Evaluation of the Tunable Diode Laser Trace Gas Analyzer System for Eddy Correlation Measurements of Methane

The development of the tunable diode laser trace gas analyzer system (TGA) has allowed in situ measurements of nitrous oxide (N2O), methane (CH4), and ammonia (NH3). The micrometeorological set-up has relied on either a mass balance or flux gradient approach. Although the data and results have proven valuable, the eddy correlation (EC) technique is considered a more direct and accurate estimate of trace gas fluxes. The response of the TGA to different frequencies was initially estimated using power spectral density analysis. In a laboratory setting, the noise of the TGA was determined by sampling air out of a barrel to ensure zero concentration fluctuations. Vertical wind fluctuations were estimated under a variety of atmospheric conditions using the neutral Kansas spectra. These data were combined with the variance of the TGA and a cospectral analysis was completed. The TGA was also subjected to large differences in CH4 concentration, ranging from ambient to upwards of 22 ppm. A random noise generator (RNG) was used to switch between the two extremes and allowed for further estimation through spectral analysis of the response of the TGA for EC measurements. Although the RNG provides the best white noise spectrum, this instrument is rarely used. In recent years, an equation estimating the standard error of the mean covariance between the concentration spectrum of the TGA and a typical vertical wind velocity spectrum has been proposed. Results from this research will provide valuable information of the EC flux detection limit of the TGA as well as ensure the validity of this proposed equation for estimating the standard error.