LI-7500 Technical Bulletin:

New Research on Off-Season CO2 Uptake Exhibited by Open-Path Analyzers in Cold Climates

LI-COR has recently conducted research to investigate an apparent uptake of CO2 during the off-season measured by open-path CO2 analyzers. Results show strong evidence that energy dissipated in the analyzer head can heat a portion of the air in the optical path, and can lead to a small reduction in the release or a small increase in the uptake of CO2. This phenomenon is manifested, at times, as an apparent CO2 uptake and may result in a systematic bias in the estimates of CO2 transport to and from the atmosphere. It is important to note that this effect is most pronounced in colder climates during the winter months, especially below -10 C, and has little impact on data collected in warmer climates.

As part of this research, a correction for the surface heating effect has been proposed and tested. Results of this correction are very promising – most of the wrong signs of off-season CO2 flux rates are eliminated, and there is a significant improvement in off-season integrations of CO2 exchange.

Burba, G., D. McDermitt, A. Grelle, D. Anderson, and L. Xu, 2008. Addressing the influence of instrument surface heat exchange on the measurements of CO2 flux from open-path gas analyzers. Global Change Biology, 14(8): 1854-1876.

Additional Term in the Webb-Pearman-Leuning Correction due to Surface Heating From an Open-Path Gas Analyzer One laboratory and two field experiments were conducted between September 2005 and September 2006 to investigate the impact of an added heat flux in the sample path of the LI-7500 CO2/H2O gas analyzer caused by the difference in temperatures between the ambient air and the surface of the instrument. Contribution of heat dissipated from the internal instrument electronics toward the instrument surface was substantial, especially in cold conditions. In the environmental chamber, surface heating ranged from about 0 °C above ambient, at air temperatures above +40 °C, to about 7 °C, at an air temperature of -25 °C. In the field, daytime temperature differences were overall smaller than in the chamber due to convective cooling by the wind and some long-wave cooling, despite the added sunlight contribution. However, considerable temperature gradients (up to 2 °C per 1mm) were still observed over the lower window of the LI-7500, suggesting strong sensible heat fluxes above the instrument surface. The nighttime situation was different due to strong long-wave cooling of some parts of the instrument, partially (and sometimes, fully) offsetting effects of the electronics heating in the other parts. The concept of an added heat flux term in the Web-Pearman-Leuning correction is revisited, and effect of the instrument surface heating on the CO2 flux measurements is examined. The proposed concept is presented in detail, along with resulted corrections to the originally computed flux. Field data are examined separately for daytime and nighttime cases, and on hourly and seasonal time scales. Significant reduction in the apparent CO2 uptake during off-season periods was observed as a result of applying correction due to the added heat, while fluxes during the growing season have not been noticeably affected. The correction also resulted in the elimination of most of the wrong signs from the off-season open- path CO2 fluxes, in considerable reduction in variability of the data, elimination of the difference between measurements made with the LI-6262 and the LI-7500, and in a significant improvement in off-season integrations of CO2 exchange. A framework was created to develop a site-specific practical correction due to instrument surface heating. The concept may provide a basis for further research in the area of instrument temperature affecting the measurement of the open-path fluxes. Proposed correction may be useful for future CO2 flux research, and it can also be applied to pre-existing data today.