Using FTIR to Measure Greenhouse Gas Emissions from Agricultural Sources.

Inventory improvement and the development of regulatory measures, trading options and mitigation strategies for greenhouse gas emissions all require precise measurements of atmospheric concentrations and fluxes of greenhouse gases.  The main emissions from agriculture are those of CH₄, N₂O, CO₂ and the indirect greenhouse gas NH₃.  Fourier Transform Infrared (FTIR) Spectroscopy identifies and quantifies concentrations of infrared absorbing gases by examining transmission spectra from a broad-band infrared source and using a non-linear least-squares algorithm that iteratively calculates the spectrum from a database of spectral line parameters until best fit to the recorded spectrum is obtained.  The relative precision (1std) of the measurements is 0.1–0.2%, giving 1std errors in CH₄, N₂O, CO₂ and NH₃ measurements of around 4 ppb, 0.5ppb, 1ppm and 0.5ppb respectively.  Attractions of FTIR spectroscopy are that it can measure the concentrations of all the infrared gases of interest simultaneously and it can be employed in both closed- and open-path systems.  Closed path systems are used for chamber, flux-gradient and backward Lagrangian stochastic dispersion (bLs) techniques of flux measurement and open-path systems are employed for measurement of fluxes from small, well-defined source areas such as feedlots, fertilised fields and grazed pastures, using the bLs technique.  The use of FTIR spectroscopy in both systems will be discussed.