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Abstract

Hydrogen peroxide (H₂O₂) plays a significant role in the oxidizing capacity of the atmosphere. It is an efficient oxidant in the liquid phase and serves as a temporary reservoir for the hydroxyl radical (OH), the most important oxidizing agent in the gas phase. Due to its high solubility, removal of H₂O₂ due to wet and dry deposition is efficient, being a sink of HO_x (OH + HO₂) radicals. In the continental boundary layer, the H₂O₂ budget is controlled by photochemistry, transport and deposition processes. Here we use in situ observations of H₂O₂ and account for chemical source and removal mechanisms to study the interplay between these processes. The data were obtained during five ground-based field campaigns across Europe from 2008 to 2014 and bring together observations in a boreal forest, two mountainous sites in Germany, and coastal sites in Spain and Cyprus. Most campaigns took place in the summer, while the measurements in the south-west of Spain took place in early winter. Diel variations in H₂O₂ are strongly site-dependent and indicate a significant altitude dependence. While boundary-layer mixing ratios of H₂O₂ at low-level sites show classical diel cycles with the lowest values in the early morning and maxima around local noon, diel profiles are reversed on mountainous sites due to transport from the nocturnal residual layer and the free troposphere. The concentration of hydrogen peroxide is largely governed by its main precursor, the hydroperoxy radical (HO₂), and shows significant anti-correlation with nitrogen oxides (NO_x) that remove HO₂. A budget calculation indicates that in all campaigns, the noontime photochemical production rate through the self-reaction of HO₂ radicals was much larger than photochemical loss due to reaction with OH and photolysis, 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 to reproduce the change in H₂O₂ mixing ratios between sunrise and midday, a variable contribution from transport (10 %-100 %) is required to balance net photochemical production and deposition loss. Transport is most likely related to entrainment from the residual layer above the nocturnal boundary layer during the growth of the boundary layer in the morning.