Air–land exchanges of CO 2, CH 4 and N 2O measured by FTIR spectrometry and micrometeorological techniques

Abstract

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 CO 2, CH 4 and N 2O 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 30 min from the ground to 22 m. 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 0–22 m layer was combined with fluxes measured at 22 m to estimate surface fluxes. Daytime fluxes for CO 2 were −0.78±0.40 (1 σ) mg CO 2 m −2 s −1. Daytime fluxes of N 2O and CH 4 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 N 2O was 17±48 ng N m −2 s −1, while the corresponding flux for CH 4 was 47±410 ng CH 4 m −2 s −1. The mean nighttime flux of CO 2 estimated using the nocturnal boundary layer method was +0.15±0.05 mg CO 2 m −2 s −1, in good agreement with chamber measurements of respiration rates. Nighttime fluxes of CH 4 and N 2O from the nocturnal boundary layer method were 109±69 ng CH 4 m −2 s −1 and 2±3.2 ng N m −2 s −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.