Speculation on the origin of sub-baseline excursions of CH4 at Cape Grim

The Advanced Global Atmospheric Gases Experiment (AGAGE) program has historically measured in situ methane (CH4 ) at Cape Grim via gas chromatography with flame ionization detection (GC-FID) in 40 minutely grab samples. By adding continuous, high precision in situ measurements of CH4 (Picarro cavity ring-down spectroscopy [CRDS]) at both Cape Grim, Tasmania, and Casey, Antarctica, a new feature has become apparent in the Cape Grim CH4 record. During the austral summer (December to February), the Cape Grim CH4 record periodically drops below baseline. For example, in Figure 1, a number of sustained episodes of depressed CH4 concentration can be seen below the baseline selected data shown in red. Notably, these episodes are also seen in the GC-FID record. In this presentation, we examine these sub-baseline excursions of CH4 . In conjunction with meteorology and a variety of other chemical species measured at Cape Grim, including radon, ozone, hydrogen and ethane, we speculate on a number of possible mechanisms that might be responsible for these dips in CH4 mixing ratio.
Origin, Hydrogen, Ethane, Radon, Allocations, Ozone
Loh, Z. M., Krummel, P. B., Gregory, R. L., Steele, L. P., Stavert, A. R., Schoot, M. V. V. D., Spencer, D. A., Mitrevski, B., Thornton, D. P., Galbally, I. E., Ward, J. Z., Somerville, N. T., Chambers, S. D., & Williams, AG. (2016). Speculation on the origin of sub-baseline excursions of CH4 at Cape Grim. Paper presented at the 44th NOAA Global Monitoring Annual Conference, Boulder Colorado. Retrieved from http://www.esrl.noaa.gov/gmd/annualconference/agenda.php