Abstract. In this study we report measurements of hydrogen peroxide (H 2 O 2 ), methyl hydroperoxide* (MHP* as a proxy of MHP based on an unspecific measurement of total organic peroxides) and formaldehyde (HCHO) from the HO x OVer EuRope (HOOVER) project (HO x = OH+HO 2 ). HOOVER included two airborne field campaigns, in October 2006 and July 2007. Measurement flights were conducted from the base of operation Hohn (Germany, 54 • N, 9 • E) towards the Mediterranean and to the subpolar regions over Norway. We find negative concentration gradients with increasing latitude throughout the troposphere for H 2 O 2 and CH 3 OOH * . In contrast, observed HCHO is almost homogeneously distributed over central and northern Europe and is elevated over the Mediterranean. In general, the measured gradients tend to be steepest entering the Mediterranean region, where we also find the highest abundances of the 3 species. Mixing ratios of these tracers generally decrease with altitude. H 2 O 2 and CH 3 OOH * show maxima above the boundary layer at 2-5 km, being more distinct over southern than over northern Europe.We also present a comparison of our data with simulations by two global 3-D-models, MATCH-MPIC and EMAC, and with the box model CAABA. The models realistically represent altitude and latitude gradients for both HCHO and hydroperoxides (ROOH). In contrast, the models have problems reproducing the absolute mixing ratios, in particular of H 2 O 2 . Large uncertainties about retention coefficients and Correspondence to: H. Fischer (horst.fischer@mpic.de) cloud microphysical parameters suggest that cloud scavenging might be a large source of error for the simulation of H 2 O 2 . A sensitivity study with EMAC shows a strong influence of cloud and precipitation scavenging on the budget of H 2 O 2 as simulations improve significantly with this effect switched off.

Abstract. In the OOMPH (Ocean OrganicsModifyingIn situ measurements of hydrogen peroxide, methylhydroperoxide and ozone were performed and are compared to simulations with the atmospheric chemistry global circulation model EMAC (ECHAM/MESSy Atmospheric Chemistry). The model generally reproduces the measured trace gas levels, but it underestimates hydrogen peroxide mixing ratios at high wind speeds, indicating too-strong dry deposition to the ocean surface. An interesting feature during the cruise is a strong increase of hydrogen peroxide, methylhydroperoxide and ozone shortly after midnight off the west coast of Africa due to an increase in the boundary layer height, leading to downward transport from the free troposphere, which is qualitatively reproduced by the model.

Abstract. In situ airborne measurements of OH and HO 2 with the HORUS (HydrOxyl Radical measurement Unit based on fluorescence Spectroscopy) instrument were performed in the summertime upper troposphere across Europe during the HOOVER 2 (HO x OVer EuRope) campaign in July 2007. Complementary measurements of trace gas species and photolysis frequencies were conducted to obtain a broad data set, which has been used to quantify the significant HO x sources and sinks. In this study we compare the in situ measurement of OH and HO 2 with simulated mixing ratios from the constrained box model CAABA/MECCA (Chemistry As A Box Model Application/Module Efficiently Calculating the Chemistry of the Atmosphere), and the global circulation model EMAC (ECHAM5/MESSy Atmospheric Chemistry Model). The constrained box model reproduces the observed OH and HO 2 mixing ratios with better agreement (obs/mod median 98 % OH, 96 % HO 2 ) than the global model (median 76 % OH, 59 % HO 2 ). The observations and the computed HO x sources and sinks are used to identify deviations between the models and their impacts on the calculated HO x budget.

In-situ airborne measurements of OH and HO₂ with the HORUS (HydrOxyl Radical measurement Unit based on fluorescence Spectroscopy) instrument were performed in the summertime upper troposphere across Europe during the HOOVER 2 (HOx OVer EuRope) campaign in July 2007. Complementary measurements of trace gas species and photolysis frequencies were conducted to obtain a broad data set, which has been used to quantify the significant HO_x sources and sinks. In this study we compare the in-situ measurement of OH and HO₂ with simulated mixing ratios from the constrained box model CAABA/MECCA (Chemistry As A Box Model Application/Module Efficiently Calculating the Chemistry of the Atmosphere), and the global circulation model EMAC (ECHAM5/MESSy Atmospheric Chemistry Model). The constrained box model reproduces the observed OH and HO₂ mixing ratios with better agreement (obs/mod median 98% OH, 96% HO₂) than the global model (median 76% OH, 59% HO₂). The observations and the computed HO_x sources and sinks are used to identify deviations between the models and their impacts on the calculated HO_x budget