Vertical distribution of carbonyl sulfide in a eucalyptus forest

Vertical profiles of carbonyl sulfide (COS) have been obtained in a eucalyptus forest in Portugal within the first 20 m above ground. Significantly lower mixing ratios were observed below the canopy level than above, indicating a net absorption of COS from the eucalyptus trees. The ratio between mean COS concentrations below and above the canopy was 0.86 in mid-afternoon, whereas in the morning and at midnight this ratio increased up to 0.97. COS mixing ratios were by more than 100 pptv higher above the canopy than below it in mid-afternoon, whereas in the morning and at midnight the difference never exceeded 25 pptv. At all levels COS presented a clear diurnal cycle with highest values around mid-afternoon. Experiments conducted in parallel with the enclosure chamber technique confirm the COS absorption from the eucalyptus trees. COS uptake reached its highest values in mid-afternoon, when photosynthetic active radiation (PAR) was maximum.