Analysis and presentation of in situ atmospheric methane measurements from Cape Ochi‐ishi and Hateruma Island

In situ measurements of atmospheric methane (CH 4 ) from the monitoring stations at Cape Ochi‐ishi (latitude 43°10′N, longitude 145°30′E) during the period from July 1995 to July 2000 and at Hateruma Island (latitude 24°03′N, longitude 123°48′E) during the period from January 1996 to January 2000 are presented. At each station a fully automated gas chromatograph equipped with a flame ionization detector measures the CH 4 mixing ratios at a frequency of more than 82 air samples per day. Over the above observation periods, average growth rates of CH 4 mixing ratios were 4.5 parts per billion (ppb) yr −1 for Ochi‐ishi and 4.7 ppb yr −1 for Hateruma, but there were considerable fluctuations in the instantaneous growth rate. The peak‐to‐peak amplitudes in the seasonal cycles determined from the smooth curve fits were 71 ppb for Ochi‐ishi and 94 ppb for Hateruma. These seasonal amplitudes are larger than those at the sampling network sites of the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory. Analysis of the back trajectories indicated that the air masses at both sites are transported from continental Asia in winter and from the Pacific Ocean in summer. Because there are strong CH 4 sources in continental Asia but no strong sources in the Pacific Ocean, the seasonality of airflow patterns can explain the enhancement of the seasonal cycles. The significantly large seasonality, especially at Hateruma, is a result of air masses originating from a wide range of latitudes, the clear seasonality of airflows, and the steep latitudinal gradient of the CH 4 mixing ratio.