EDDY COVARIANCE MEASUREMENT OF ISOPRENE FLUXES

A system has been developed to directly measure isoprene flux above a forest canopy by eddy covariance using the combination of a fast respo nse, real-time isoprene sensor and sonic anemometer. This system is su itable for making nearly unattended, long-term, and continuous measure ments of isoprene fluxes. Isoprene detection is based on chemiluminesc ence between isoprene and reactant ozone, which produces green light a t 500 nm. The sensor has a noise level (1 sigma) of 450 pptv for a 1-s integration which is dominated by random high-frequency noise that do es not significantly degrade eddy covariance flux measurements. Interf erence from the flux of other compounds is primarily due to the emissi on of monoterpenes, propene, ethene, and methyl butenol and the deposi tion of methacrolein and methyl vinyl ketone. The average total interf erence for North American landscapes in midday summer is estimated to be about 5% for emissions and -3% for deposition fluxes. In only a few North American landscapes, where isoprene emissions are very low and methyl butenol emissions are high, are interferences predicted to be s ignificant. The system was field tested on a tower above a mixed decid uous forest canopy (Duke Forest, North Carolina, U.S.A.) dominated by oak trees, which are strong isoprene emitters. Isoprene fluxes were es timated for 307 half-hour sampling periods over 10 days. Daytime fluxe s ranging from 1 to 14 mg C m(-2) h(-1) were strongly correlated with light and temperature. The daytime mean flux of 6 mg C m(-2) h(-1) is similar to previous estimates determined by relaxed eddy accumulation by Geron ed al. [1997] at this site. Nighttime fluxes were near zero ( 0.01 +/- 0.03 mg C m(-2) h(-1)).