In-path heat exchange affects open-path CO2 flux measurements

1. INTRODUCTION

Open-path infrared gas analyzers are widely used around the world for measuring CO2 exchange. There have been many comparisons between open-path and closed-path sensors demonstrating substantially similar CO2 flux measurements. However, there is growing evidence that shows differences between open-path and closed-path measurements, especially in the form of apparent off-season CO2 uptake when it is physiologically unreasonable to expect it. Measurements made with closed-path analyzers, chambers, and gradient methods have consistently shown small releases of CO2. To address this issue, laboratory and field experiments were conducted to examine the influence of instrument surface temperature and heat exchange on the open-path CO2 fluxes.

2. APPROACH

The conceptual framework (Burba et al. 2005a,b; 2006 a,b) is fairly straightforward: if the instrument surface temperature is different from the ambient temperature (i.e., measured outside the instrument open path), it could cause sensible heat fluxes inside the open-path cell to be different from the ambient, thus affecting CO2 densities. This phenomenon should be accounted for in Webb-Peraman-Leuning term (WPL, Webb et al. 1980). With these issues in mind, this investigation is focused on the following specific questions:

i) Is the instrument surface temperature significantly warmer or colder as compared to the ambient air?

ii) Does the resulting instrument surface heat exchange correlate with the vertical wind speed to produce a sensible heat flux inside the path which is different from the ambient sensible heat flux?

iii) Can the effect of this heat exchange be eliminated by enclosing an open-path gas analyzer?

iv) Can a sensible heat flux measured directly inside the path be used in WPL to correct unreasonable open-path fluxes to match closed-path fluxes?

v) Can the instrument surface heat exchange be estimated from co-located meteorological data to correct previously measured open-path fluxes?

vi) What are the consequences of assuming the instrument surface heating effect is negligible?

4. RESULTS AND DISCUSSION

Instrument surface heating and its effect on open-path CO2 fluxes were tested in laboratory and field experiments covering four ecosystems: ryegrass, forest clear-cut, maize, and soybean. The surface temperatures of the open-path infrared gas analyzer adjacent to the sampling volume were up to 6oC warmer than the ambient air during daytime due to heating from the sensor head electronics and solar loading.

The heat exchange inside the path was correlated with vertical wind speed. This produced sensible heat fluxes that were on average 14% larger than in the ambient air, at mild air temperatures. This percentage increased further with colder air temperatures and associated increase in sensor head instrument heating by on-board thermoelectric devices.

The effect of sensible heat on CO2 flux measurements was effectively eliminated by enclosing an open-path sensor. Fluxes measured with an enclosed LI-7500 matched closed-path references to within 0.3%. At the same time, the open-path LI-7500 with a traditionally computed WPL term led to a 33% underestimation of CO2 losses from the ecosystem, with a large number of apparent CO2 uptake hours over frozen and senesced ecosystem.

The sensible heat flux measured inside the open path by a fine-wire Platinum Resistance Thermometer (PRT) was used instead of the ambient sensible heat flux in the WPL term to correct open-path CO2 fluxes. Open-path fluxes computed this way matched the closed-path reference to within 4% in contrast to a 19% error by a traditional WPL term.

The heat fluxes added by sensor head instrument surfaces adjacent to the open cell volume were estimated from standard meteorological data. On a long-term basis, the use of an estimated heating correction led to a match between the open-path fluxes and the closed-path references to within 0-10% in all four tested ecosystems. As a result, underestimation of the off-season CO2 release was reduced from 36-81% to a few percent, and overestimation of the annual CO2 uptake was reduced from 14-16% to 0-1%.

5. CONCLUSIONS

Among the tested techniques the best performance was yielded by an enclosed LI-7500, followed by the fine-wire PRT technique, and by the estimated heating correction technique utilizing measured Ts, then by Ts evaluated by linear regression with Ta and by a multiple regression with weather variables. The worst performance was yielded by a traditional approach that uses solely ambient sensible heat flux in WPL term, resulting in a significant underestimate of long-term CO2 release.

REFERENCES

Burba GG, Anderson DJ., Xu L, McDermitt DK (2005a) Solving the off-season uptake problem: correcting fluxes measured with the LI-7500 for the effects of instrument surface heating. Progress report of the ongoing study. PART I: THEORY. Poster presentation. AmeriFlux 2005 Annual Meeting, Boulder, Colorado.

Burba GG, Anderson DJ., Xu L, McDermitt DK (2005b) Solving the off-season uptake problem: correcting fluxes measured with the LI-7500 for the effects of instrument surface heating. Progress report of the ongoing study. PART II: RESULTS. Poster Presentation. AmeriFlux 2005 Annual Meeting, Boulder, Colorado.

Burba GG, Anderson DJ., Xu L, McDermitt DK (2006a) Correcting apparent off-season CO2 uptake due to surface heating of an open-path gas analyzer: progress report of an ongoing experiment. Proceedings of 27th Annual Conference of Agricultural and Forest Meteorology, San Diego, California, 13 pp.

Burba GG, Anderson DJ., Xu L, McDermitt DK (2006b), Additional Term in the Webb-Pearman-Leuning Correction due to Surface Heating From an Open-Path Gas Analyzer, EOS Trans. AGU, 87(52).

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