COS AND H2S FLUXES OVER A WET MEADOW IN RELATION TO PHOTOSYNTHETIC ACTIVITY - AN ANALYSIS OF MEASUREMENTS MADE ON 6 SEPTEMBER 1990

Micrometeorological measurements of the fluxes of carbonyl sulfide (COS) and hydrogen sulfide (H2S) were performed over wet meadow vegetation in southern Germany during September 1990. This experiment was conducted in order to verify that vegetation is an important sink for atmospheric COS. A cryo-sampling technique for continuously collecting these trace gases was applied at four different levels above the ground, combined with eddy correlation measurement of the meteorological fluxes and a micrometeorological profile technique. From these data the trace gas fluxes were determined, based on the assumption that chemical constituents are transported in a similar fashion to meteorological parameters. The observed diel variation of the COS and H2S fluxes exhibits peak values of (-360+/-130) ng S(COS) m-2 s-1 (deposition) and (+6.7 +/-2.4) ng S(H2S) m-2 s-1 (emission). Using a modified budget model which includes horizontal advection, it is shown that the observed mean profile of COS is consistent with the assumption of a sink for COS at the canopy level. The observed strong influence of light intensity fluctuations on the fluxes is explained by variations in the bulk Richardson stability number of the atmospheric boundary layer (ABL), which are the result of equally strong variability of the buoyancy-generated turbulent transport. The chemical potential gradients of COS and CO2 in the canopy layer indicate that the COS flux observed in the ABL is related to the COS nonequilibrium distribution across the canopy. COS deposition is interpreted on the basis of COS consumption by the carboxylating plant enzymes. Further, the light-driven H2O emission supports the linkage between trace gas flux and plant photosynthetic CO2 assimilation. Finally, the observed H2S emission may be explained by soil biochemistry.