Abstract
Atmospheric methane concentration was continually monitored for two years (April 1992-March 1994) at 29 stations in Aichi Prefecture. Possible causes for variability in the data were evaluated. The annual means of methane concentration were higher by about 0.1 ppm than the value of background concentration at Ryori. Monthly mean concentrations showed two distinctive peaks a year, in July and early winter. The former peak was presumed to result mainly from rice cultivation in southwestern Owari, while the latter was due to urban activities around Nagoya City. At the stations with high mean concentration, a clear diurnal variation was observed; methane concentration increased from midnight to early morning, corresponding well with a decrease in wind speed. The night peaks appeared concurrently over an area of some 10 km2 with an irregular periodicity of several days. The periodicity seemed to be affected by rainfall as well as wind speed; the night peak tended to disappear just after rainfall of more than about 10 mm/d. From the annual variation pattern of high methane levels at night, the 29 stations could roughly be classified into 4 types: background, paddy field, semi-urbanized (industrial), and highly urbanized. Each was considered to have been affected by different sources of methane.
