Mapping and Characterizing the Alaskan North Slope Methane Flux With Airborne Eddy-Covariance Flux Measurements

Abstract

Understanding and forecasting the progression of climate change imposes new observational challenges that require broadening of both spatial and temporal coverage as well as resolution. Technological advances in molecular spectroscopy have improved the precision and accuracy of greenhouse gas measurements, miniaturized their footprint, and thereby enhanced their in situ measurement capabilities.

Described here are two off-axis integrated cavity output spectroscopy instruments I developed as part of an effort to perform airborne iin situ concentration and airplane flux measurements of methane aboard the Flux Observations of Carbon on an Airborne Laboratory (FOCAL) platform. One instrument is a high temporal resolution CH4 and H2O instrument for pairing with vertical wind measurements to make eddy-covariance methane flux measurements. The other is a real time, in situ delta13C-CH4 instrument for high spatial resolution source discrimination. These ICOS-instruments were deployed in August, 2013 over the Alaskan North Slope (ANS) to investigate spatial heterogeneity of methane emissions and demonstrate the capability of making direct methane flux measurements from an aircraft. The turbulent boundary layer height over the ANS typically ranged between 100 to 200-ft, and observations were made in the turbulent boundary layer to ensure the highest fidelity of the flux observations with ground base processes.

The results of the concentration observations during the August 2013 FOCAL campaign indicate that there is a large dynamic range in methane concentration values observed (1.8-2.5ppm), and most of this variability occurs near the surface. Methane flux observations spanned 10-151 mg CH4m-2day-1, which is in the range of other ground based methane flux observations, but larger than published flux estimates for the Alaskan North Slope using aircraft observations and inverse modelling. In situ delta13C-CH4 measurements indicate that the ANS is an open system with multiple sources and sinks of methane. Therefore more tracers (delta D-CH_4, CO, O_3, etc.) would be necessary to better constrain sources and sinks.

Ground based eddy covariance flux tower observations used for validation of the aircraft fluxes produced 2 summers of methane and carbon dioxide fluxes, and provided a critical role in the aircraft flux observation campaign. A time series and carbon balance analysis of the tower flux data suggests conditions leading to warming of the soil during dark or low light conditions are the most likely to tip the soil carbon balance in favor of large scale methane release, although their greatest contribution to warming is likely to occur over the next two decades compared to the next century.