Assessment of Sea-Air Fluxes of Methane from the East Siberian Arctic Shelf Based on Multi-Year High-Precision Observations in the Atmospheric Boundary Layer and Seawater

Abstract:

The latitude specific monthly mean (LSMM) methane (CH₄) mixing ratio in the Arctic region atmospheric boundary layer (ABL) is established at 1.85 ppm (monitoring station at Barrow, Alaska, USA). This LSMM is about 10% higher than in Antarctica and about 5-7% higher than in the mid-latitudes, where the majority of anthropogenic activity occurs. So far, there is no reasonable explanation for the atmospheric CH₄ maximum above the Arctic and no models have succeeded in satisfactorily reproducing the inter-polar gradient, due to the assumption that there are no Arctic region CH₄ sources sufficient to serve year-round to maintain the existing maximum. The East Siberian Arctic Shelf (ESAS), from which extensive CH₄ release was recently reported, has never been considered to be an atmospheric source of CH₄, because no knowledge existed about this area until very recently. One focus of our multi-year ESAS investigations was to collect a comprehensive data set of continuous ABL CH₄ mixing ratio measurements along with continuous measurements of dissolved CH₄ in the surface water. When collecting the data set, we aimed to assess CH₄ fluxes by using top-down and bottom-up approaches. Achieving representative spatial coverage of the study area with high-resolution, high-accuracy measurements was our priority. We collected data from 2006-2012 with a high-accuracy fast CH₄ analyzer; two sonic anemometers measured the 3D wind vector and sonic temperature; a meteorological station measured wind speed and direction, moisture, and temperature; and an open path infrared gas analyzer measured H₂O and CO₂. Other instruments included a pressure transducer and a motion package that measured 6 components of ship’s motion and also acceleration, magnetic field, and position. In 2011 and 2012 we performed such measurements in the outer ESAS seepage area. Data were synchronized in time and space to demonstrate correspondence of increased atmospheric CH₄ levels (up to 3.2 ppm) to observed images of CH₄ gas flares and measured levels of dissolved CH₄ in the water column. However, these data cannot be considered conclusive for flux calculations until increased background levels of Arctic atmospheric CH₄ are established as Arctic source-induced.