Development of Optical Sensors for Assessment of Greenhouse Gas Concentrations Above Thawing Permafrost at the Bonanza Creek Experimental Forest near Fairbanks, Alaska

Beyond anthropogenic carbon emissions, the increase in atmospheric carbon from natural feedbacks, such as thawing permafrost, poses a risk to the global climate as global temperatures continue to increase. Permafrost, formally defined as soil that is continuously frozen for 24 consecutive months, comprises nearly twenty-five percent of the Earth's terrestrial surface and possesses twice the amount of carbon currently in the atmosphere. Continuous collection of carbon dioxide (CO₂) and methane (CH₄) concentrations is imperative in understanding seasonal and inter-annual variability of carbon feedbacks above thawing permafrost. A multi-year collaborative effort was undertaken to monitor these feedbacks near Fairbanks, Alaska.

Open-path tunable diode laser absorption spectrometers were developed and deployed for measurement of CO₂ and CH₄ concentrations above thawing permafrost at a young thermokarst collapse scar bog and a rich fen site. The open-path instrument (OPI) is a relatively inexpensive, low-power sensor that collects spatially-integrated measurements of target molecules approximately 1.5 meters above ground level. The instrument sweeps across an absorption feature at 500 Hz and reports an averaged spectrum every 10 seconds. The OPI achieves 6 ppm precision for measurements of CO₂, and 10s of ppb for CH₄, over a 2.5 minute reporting interval. Here we report on initial retrieval of diurnal cycles from each field site and compare our spatially-integrated measurements of CO₂ and CH₄. For CO₂, the magnitude of the diurnal cycles show a strong dependence on daily weather at both field sites. CH₄ diurnal cycles show short-term enhancements overnight compared to CO₂. We also present an auto-alignment scheme for the OPI to mitigate effects from ground-surface instability and hysteresis present in the alt-azi motor mount.

These laser measurements are complemented by point measurements of CO₂ via non-dispersive infrared (NDIR) sensors combined with temperature, pressure, and humidity measurements along the laser's optical path. The point-based sensors (referred to as "LuftSinn" sensors) collect measurements approximately 0.6 meters above the surface. Reported CO₂ concentrations from LuftSinn sensors show good agreement with OPI measurements.