Fine-wire thermometer to correct CO2 fluxes by open-path analyzers for artificial density fluctuations

A fine-wire thermometer was applied to measure high-frequency air temperature fluctuations averaged over the length of the optical path of an LI-7500 open-path gas analyzer that was used for CO2 flux measurements over a forest clear-cut in Sweden. Heat fluxes derived by the fine-wire temperature data were used to correct the CO2 fluxes for density fluctuations within the optical path. As this method incorporates both natural heat fluxes, accounted for in the conventional Webb–Pearman–Leuning term, and heat fluxes caused by analyzer surface heating, it provides a means to minimize or eliminate the bias in open-path fluxes that could lead to apparent CO2-uptake in ecosystems during the off-season period, and to a general underestimation of carbon losses in other periods. The proposed technique was compared to two other methods: the conventional WPL-correction and a theoretical correction for the instrument surface heating with empirically estimated instrument surface temperatures. All methods were referenced to a closed-path LI-6262 system because high frequency temperature fluctuations in that system were attenuated by the long intake tube. Measurements demonstrated that the fine-wire correction yielded integrated fluxes that were within 4% of the LI-6262-derived fluxes. The theoretical correction with empirically estimated temperatures led to integrated fluxes being within 6% of those measured using the LI-6262. For the same time period, the conventional WPL-correction underestimated CO2 losses by 19%. During another period with external heating applied to the LI-7500, the conventional WPL-correction led to an underestimation of the CO2 efflux by 66%, while the PRT-correction yielded fluxes within 8% of the LI-6262. The experiment confirmed that the bias in the open-path CO2 flux measurements is mostly caused by instrument surface heating, and that such heat should be accounted for in the density corrections. The experiment also demonstrated that a fine-wire thermometer provided a reliable instrumental solution for the open-path heating phenomenon, and could give adequate corrections on a half-hourly and seasonal basis even when external heat was applied to the open-path analyzer.