Early results from the development of a miniature tunable diode laser gas cell for measuring CO2 isotopologue fluxes in situ

In order to accurately predict how ecosystems will respond to climate change, it is necessary to separate the response of respiration and photosynthetic uptake individually to environmental conditions. Currently, the net ecosystem exchange of CO₂ is measured continuously at various ecosystems around the world. Net CO­₂ flux can be partitioned into the primary components using either models or measurements of ¹³C/¹²C in the CO₂ flux. We introduce recent technological developments toward in situ, rapid, continuous measurements of fluxes of ¹³CO₂ and ¹²CO₂.

We describe a unique approach to achieving 10Hz measurements of CO₂ using tunable diode laser gas absorption spectroscopy in a multi-pass White cell capable of being deployed directly to a canopy. We will first discuss proof-of-concept characterization of the technique using wave modulation spectroscopy with a laser tuned to detect ¹²CO₂ fluxes_. We show the sensitivity of the 2w component of a wave-modulated signal to CO₂ concentration, the precision, and the accuracy of the sensor as well as the stability of the sensor under normal ranges of ambient temperature and humidity in an environmental chamber. We then show preliminary results of sensor performance with a laser tuned to measure ¹³CO₂ and ¹²CO₂ fluxes. We discuss our approach to reliably measuring multiple peaks of gas absorption while maintaining the rapid sampling rates necessary for flux calculations. We will also discuss considerations for extending the sensor from the lab to being directly deployed into a canopy for in situ measurements.

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS- 658355