Hydrogen peroxide (H₂O₂) is an important chemical constituent of the troposphere. Its production occurs mainly in the gas phase by recombination of two HO₂ radicals. The primary sink for H₂O₂ is the uptake into existing aerosols acting as an oxidant, which is mainly responsible for the conversion of SO₂ into sulphuric acid (H₂SO₄). Near the surface dry deposition also represents a major sink for H₂O₂ due to its high solubility and reactivity. Recent reports indicate another possible source of H₂O₂: the ozonolysis of both anthropogenic and biogenic alkenes. A study of the ozonolysis of isoprene has shown that H₂O₂ is produced by the reaction of the Criegee intermediate with water vapour in the absence of light. Measured H₂O₂ yields from a series of anthropogenic and biogenic hydrocarbons have already been used to predict the relevance of the ozonolysis reactions for tropospheric chemistry.

The first measurements of H₂O₂ surface fluxes applying the Relaxed Eddy Accumulation Technique (REA) were successfully carried out above the canopy of a spruce forest in Germany. The REA technique was specially adapted for the measurements of H₂O₂ and organic peroxides. The results presented here focus on the H₂O₂ measurements. Measured concentrations and fluxes show a clear diurnal cycle. The fluxes were mainly downward with deposition velocities of about 4 ± 1 cm s^-1. However, upward fluxes (emission) were also observed. These measurements were made in order to determine the importance of the ozonolysis reaction for the H₂O₂ surface exchanges.