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Measurements of OH, total peroxy radicals, non-methane hydrocarbons(NMHCs) and various other trace gases were made at the Meteorological Observatory Hohenpeissenberg in June 2000. Thedata from an intensive measurement period characterised by high solar insolation(18-21 June) are analysed. The maximum midday OH concentration ranged between4.5x106 molecules cm-3 and 7.4x106molecules cm-3. The maximum total ROx (ROx=OH+RO+HO2+RO2) mixing ratio increased from about 55pptv on 18 June to nearly 70 pptv on 20 and 21 June. A total of 64 NMHCs, including isoprene andmonoterpenes, were measured every 1 to 6 hours. The oxidation rate of the NMHCs by OH was calculated and reached a total of over14x106 molecules cm-3 s-1 on two days. A simple photostationary state balance model was used to simulate the ambient OH and peroxyradical concentrations with the measured data as input. This approach was able to reproduce the mainfeatures of the diurnal profiles of both OH and peroxy radicals. The balance equations were usedto test the effect of the assumptions made in this model. The results proved to be most sensitive toassumptions about the impact of unmeasured volatile organic compounds (VOC), e.g. formaldehyde(HCHO), and about the partitioning between HO2 and RO2. The measured OH concentration and peroxy radicalmixing ratios were reproduced well by assuming the presence of 3 ppbv HCHO as a proxy for oxygenatedhydrocarbons, and a HO2/ RO2 ratio between 1:1 and 1:2. The most important source of OH, and converselythe greatest sink for peroxy radicals, was the recycling of HO2radicals to OH. This reaction was responsible for the recycling of more than45x106 molecules cm-3 s-1 on two days. The most important sink for OH, and the largest source of peroxy radicals, was the oxidation ofNMHCs, in particular, of isoprene and the monoterpenes

Hohenpeissenberg Photochemical Experiment (HOPE 2000): Measurements and photostationary state calculations of OH and peroxy radicals