ATMOSPHERIC METHANE AT CAPE-MEARES - ANALYSIS OF A HIGH-RESOLUTION DATA-BASE AND ITS ENVIRONMENTAL IMPLICATIONS

Between 1979 and 1992 we took some 120,000 measurements of atmospheric methane at Cape Meares on the Oregon coast The site is representative of methane concentrations in the northern latitudes (from 30-degrees-N to 90-degrees-N). The average concentration during the experiment was 1698 parts per billion by volume (ppbv). Methane concentration increased by 190 ppbv (or 11.9%) during the 13-year span of the experiment. The rate of increase was about 20 +/- 4 ppbv/yr in the first 2 years and 10 +/- 2 ppbv/yr in the last 2 years of the experiment suggesting a substantial decline in the trend at northern middle and high latitudes (-1 ppbv/yr2). Prominent seasonal cycles were observed. During the year, the concentration stays more or less constant until May and then starts falling, reaching lowest levels in July and August, then rises rapidly to nearly maximum concentrations in October. The average amplitude of this cycle is about 30 +/- 7 ppbv and has increased during the course of the experiment. Interannual variations with small amplitudes of 2-3 ppbv occur with periods of 1.4 and 6.5 years. The residual concentrations, after accounting for the trends and cycles, have a standard deviation of 6 ppbv for monthly averaged data and 12 ppbv for the daily data. Mass balance calculations show that to explain the observed seasonality of concentrations, the emissions must peak in late summer and early fall (August-September). No increases in regional annual emissions are required over the last decade to explain the data. For further research, readers may obtain the complete and averaged data from the archives.