Data Processing for Making Eddy Covariance Methane Flux Measurement with Laser-Based CH4 Gas Analyzer

Abstract:

First we will discuss the fundamental difference in the theory of operation between NDIR (Non-dispersive Infrared) based and laser-based gas analyzer. Taking LI-7500A (an open-path CO₂ gas analyzer) as an example for a NDIR-based gas analyzer, the wavelength of the infrared radiation for making the gas concentration measurement is from 4.20 to 4.34 μm which includes many absorption lines. While the LI-7700 (an open-path methane gas analyzer) is a laser-based analyzer. It uses a single absorption line at 1.651 μm to make the methane concentration measurement. It employs a Herriott cell configuration with mirror spacing of 0.47 m and a total optical path length of 28.2 m. Methane density is measured using wavelength modulation spectroscopy. As a result, the measured methane density is affected by sensible heat and latent heat flux, and also by spectroscopic effects (e.g., line broadening) due to changes in temperature and water vapor content. Here we propose a new procedure to account for spectroscopic effects. Since both density effects and spectroscopic effects are predictable with the ideal gas law and HITRAN respectively, the spectroscopic effect can be incorporated into WPL correction. In this paper, we will discuss the details of this new procedure to account for the spectroscopic effect in the methane flux calculation. Field experiment results will be presented to show the accuracy of this new procedure.