Kirkby, S. Mathot, P. Minginette, A. Onnela, H. Walther, A. Wasem, A. Hansel, CLOUD Team |
| 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. 7 ; Nr. 7, no. 7 (2014-07-18), S.2159-2168 |
| Datensatznummer |
250115850
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| Publikation (Nr.) |
 copernicus.org/amt-7-2159-2014.pdf |
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| Zusammenfassung |
| The CLOUD experiment (Cosmics Leaving OUtdoor
Droplets) investigates the nucleation of new particles and how this
process is influenced by galactic cosmic rays in an electropolished,
stainless-steel environmental chamber at CERN (European Organization for
Nuclear Research). Since volatile organic compounds (VOCs) can act as
precursor gases for nucleation and growth of particles, great efforts have
been made to keep their unwanted background levels as low as possible and to
quantify them. In order to be able to measure a great set of VOCs
simultaneously in the low parts per trillion (pptv) range,
proton-transfer-reaction mass spectrometry (PTR-MS) was used. Initially the
total VOC background concentration strongly correlated with ozone in the
chamber and ranged from 0.1 to 7 parts per billion (ppbv). Plastic used as
sealing material in the ozone generator was found to be a major VOC source.
Especially oxygen-containing VOCs were generated together with ozone. These
parts were replaced by stainless steel after CLOUD3, which strongly reduced
the total VOC background. An additional ozone-induced VOC source is
surface-assisted reactions at the electropolished stainless steel walls. The
change in relative humidity (RH) from very dry to humid conditions increases
background VOCs released from the chamber walls. This effect is especially
pronounced when the RH is increased for the first time in a campaign. Also
the dead volume of inlet tubes for trace gases that were not continuously
flushed was found to be a short but strong VOC contamination source. For
lower ozone levels (below 100 ppbv) the total VOC contamination was usually
below 1 ppbv and therewith considerably cleaner than a comparable Teflon
chamber. On average about 75% of the total VOCs come from only five exact
masses (tentatively assigned as formaldehyde, acetaldehyde, acetone, formic
acid, and acetic acid), which have a rather high vapour pressure and are
therefore not important for nucleation and growth of particles. |
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