A laser‐based open‐path analyzer with low-temperature corrections for eddy covariance CH4 flux measurements

This work presents an open-path methane (CH₄) analyzer (Model HT8600, HealthyPhoton Co., Ltd.) suitable for eddy covariance (EC) flux measurements based on the tunable diode laser absorption spectroscopy (TDLAS) technology. As discussed in the previous literature, EC flux measurements based on open-path analyzers are subject to temperature-correlated corrections, including the density and spectroscopic effects. The HT8600 utilizes an interband cascade laser (ICL) to probe the mid-infrared transition of CH₄ at ~3221.1 nm. The chosen absorption peak has the advantage that the density and spectroscopic effects compensate for each other, resulting in low temperature-related corrections in EC flux measurements [1].

 

The HT8600 has a weight of ~15 kg and dimensions of 84 cm (length) and 20 cm (diameter). Multiple laser beam reflections enable an optical path length of 17 m. The total power consumption is 30 W, which lithium batteries can supply for continuous measurements. Laboratory experiments showed that the HT8600 has a noise level of 1.36 ppbv at a 10-Hz data rate. A long-term field experiment is ongoing to compare the performance of the HT8600 against another commercial open-path CH₄ analyzer. We used the fluxes measured by the commercial analyzer with complete temperature corrections as the reference. Preliminary results showed that raw fluxes measured by the HT8600 achieved high consistency with the one from the commercial analyzer, which proved our initiative that the chosen absorption line is subject to low-temperature biases.

 

As the inter-comparison is ongoing, we are collecting more field results for formal analysis of the performance of HT8600 under a wide dynamic range of temperatures. Detailed analysis of the correction factors will be presented at the conference. Meanwhile, as there is a nearby absorption line of water (H₂O) at 3222.7 nm, the work extends the analyzer’s capability of measuring H₂O and CH₄ simultaneously, enhancing its versatility for field CH₄ flux monitoring.

Reference

• Pan, D., Gelfand, I., Tao, L., Abraha, M., Sun, K., Guo, X., Chen, J., Robertson, G. P., andZondlo, M. A. (2022). A new open-path eddy covariance method for nitrous oxide and other trace gases that minimizes temperature corrections. Global Change Biology, 28, 1446– 1457. Doi: https://doi.org/10.1111/gcb.15986.