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Abstract. Gas-phase H 2 O 2 , organic peroxides and carbonyl compounds were determined at various sites from Mid-July to early August 1998 during the BERLIOZ campaign in Germany. The sites were located northwest of Berlin and were chosen to determine pollutants downwind of the city emissions during a summer smog episode. Hydrogen peroxide (H 2 O 2 ), methyl hydroperoxide (MHP, CH 3 OOH) and occasionally hydroxymethyl hydroperoxide (HMHP, HOCH 2 OOH) were quantified in air samples by commercial fluorimetric methods and classical HPLC with post-column derivatisation by horseradish peroxidase/p-hydroxyphenyl acetic acid and fluorimetric detection. Carbonyl compounds were determined in ambient air by a novel method based on O-pentafluorobenzyl hydroxylamine as derivatisation agent. Mixing ratio profiles of the hydroperoxides and the carbonyl compounds are reported for the intensive phase of the campaign, 20-21 July, 1998. Peroxides showed pronounced diurnal variations with peak mixing ratios in the early afternoon. At times, a second maximum was observed in the late afternoon. The major part of the H 2 O 2 was formed through recombination reactions of HO 2 radicals, but there is some evidence that H 2 O 2 is also formed from ozonolysis of anthropogenic and/or biogenic alkenes. Diurnal variations of mixing ratios of various carbonyl compounds are reported: alkanals (C 2 to C 10 , isobutanal), unsaturated carbonyl compounds (methacrolein, methylvinylketone, acrolein), hydroxycarbonyl (glycolaldehyde, hydroxyacetone) and dicarbonyl compounds (glyoxal, methylglyoxal, biacetyl), aromatic compounds (benzaldehyde, o-and m-tolylaldehyde) and pinonaldehyde.

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