Flux measurements of NH3, N2O and CO2 using dual beam FTIR spectroscopy and the flux–gradient technique

We describe the application of a dual beam Fourier transform infrared (FTIR) spectrometer and sampling system for simultaneous measurements of fluxes of several trace gases between the earthʹs surface and the atmosphere. The spectrometer is based on a commercial dual-output FTIR spectrometer with two long-path absorption cells, fully automated gas handling, data acquisition and quantitative spectrum analysis. The spectrometer may be operated in single or dual beam (optical subtraction) modes; the advantages and disadvantages of the two modes are tested and discussed. Measurements of fluxes of N2O, CO2 and NH3 from agricultural landscapes were made by the flux–gradient technique in two field trials in Sweden and Denmark in 1993. Fluxes of NH3 were determined following liquid manuring of a young wheat crop in early summer, and N2O and CO2 fluxes were measured from a recently harvested wheat stubble on an unfertilised organic soil in late summer. NH3 fluxes of more than 5 μgN m−2 s−1 (4 kg ha−1 d−1) were measured a day after fertilisation, decreasing to <0.5 μgN m−2 s−1 two days later. N2O fluxes averaged 42 ngN m−2 s−1 (36 g ha−1 d−1) over the six days of measurement and showed no significant diurnal or longer term variability. Minimum fluxes of 500 and 20 ngN m−2 s−1 for NH3 and N2O, respectively, were detectable with 20 min time resolution. The system is readily extendable to CH4 measurement. Improvement for the future should improve the minimum detectable fluxes.