New low-power and light-weight solutions for fast measurements of CO2 and H2O ecosystem exchange

Background/Question/Methods   Closed-path and open-path configurations of the fast gas analyzers are well-established and widely utilized for measurements of ecosystem fluxes. Each configuration has advantages and deficiencies. Open-path analyzers have excellent frequency response, long-term stability, and low sensitivity to window contamination. They are pump-free and require infrequent calibrations. Yet they are susceptible to data loss during precipitation and icing, and may need surface heat flux correction in extremely cold conditions. Closed-path analyzers can collect data during precipitation, can be climate-controlled, and are not susceptible to surface heating. Yet they experience significant frequency loss in long intake tubes, especially problematic for H2O flux. They may also require periodic calibrations and powerful pump. In this presentation two instruments utilizing novel designs are discussed. One is LI-7200, a new enclosed short-tube enabled design, and the other one is LI-7500A, a modified open-path low temperature controlled design. These are compared to the traditional approaches in terms of field performance for Eddy Covariance flux measurements.

Results/Conclusions   Four prototypes of LI-7200 were extensively field-tested in 2006-2010. Instantaneous temperature fluctuations were attenuated, on average, by about 90-95% with 1 m intake tube, minimizing Webb-Pearman-Leuning term. The remainder was measured directly. Fast temperature and pressure measured inside the cell of LI-7200, and low sensitivity to window contamination allowed for short intake tubes (0.5-1.0 m or less), leading to low power demand for the entire system. New LI-7200 gas analyzer utilized strengths of both open-path and closed-path designs. Similar to the closed-path analyzers, it has minimal data loss during precipitation/icing, and does not have surface heating issues. Similar to the open-path analyzers, the LI-7200 has good frequency response due to small flux attenuation loss in short intakes, does not need frequent calibration, requires minimal maintenance and low power. Two prototypes of LI-7500A were field-tested in 2009-2010. Two regiments of the temperature control for internal electronics were examined: (i) the traditional control at about 30C, and (ii) new control at 5C. Both external heat dissipation and the system power demand were significantly reduced when 5C regiment was activated under extremely cold conditions. The availability of a new low-temperature control improved the performance of the open-path analyzer in extremely cold conditions, which allows continued and expanded use of this ultimately lowest-power remote solution for fast measurements of CO2 and H2O. Both new designs provide new and improved tools for ecological community to measure and study the ecosystem exchange of CO2 and H2O.