The effect of water vapour broadening on methane eddy correlation flux measurements

Abstract

A high resolution tunable diode laser absorption spectrometer (TDLAS) was used to measure the broadening effect of water vapor and other gases (dry air, nitrogen, oxygen, hydrogen and helium) on three methane lines in the v4 fundamental. The effects on methane eddy correlation flux measurements amount to a few percent for the least broadened line for expected H20 fluxes, to ~-10% for the most broadened line for higher H20 and lower CH 4 fluxes likely to be encountered. The broadening coefficients of methane measured for air, N 2, 02, and He are in good agreement with recently published values.

Introduction of Water‐Vapor Broadening Parameters and Their Temperature‐Dependent Exponents Into the HITRAN Database: Part I—CO2, N2O, CO, CH4, O2, NH3, and H2S

“…γ H2O for methane. (a) Black circles represent the water‐vapor broadening Lorentz half‐widths of CH 4 in the mid‐infrared and near‐infrared region by (Delahaye et al, ) with the red line for the fourth‐order polynomial fitting (not used in this work), blue triangles from the ν 4 P(5), P(9), and P(10) manifolds by Lübken et al (), and the 2ν 3 R(3) and R(4) manifolds in green diamonds by Gharavi and Buckley (). (b) The ratio of the water‐vapor broadening half‐widths of methane transitions to their corresponding air‐broadening half‐widths in HITRAN, as well as the average value of the those ratios plotted here.…”

“…In that study 76 ro‐vibrational transitions were measured. Prior to that, only the 2ν 3 R(3) and R(4) manifolds located in the 1.6‐μm region were studied from 316 to 580 K by diode laser absorption spectroscopy in (Gharavi & Buckley, ), and the ν 4 P(5), P(9), and P(10) manifolds located in the 8‐μm region were measured by Tunable Diode Laser Absorption Spectrometer at room temperature in Lübken et al ().…”

“…Since the rotational dependence of the water‐vapor broadening effects (including the effect of rotational symmetry) for methane is still not quite clear based on our analysis of the available measurements, the water‐vapor broadening coefficients at room temperature were then generated from the scaled value of their corresponding air‐broadening parameters. The scaling factor of 1.36 (with a standard deviation of 0.10) was used based on the measurements from Delahaye et al (), Lübken et al (), and Gharavi and Buckley () to their corresponding air‐broadening parameters in HITRAN. As for their temperature‐dependent exponents, the scaling factor of 1.26 to the air‐broadening temperature‐dependent exponents was used as well, which also came from the experimental measurement of the 2ν 3 R(3) and R(4) manifolds in Gharavi and Buckley ().…”