Development of Optical Sensors for Assessment of Greenhouse Gas Concentrations Above Thawing Permafrost
Open AccessBeyond 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 at the Bonanza Creek Long Term Ecological Research Site for measurement of CO2 and CH4 concentrations. 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. 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. Retrievals of CO2 and CH4 concentrations are reported here for a multi-year study at a young thermokarst collapse scar bog and an exploratory campaign in a rich fen site. Observed diurnal cycles are shown to be highly dependent on local weather conditions.
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