Flux correction for closed-path laser spectrometers without internal water vapor measurements

Abstract. Recently, instruments became available on the market that provide the possibility to perform eddy covariance flux measurements of CH₄ and many other trace gases, including the traditional CO₂ and H₂O. Most of these instruments employ laser spectroscopy, where a cross-sensitivity to H₂O is frequently observed leading to an increased dilution effect. Additionally, sorption processes at the intake tube walls modify and delay the observed H₂O signal in closed-path systems more strongly than the signal of the sampled trace gas. Thereby, a phase shift between the trace gas and H₂O fluctuations is introduced that dampens the H₂O flux observed in the sampling cell. For instruments that do not provide direct H₂O measurement in the sampling cell, transfer functions from externally measured H₂O fluxes are needed to estimate the effect of H₂O on trace gas flux measurements. The effects of cross-sensitivity and the damping are shown for an eddy covariance setup with the Fast Greenhouse Gas Analyzer (FGGA, Los Gatos Research Inc.) that measures CO₂, CH₄, and H₂O fluxes. This instrument is technically identical with the Fast Methane Analyzer (FMA, Los Gatos Research Inc.) that does not measure H₂O concentrations. Hence, we used measurements from a FGGA to derive a modified correction for the FMA accounting for dilution as well as phase shift effects in our instrumental setup. With our specific setup for eddy covariance flux measurements, the cross-sensitivity counteracts the damping effects, which compensate each other. Hence, the new correction only deviates very slightly from the traditional Webb, Pearman, and Leuning density correction, which is calculated from separate measurements of the atmospheric water vapor flux.