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 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 (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 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.
RuIII(edta) complexes as molecular redox catalysts in chemical and electrochemical reduction of dioxygen and hydrogen peroxide: inner-sphere versus outer-sphere mechanism
