Ground-Level Concentrations of Atmospheric Methane in Southwest Greenland Evaluated Using Open-Path Laser Spectroscopy and Cavity-Enhanced Absorption Spectroscopy

Abstract

Portable open-path laser (OPL) instruments provide a way to measure real-time, in situ atmospheric methane (CH4) concentrations at a variety of spatial and temporal scales. Our results suggest that OPL instruments may be simple and effective tools for monitoring atmospheric CH4 dynamics near ground-level in Arctic landscapes. We used a CH4 detecting OPL instrument at seven different field sites in southwest Greenland during the summer of 2012 to monitor atmospheric CH4 concentrations in the presence of local CH4 sources and sinks. Measurements with the OPL were conducted for deployments between 0.4 and 40 h over path lengths between 10 and 300 m. These measurements were checked against periodic grab samples analyzed in the field lab using cavity-enhanced absorption spectroscopy. Mean concentrations of atmospheric CH4 measured with the OPL across all field sites ranged from 1.72 (±0.12) to 1.88 (±0.15) ppm by volume (ppmv). The median atmospheric CH4 concentration measured using cavity-enhanced absorption spectroscopy was 1.82 ± 0.04 ppmv. Slightly elevated CH4 concentrations were measured downwind of one lake and in a subglacial cave. OPL measurements of atmospheric CH4 concentrations agreed with results from cavity-enhanced absorption spectroscopy (<3% difference). OPL technology may be a useful tool for extended CH4 monitoring studies in the Arctic and may help in our understanding of atmospheric CH4 concentrations at spatial and temporal scales not accessible to other forms of measurement.