| Summary: | The hydrocarbon measurements made during the ARCTOC 95 field campaign (16 April – 10 June 1995) at Ny Ålesund, Spitsbergen (78°55''N, 11°56'E) allow the estimation of Cl and Br atom concentrations during periods of low ozone concentrations in the lower troposphere. In-situ analyses for nonmethane hydrocarbons and selected halocarbons were carried out with two GC-FID/ECD systems allowing a time resolution of about 2 hours and detection limits in the lower ppt range. A third GC was used to monitor the CO mixing ratio with a 5 min time resolution. Additionally, about 70 whole air samples were collected in stainless steel canisters and analysed for hydrocarbons (C2-C8), some halocarbons, methane, CO, and CO2 at the laboratory in Jülich. 15 of those samples were taken at the measurement site, the others were collected on top of Zeppelin Mountain (474 m a.s.l., distance from measurement site 1000 m). In-situ measurements as well as canister samples showed that all nonmethane hydrocarbons were present at low mixing ratios. CO levels were found to be between 100 and 140 ppb showing a nearly constant decrease from April to June. During an ozone depletion event (18-24 April) the hydrocarbon pattern changed drastically. The mixing ratios of alkanes and ethyne decreased together with ozone, whereas benzene, chloromethane, and CO mixing ratios remained nearly constant. From the ratio of NMHC background mixing ratios and those during low ozone periods the time integrated Cl and Br atom concentrations were calculated to some 1010 moleculesċsċcm−3 and 5ċ1012 moleculesċsċcm−3, respectively.
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