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

Abstract. Hydrogen peroxide (H 2 O 2 ) 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 H 2 O 2 due to wet and dry deposition is efficient, being a sink of HO x (OH + HO 2 ) radicals. In the continental boundary layer, the H 2 O 2 budget is controlled by photochemistry, transport and deposition processes. Here we use in-situ observations of H 2 O 2 , 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 H 2 O 2 are strongly site-dependent and indicate a significant altitude dependence. While boundary layer mixing ratios of H 2 O 2 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 (HO 2 ), and shows significant anti-correlation with nitrogen oxides (NO x ) that remove HO 2 . A budget calculation indicates that in all campaigns, the noontime photochemical production rate through the self-reaction of HO 2 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 H 2 O 2 by vegetation. In order to reproduce the change in H 2 O 2 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.