Measurement of trace gas fluxes using tunable diode laser spectroscopy

Abstract

A fast-response, high precision, tunable diode laser spectrometer has been developed for field measurements of methane and nitrous oxide fluxes using the eddy correlation method. The instrument uses a novel multiple-pass absorption cell with astigmatic mirrors to provide a long absorption path length (36 m) in a small volume (0.3 l). The combination permits rapid response with sufficient sensitivity for eddy correlation flux measurements over a wide range of meteorological conditions. Signal processing uses a least-squares fitting algorithm to determine absolute trace gas concentrations directly from the absorption spectra using spectral line parameters at data rates up to 20 Hz. The precision for methane and nitrous oxide is 0.1% of ambient levels with a 1 s averaging time. Extensions of the method to measurements of surface-reactive gases such as nitric acid and ammonia are possible using rapid sampling techniques to minimize surface interactions.

A compact and stable eddy covariance set-up for methane measurements using off-axis integrated cavity output spectroscopy

“…A field technique for eddy covariance using a multipass absorption cell was introduced in 1995 (Zahniser et al, 1995). Unfortunately, serious problems of drift and low sensitivity effects occurred.…”

“…The limiting factor of the maximum sampling rate is often τ , which could be determined by the volume of the measurement cell (0.55×10 −3 m 3 ) divided by the actual pumping speed (5.50×10 −3 m 3 s −1 ) giving a flow response of 0.10 s. Additionally, τ was determined by applying a step change in concentrations at 20 Hz sampling rate (Fig. 5) (Moore, 1986;Zahniser et al, 1995;Nelson et al, 2004). Each data point was the average mixing ratio of multiple step change events at a certain t (time elapsed since step change in concentration).…”

A Widely Tunable Infrared Laser Spectrometer for Measurements of Isotopic Ratios of Carbon Cycle Gases

“…measurements for both mapping atmospheric concentration distributions and for micrometerological or tracer ratio measurements of surface fluxes (e.g. [Poldolski and Lowenstein, 1993;Webster et al, 1994;Weinhold et al, 1994;Lamb et al, 1995;Zahniser et al, 1995;Güllük et al, 1997;Nelson, 1998;Weinhold et al, 1998;Mosher et al, 1999]). In our laboratory at Aerodyne Research, Inc. (ARI), we have extensive experience in fielding both tunable lead salt diode laser systems (e.g.…”

A New Laser Based Approach for Measuring Atmospheric Greenhouse Gases

“…Although several other open path techniques exist [7][8][9][10][11][12][13][14], LAnTeRN is a different implementation which simultaneously transmits and receives both the on and the offline wavelengths. This key difference makes many of the noise sources including scintillation common to both wavelengths and since this is a ratiometric measurement they cancel in the ratio.…”