Methanol and other VOC fluxes from a Danish beech forest during springtime

Abstract. In-canopy mixing ratio gradients and above-canopy fluxes of several volatile organic compounds (VOCs) were measured using a commercial proton transfer reaction mass spectrometer (PTR-MS) in a European beech (Fagus sylvatica) forest in Denmark. Emission fluxes of methanol occurred dominantly late at night, which was supported by highest mixing ratios in the crown region, and is in line with recent controlled laboratory experiments. Also confirming previous measurements, monoterpene emissions showed a diurnal cycle consistent with light-dependent emissions, supported by highest mixing ratios in the canopy space during early afternoon. Also emitted was acetone, but only at ambient temperatures exceeding 20°C. Deposition dominated at lower temperatures. Deposition fluxes occurred also for methanol but seemingly as a result of high ambient methanol mixing ratios. Our in-canopy gradient measurements contrasted earlier results from tropical and pine forest ecosystems in that they did not show this beech ecosystem to be a strong sink for oxygenated VOCs. Instead, their gradients were flat and only small deposition velocities (<0.1 cm s^–1) were observed to the onsite soil. However, as soil uptake was consistent and appeared to be related to soil moisture, more measurements are needed to evaluate the soil sink strength. In turn, as canopy scale fluxes are net fluxes with emissions from photosynthesizing leaves affecting potential oxygenated VOC uptake, only independent, controlled laboratory experiments may be successful in separating stomatal from non-stomatal fluxes, and emission from deposition.