Atmospheric hydrogen peroxide (H 2 O 2 ) and organic hydroperoxides were measured from 18 to 30 July in 2006 during the PRIDE-PRD'06 campaign at Backgarden, a rural site located 48 km north of Guangzhou, a mega-city in southern China. A groundbased instrument was used as a scrubbing coil collector to sample ambient air, fol-5 lowed by on-site analysis by high-performance liquid chromatography (HPLC) coupled with post-column derivatization and fluorescence detection. The H 2 O 2 mixing ratio

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... the 13 days ranged from below the detection limit to a maximum of 4.6 ppbv, with a mean (and standard deviation) of (1.26±1.24) ppbv during the daytime (08:00-20:00 LT). Methyl hydroperoxide (MHP), with a maximum of 0.8 ppbv and a mean (and 10 standard deviation) of (0.28±0.10) ppbv during the daytime, was the dominant organic hydroperoxide. Other organic peroxides, including bis-hydroxymethyl hydroperoxide (BHMP), peroxyacetic acid (PAA), hydroxymethyl hydroperoxide (HMHP), 1-hydroxyethyl hydroperoxide (1-HEHP) and ethyl hydroperoxide (EHP), were detected occasionally. The concentration of H 2 O 2 exhibited a pronounced diurnal variation on sunny 15 days, with a peak mixing ratio in the afternoon (12:00-18:00 LT), but lacked an explicit diurnal cycle on cloudy days. Sometimes a second peak mixing ratio of H 2 O 2 was observed during the evening, suggesting that H 2 O 2 was produced by the ozonolysis of alkenes. The diurnal variation profile of MHP was, in general, consistent with that of H 2 O 2 . The estimation indicated that in the morning the H 2 O 2 detected was formed 20 mostly through local photochemical activity, with the rest probably attributable to vertical transport. It is notable that relatively high levels of H 2 O 2 and MHP were found in polluted air. The unexpectedly high level of HO 2 radicals detected in this region can account for the production of hydroperoxides, while the high level of NO x suppressed the formation of hydroperoxides significantly. High concentrations of hydroperoxides 25 were detected in samples of rainwater collected in a heavy shower on 25 July when a typhoon passed through, indicating that a considerable mixing ratio of hydroperoxides, particularly MHP, resided above the upper boundary layer, which might be transported 10483 Abstract ACPD