Scientific questions relating to FLUXNET
Provided by: Jonas Ardö on 07/28/2010
I got 6-7 responses where all but one said it was OK to use N gas with 400 ppm CO2 for calibration of a LI-7500. The person answering that N-gas was not suitable refereed to Agricultural and Forest Meteorology 118 (2003) 1–19, (see especially Bischof and Griffith refs). I also asked LICOR and they answered: You can certainly use nitrogen gas for calibration. CO2 in air is slightly more accurate because it includes the balance of oxygen present in normal air (which can have a slight effect on the infrared response), but nitrogen gas is entirely usable and quite a few customers do use it. (The main issue was about the N-gas and not regarding the 400 ppm, it was maybe badly formulated)
Provided by: Ray Leuning on 05/13/2010
My first comment is that these sensors are not sufficiently accurate or precise to give you good profile information unless the concentration within the canopy is substantially greater than background during the periods of interest i.e. >> than 8 ppm resolution. However, these problems can be minimized by comparing the sensors at the same height over a range of temperatures to obtain relative calibrations. Such a comparison may be sufficient for evaluating the change in storage term of the mass balance.
I have just placed an order for 8 sensors for another purpose. I will be comparing their performance to an FTIR spectrometer for several weeks/months and will also do an intensive set of calibrations in the lab to determine their accuracy, precision and temperature sensitivity. I plan to publish the results in due course.
Provided by: Bai Yang, ORNL on 05/25/2009
I assume that you are trying to measure the water vapor flux (latent heat) at this shrub site.
(1) In addition to means from CSAT3 and KH20, did you also save the variances and co- variances (such as w'w', q'q' and w'q' etc; w--vertical velocity, q-- water vapor concentration)? If not, these data are hardly useful because, by definition, water vapor flux is the covariance between w and q.
(2) Using eddy covariance measurement, people always try to minimize the mean vertical velocity. In doing so, two dimensional coordinate rotation is normally performed at flux sites with a non-flat terrain. The purpose is to force the mean vertical velocity to zero. If you want to perform this 2-D coordinate rotation, you also need to save variances for u and v (u--velocity in x direction, v--velocity in y direction) and covariance among u, v, w and q. This document might be helpful, http://public.ornl.gov/ameriflux/sop.shtml.
(3) A correction on flux terms at most sites is the WPL correction (Webb EK, Pearman GI, and Leuning R,1980. Correction of flux measurements for density effects due to heat and water vapor transfer. Quarterly Journal of the Royal Meteorological Society, 106, 85-100). You need to measure sensible heat flux (w'T'; T--- air temperature) for the purpose of WPL correction.
(4) I understand you have not saved 10-hz data because of insufficient storage in CR23X. However, we often find that it is necessary to go back and re-process our flux terms from 10-Hz data at a site. In this case, it is wise to save 10-Hz data if possible. You may want to buy external memory module or memory card from Campbell Sci so that you can expand your data storage size to a few GB. Alternatively, you can use a laptop computer to collect the 10 Hz data and perform an off-line flux computation.
(5) How to use these flux data largely depends on what you want to get from these data. If you want to get the water vapor flux for the entire ecosystem, what you measure provides an answer. If you want to get water vapor flux from shrubs only (evapotranspiration from shrubs), you need to split your footprint into sectors and only use the data when wind comes from the sector fully covered with shrubs.
(6) Are you a member of ChinaFlux (http://www.chinaflux.org/index/index.asp)? Their experience and training courses might be helpful to you too.
Provided by: Bob Cook, ORNL on 2/26/2009
We have instrument information about open-path and closed path sensors at 78 towers.
Of these, 59 towers have minimum air temperature below zero centigrade; these towers have the following instruments.
29 Open Path (LI-7500)
19 Closed Path (LI-820, LI-6251, LI-6262, LI-7000,)
11 Both types
Response 1:
The paper by Yang et al. (2007, JGR, http://www.agu.org/journals/jd/jd0720/2006JD008243/2006JD008243.pdf) and discussions with others suggest that averaging storage fluxes over 30 minutes underestimates the true storage (up to 50%). It seems the cycle should be as short as physically possible given your sampling setup, preferably 5 minutes or less.
Response 2:
Provided by: Steven Wofsy on 6/19/2008
In order to do the storage measurement most efficiently and accurately, you would want to implement exponential averaging volumes (2 minute time constant) on each line. Alternatively, you can make one measurement by bringing all the tubes from the profile inlets into a single averaging volume. These setups are not too hard to do, but care is required to ensure appropriate flow rates in each tube--proportional to Delta-Z if the inlets are not evenly spaced. A good exponential averaging volume is a glass flask with a tiny muffin fan inside.
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