Air-land exchanges of CO2, CH4 and N2O measured by FTIR spectrometry and micrometeorological techniques

Micrometeorological flux-gradient and nocturnal boundary layer methods were combined with Fourier transform infrared (FTIR) spectroscopy for high-precision trace gas analysis to measure fluxes of the trace gases CO₂, CH₄ and N₂O between agricultural fields and the atmosphere. The FTIR measurements were fully automated and routinely obtained a precision of 0.1-0.2% for several weeks during a measurement campaign in October 1995. In flux-gradient measurements, vertical profiles of the trace gases were measured every 30min from the ground to 22m. When combined with independent micrometeorological measurements of water vapour fluxes, trace gas fluxes from the underlying surface could be determined. In the nocturnal boundary layer method the rate of change in mass storage in the 0-22m layer was combined with fluxes measured at 22m to estimate surface fluxes. Daytime fluxes for CO₂ were -0.78±0.40 (1σ) mg CO₂ m^-2s^-1. Daytime fluxes of N₂O and CH₄ were very small and difficult to measure reliably using the flux-gradient technique, despite the high precision of the concentration measurements. Mean daytime flux for N₂O was 17±48ng Nm^-2s^-1, while the corresponding flux for CH₄ was 47±410ng CH₄m^-2s^-1. The mean nighttime flux of CO₂ estimated using the nocturnal boundary layer method was +0.15±0.05mg CO₂m^-2s^-1, in good agreement with chamber measurements of respiration rates. Nighttime fluxes of CH₄ and N₂O from the nocturnal boundary layer method were 109±69ng CH₄ m^-2s^-1 and 2±3.2ng Nm^-2s^-1, respectively, in good agreement with chamber measurements and inventory estimates based on the sheep and cattle stocking rates in the region. The suitability of FTIR-based methods for long term monitoring of spatially and temporally averaged flux measurements is discussed.