AUTOMATED GAS-CHROMATOGRAPHIC ANALYSIS SYSTEM FOR MICROMETEOROLOGICAL MEASUREMENTS OF TRACE GAS FLUXES

High-resolution analytical techniques are required for micrometeorological measurement of trace gas fluxes. By adding to a standard gas chromatograph (GC), a programmable autoinjector built in the laboratory at a total material cost of about Pound 5k ($8k), we were able to attain the precision necessary to detect N2O and CH4 fluxes of the order of 2 nmol m-2 s-1. Ten to fifteen repeated analyses gave resolutions around 1.0 parts per billion by volume (ppbv) for N2O and 0.5 ppbv for CH4. The two gases could be analyzed simultaneously. Fluctuations in ambient temperature limited the precision attainable, especially for N2O. Under these circumstances, analyses based on differences between successive measurements were more precise than those based on the measurements themselves. The system was used to measure N2O concentration gradients in the atmosphere above a recently fertilized grassland site in Stirling, central Scotland, in April 1992. Gas-tight bags were filled from a number of heights up a mast and returned to the laboratory for analysis; coherent gradients were discerned on more than half the sampling occasions.