Reply to referee 1

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 HOx (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 southwest 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 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 (NOx) 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 approx. 1 and 6 cm/s, 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.