Abstract. In this study we report measurements of hydrogen peroxide (H₂O₂), 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+HO₂). 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 H₂O₂ and CH₃OOH*. 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. H₂O₂ and CH₃OOH* 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 H₂O₂. Large uncertainties about retention coefficients and cloud microphysical parameters suggest that cloud scavenging might be a large source of error for the simulation of H₂O₂. A sensitivity study with EMAC shows a strong influence of cloud and precipitation scavenging on the budget of H₂O₂ as simulations improve significantly with this effect switched off.