Disjunct eddy covariance technique for trace gas flux measurements

A new approach for eddy covariance flux measurements is developed and applied for trace gas fluxes in the atmospheric surface layer. In disjunct eddy covariance technique, quick samples with a relatively long time interval between them are taken instead of continuously sampling air. This subset of the time series together with vertical wind velocity data at corresponding sampling times can be correlated to give a flux. The disjunct eddy sampling gives more time to analyze the trace gas concentrations and thus makes eddy covariance measurements possible using slower sensors. In this study a proton-transfer-reaction mass spectrometer with response time of about 1 second was used with a disjunct eddy sampler to measure fluxes of volatile organic compounds from an alfalfa field. The measured daytime maximum methanol fluxes ranged from 1 mg m(-2) h(-1) from uncut alfalfa to 8 mg m(-2) h(-1) from freshly cut alfalfa. Night-time fluxes were around zero.