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. Hydrogen peroxide (H2O2) plays a significantrole in the oxidizing capacity of the atmosphere. It is an efficient oxidantin the liquid phase and serves as a temporary reservoir for the hydroxylradical (OH), the most important oxidizing agent in the gas phase. Due toits high solubility, removal of H2O2 due to wet and dry depositionis efficient, being a sink of HOx (OH+HO2) radicals. In thecontinental boundary layer, the H2O2 budget is controlled byphotochemistry, transport and deposition processes. Here we use in situobservations of H2O2 and account for chemical source and removalmechanisms to study the interplay between these processes. The data wereobtained during five ground-based field campaigns across Europe from 2008 to2014 and bring together observations in a boreal forest, two mountainoussites in Germany, and coastal sites in Spain and Cyprus. Most campaigns tookplace in the summer, while the measurements in the south-west of Spain tookplace in early winter. Diel variations in H2O2 are stronglysite-dependent and indicate a significant altitude dependence. Whileboundary-layer mixing ratios of H2O2 at low-level sites showclassical diel cycles with the lowest values in the early morning and maximaaround local noon, diel profiles are reversed on mountainous sites due totransport from the nocturnal residual layer and the free troposphere. Theconcentration of hydrogen peroxide is largely governed by its mainprecursor, the hydroperoxy radical (HO2), and shows significantanti-correlation with nitrogen oxides (NOx) that remove HO2. Abudget calculation indicates that in all campaigns, the noontimephotochemical production rate through the self-reaction of HO2 radicalswas much larger than photochemical loss due to reaction with OH andphotolysis, and that dry deposition is the dominant loss mechanism.Estimated dry deposition velocities varied between approximately 1 and 6 cm s−1,with relatively high values observed during the day in forested regions,indicating enhanced uptake of H2O2 by vegetation. In order toreproduce the change in H2O2 mixing ratios between sunrise andmidday, a variable contribution from transport (10 %–100 %) is requiredto balance net photochemical production and deposition loss. Transport ismost likely related to entrainment from the residual layer above thenocturnal boundary layer during the growth of the boundary layer in themorning.

Diurnal variability, photochemical production and loss processes of hydrogen peroxide in the boundary layer over Europe