HO_x measurements in the summertime upper troposphere over Europe: a comparison of observations to a box model and a 3-D model

Abstract. 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 (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 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.