Response 1:
I'm Dave Billesbach at the University of Nebraska. I was the CH4 instrument manager for Shashi Verma's group back in the 1990 when we were looking at CH4 fluxes from wetlands. The technology that you would be using is some version of TDLAS (tunable diode laser absorption spectroscopy). Back then, there were only a couple of options, and there really aren't too many more now (YET!). Using current technology, you will be doing "closed path" measurements and will need to provide an appropriate shelter for the TDLAS instrument. Also, because the current crop of instruments are all closed path, you will need to pump sample air at an appropriate rate (and pressure) from a point near your sonic anemometer to the instrument in the shelter. In short, the physical setup is almost the same as if you were measuring CO2 fluxes with a closed path IRGA (i.e. the LiCor LI-7000 or LI-6262). One of the challenges is processing the data. Since CH4 fluxes are usually so small, they can easily be overwhelmed by WPL terms. These must be very carefully calculated or they must be eliminated by thermally equilibrating and drying your sample stream. If you have done closed-path CO2 fluxes, the CH4 problem will be familiar to you. As for the cost.... you're right that it is more expensive. An open path CO2 sensor like the LiCor LI-7500 can be purchased for about $14,400 USD, however a CH4 analyzer will cost you a minimum of $30,000 USD and can be much more expensive. We bought one of the first Campbell TGAS sensors, and it was quite expensive. Also some of the cheaper options for TDLAS systems might require the use of liquid nitrogen to cool the laser and optical detectors, although people are moving away from these systems to others that use lasers and detectors that run at warmer temperatures. Good luck with your project
Response 2:
This paper describes the use of the los Gatos instrument in an eddy covariance setup. A compact and stable eddy covariance set-up for methane measurements using off-axis integrated cavity output spectroscopy D. M. D. Hendriks, A. J. Dolman, M. K. van der Molen, and J. van Huissteden Atmos. Chem. Phys., 8, 431-443, 2008. The paper is freely available from the web site of ACP. Regards, Han Dolman