Atmospheric methane over the North Pacific from 1987 to 1993.

Abstract

Atmospheric methane mixing ratios were measured over the North Pacific during the winter season from 1987 to 1993 to extend our methane record since 1978. The latitudinal distribution of methane mixing ratio showed a north-to-south gradient from mid-latitudes to the equator every year. A sharp mixing ratio gradient often appeared at the boundary between the winter monsoon and the trade wind regions around 20°N. No significant longitudinal gradient was found during the winter season, although methane levels along the equator showed a large difference between the western and eastern Pacific. The overall methane increase rate in the western Pacific was estimated to be 13 ppb/yr on the basis of the long-term record for 15 years from 1978 to 1993. This record indicates that the methane growth rate over this Pacific region was gradually slowing down until 1990, followed by no significant increase in the 1990's. The overall deceleration of the growth rate was more rapid in the middle latitudinal zone (20°N 30°N) than in the lower latitudinal zone (3°N-20°N). This latitudinal difference suggests a rapid reduction of methane emissions from the continental regions. The methane growth rate showed an interannual variation with an increasing trend around 1983 and 1987, which was roughly related to the El Niflo events. It is suggested that the methane growth rate was affected by a change of interhemispheric transport due to the ENSO events. INTRODUCTIONAtmospheric methane is known as an important greenhouse gas that influences the radiative bal ance and climate of the earth by warming the troposphere and cooling the stratosphere. In addi tion, atmospheric methane is closely related to the chemical cycles of carbon monoxide, ozone, and water vapor by reaction with hydroxyl radicals. The main sources of atmospheric methane are es timated to be enteric fermentation, rice paddies, and wetlands (Cicerone and Oremland, 1988;Fung et al., 1991). Methane is also emitted to the at mosphere from natural gas leakage, biomass burning, and termites. On the other hand, the main sink is oxidative reaction with hydroxyl radicals (Levy, 1971). A minor sink is methane oxidation by bacteria in the soil. However, large uncertain ties in global estimation of their sources and sinks still remain (Fung et al., 1991).Ice-core records indicate that methane mixing ratio in the atmosphere has increased by about a factor of two in the last 150 years due to human activity (e.g. Etheridge et al., 1992). In the late 1970's, accurate measurements of atmospheric methane started using a gas chromatograph with a flame ionization detector (GC/FID) (Fraser et al., 1986;Khalil and Rasmussen, 1983;Blake and Rowland, 1986;Matsueda et al., 1992). These measurements clearly indicate that atmospheric methane had continued to increase at the rate of about 1%/yr around 1980. More recently, methane levels have been observed regularly at many re mote sites to globally determine the increasing trend (Steele et al., 1987;Blake and Rowland, 1988;Scheel et al., 1990; Aoki et a...

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