[1] Gas-phase H 2 O 2 , organic peroxides, carbonyl compounds, and carboxylic acids were measured from mid-July to early August 1998 during the Berlin ozone (BERLIOZ) campaign in Pabstthum, Germany. The rural site, located 50 km northwest from Berlin, was chosen to measure the pollutants downwind during a summer smog episode. The hydroperoxides showed pronounced diurnal variations with peak mixing ratios in the early afternoon. The maximum mixing ratios were 1.4 ppbv (H 2 O 2 ), 0.64 ppbv (methylhydroperoxide), and 0.22 ppbv (hydroxymethyl-hydroperoxide). H 2 O 2 was formed through photochemical activity, but originated also from vertical transport from air residing above the local inversion layer in the morning hours. Sometimes a second maximum was observed in the late afternoon-evening: This H 2 O 2 might be formed from ozonolysis of biogenic alkenes. Ratios of H 2 O 2 /HNO 3 were used as indicators for the determination of NO x -sensitive versus volatile organic compound (VOC)-sensitive regimes for photochemical production of ozone. Diurnal profiles were measured for alkanals (C 2 -C 10 ), showing maximum mixing ratios decreased from C 2 (0.6 ppbv) to C 5 (0.1 ppbv) alkanals, which originate primarily from anthropogenic hydrocarbon degradation processes. However, higher C 6 , C 9 , and C 10 alkanals show strong fluctuations (0.25, 0.17, and 0.13 ppbv, respectively), showing evidence of biogenic emissions. Both primary unsaturated carbonyl (methyl vinyl ketone, methacrolein) and secondary oxidation products of isoprene (hydroxyacetone and glycolaldehyde, up to 0.16 and 0.20 ppbv, respectively) showed excellent correlation. Diurnal profiles of glyoxal, methylglyoxal, biacetyl, benzaldehyde, and pinonaldehyde were also obtained. Formaldehyde was measured continuously by longpath DOAS and by an instrument based on the ''Hantzsch'' reaction; however, mixing ratios measured by DOAS (maximum 7.7 ppbv) were systematically larger by a factor of 1.3 on average, but by a factor of 1.7 during high photochemical activity. Homologous series of monocarboxylic acids were determined: Formic and acetic acid varied between 0.6 and 3.0 ppbv. The mixing ratio of the other dropped from 0.1 to 0.2 ppbv for C 3 to typical 0.01 to 0.03 ppbv for C 6 , and from 0.01 to 0.002 ppbv for C 7 to C 9 monocarboxylic acids.
