Emission fluxes and atmospheric degradation of monoterpenes above a borealforest: field measurements and modelling

The contribution of monoterpenes to aerosol formation processes within and above forests is not well understood. This is also true for the particle fo rmation events observed during the BIOFOR campaigns in Hyytiala, Finland. T herefore, the diurnal variation of the concentrations of several biogenic v olatile organic compounds (BVOCs) and selected oxidation products in the ga s and particle phase were measured on selected days during the campaigns in Hyytiala, Finland. alpha -pinene and Delta (3)-carene were found to repres ent the most important monoterpenes above the boreal forest. A clear vertic al gradient of their concentrations was observed together with a change of the relative monoterpene composition with height. Based on concentration pr ofile measurements of monoterpenes, their fluxes above the forest canopy we re calculated using the gradient approach. Most of the time, the BVOC fluxe s show a clear diurnal variation with a maximum around noon. The highest fl uxes were observed for alpha -pinene with values up to 20 ng m(-2) s(-1) in summer time and almost 100 ng m(-2) s(-1) during the spring campaign. Furt hermore, the main oxidation products from a-pinene, pinonaldehyde, and from beta -pinene, nopinone, were detected in the atmosphere above the forest. In addition to these more volatile oxidation products, pinic and pinonic ac id were identified in the particle phase in a concentration range between 1 and 4 ng m(-3). Beside these direct measurement of known oxidation product s, the chemical sink term in the flux calculations was used to estimate the amount of product formation of the major terpenes (alpha -pinene, beta -pi nene, Delta (3)-carene). A production rate of very low volatile oxidation p roducts (e.g., multifunctional carboxylic) from . OH- and O-3-reaction of m onoterpenes of about 1.3 (.) 10(4) molecules cm(-3) s(-1) was estimated for daylight conditions during summer time. Additionally, model calculations w ith the one-dimensional multilayer model CACHE were carried out to investig ate the diurnal course of BVOC fluxes and chemical degradation of terpenes.