Biogenic carbonyl compounds within and above a coniferous forest in Germany

Diurnal mixing ratios of aldehydes and ketones were investigated during two joint experiments in summer months to identify biogenic contributions from coniferous forests to tropospheric chemistry. In a Norway spruce forest, the diurnal variation of carbonyl compounds was measured at 12 m (in the treetop) and at 24 m (above the canopy). The main findings of the experiment are that acetone (up to 9.1 ppbv), formaldehyde (up to 6.5 ppbv), acetaldehyde (up to 5.5 ppbv) and methyl ethyl ketone (MEK, up to 1.8 ppbv) were found in highest concentrations. For all major compounds with the exception of MEK, primary emissions are supposed. From α-pinene oxidation, pinonaldehyde was found with its peak concentrations (up to 0.15 ppbv) during the early morning hours. The diurnal variation of concentrations for most other compounds shows maximum concentrations near midday in 2,4-dinitrophenylhydrazine (DNPH) measurements but not for proton-transfer reaction mass spectrometry (PTR-MS) measurements of acetaldehyde and acetone. A clear correlation of carbonyl compound concentration to the radiation intensity and the temperature (R2=0.66) was found. However, formaldehyde did not show distinct diurnal variations. A very high correlation was observed for both heights between mixing ratios of acetaldehyde and acetone (R2=0.84), acetone and MEK (R2=0.90) as well as acetaldehyde and MEK (R2=0.88) but not for formaldehyde and the others. For the most time, the observed carbonyl compound concentrations above the canopy are higher than within the forest stand. This indicates an additional secondary formation in the atmosphere above the forest. The differences of acetone and acetaldehyde mixing ratios detected by DNPH technique and the PTR-MS could not be fully clarified by a laboratory intercomparison.