Design, construction and operation of flux measurement systems using the conditional sampling technique

The design, construction and field operation of computer-based systems for the measurement of trace gas fluxes using the conditional sampling technique are described. A simple system which sampled air into sampling bags was used to measure CH4 and N2O fluxes from peatland and agricultural land, respectively. The system was subsequently automated by making real-time measurements of the gas mixing ratios in the sampling lines thus providing continuous measurements for periods of up to several days. Minor modifications enabled measurement of total hydrocarbons, CO2 and non-methane hydrocarbon fluxes from a forest ecosystem. Intercomparison of conditional sampling with other techniques (eddy covariance, gradient and aircraft boundary layer budget) was encouraging with good agreement between flux measurements of CH4, CO2, N2O and sensible heat. The system has been developed in a sufficiently simple and robust manner to enable extended field measurements. A number of theoretical problems remain including the absolute accuracy of the gas analysis procedures and real-time coordinate rotation procedures to deal with non-uniform terrain.