Measurements of methyl chloride in air trapped in an Antarctic ice core

Methyl chloride (CH₃Cl), the most abundant atmospheric halocarbon, is emitted into the atmosphere from various natural sources, and is considered to contribute to the stratospheric ozone depletion. In order to deduce past variations of the atmospheric CH₃Cl mixing ratio, air samples extracted from an ice core were analyzed for CH₃Cl. The ice core was collected at Dome Fuji (77\deg 19' S, 39\deg 42' E, 3810 m a.s.l.) in the Antarctica. Air was extracted from the ice core samples of about 300 g by milling the samples by a cutter into fine powder under vacuum at -20 \deg C. The extracted air was dried by passing through a glass trap held at -100 \deg C and then collected into a stainless steel tube cooled at about -263 \deg C. The CH₃Cl mixing ratios in the air samples were determined using a gas chromatography/mass spectrometer system with prior cryogenic pre-concentration. The measured mixing ratios of CH₃Cl in the Holocene were found to be similar to the present day atmospheric mixing ratios. However, the CH₃Cl levels were much higher in last glacial period, presumably responding to changes in the natural sources and sinks due to low air temperature. These results were compared to other climatic and environmental signals reconstructed from the Dome Fuji ice core and the cause of the glatial-interglatial variation of the atmospheric CH₃Cl will be discussed.