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The CLOUD experiment (Cosmics Leaving OUtdoorDroplets) investigates the nucleation of new particles and how thisprocess is influenced by galactic cosmic rays in an electropolished,stainless-steel environmental chamber at CERN (European Organization forNuclear Research). Since volatile organic compounds (VOCs) can act asprecursor gases for nucleation and growth of particles, great efforts havebeen made to keep their unwanted background levels as low as possible and toquantify them. In order to be able to measure a great set of VOCssimultaneously in the low parts per trillion (pptv) range,proton-transfer-reaction mass spectrometry (PTR-MS) was used. Initially thetotal VOC background concentration strongly correlated with ozone in thechamber and ranged from 0.1 to 7 parts per billion (ppbv). Plastic used assealing material in the ozone generator was found to be a major VOC source.Especially oxygen-containing VOCs were generated together with ozone. Theseparts were replaced by stainless steel after CLOUD3, which strongly reducedthe total VOC background. An additional ozone-induced VOC source issurface-assisted reactions at the electropolished stainless steel walls. Thechange in relative humidity (RH) from very dry to humid conditions increasesbackground VOCs released from the chamber walls. This effect is especiallypronounced when the RH is increased for the first time in a campaign. Alsothe dead volume of inlet tubes for trace gases that were not continuouslyflushed was found to be a short but strong VOC contamination source. Forlower ozone levels (below 100 ppbv) the total VOC contamination was usuallybelow 1 ppbv and therewith considerably cleaner than a comparable Teflonchamber. On average about 75% of the total VOCs come from only five exactmasses (tentatively assigned as formaldehyde, acetaldehyde, acetone, formicacid, and acetic acid), which have a rather high vapour pressure and aretherefore not important for nucleation and growth of particles

Characterisation of organic contaminants in the CLOUD chamber at CERN