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 earth's surface and the atmosphere. The spectrometer is based on a commercial dual-output FTIR spectrometer with tw o long-path absorption cells, fully automated gas handling, data acquisitio n and quantitative spectrum analysis. The spectrometer may be operated in s ingle or dual beam (optical subtraction) modes; the advantages and disadvan tages of the two modes are tested and discussed. Measurements of fluxes of N2O, CO2 and NH3 from agricultural landscapes were made by the flux-gradien t technique in two field trials in Sweden and Denmark in 1993. Fluxes of NH 3 were determined following liquid manuring of a young wheat cl-op in early summer, and N2O and CO2 fluxes were measured from a recently harvested whe at stubble on an unfertilised organic soil in late summer. NH3 fluxes of mo re than 5 mu gN m(-2) s(-1) (4 kg ha(-1) d(-1)) were measured a day after f ertilisation, decreasing to < 0.5 mu gN m(-2) s(-1) two days later. N2O flu xes averaged 42 ngN m(-2) s(-1) (36 g ha(-1) d(-1)) over the six days of me asurement and showed no significant diurnal or longer term variability. Min imum fluxes of 500 and 20 ngN m(-2) s(-1) for NH3 and N2O, respectively, we re detectable with 20 min rime resolution. The system is readily extendable to CH4 measurement. Improvement for the future should improve the minimum detectable fluxes. (C) 2000 Elsevier Science Ltd. All rights reserved.