We have compiled a unique high-resolution ambient-air methane data set consisting of approximately 125,000 independently measured data points for the years 1991--1995 that have been collected at a site in the northeastern United States. The annual median mixing ratio of methane for all measurements was 1808 ppbv in 1992, increasing at a variable rate to 1837 ppbv in 1995. The lower 10--30% of the data from each month were defined as representative of background air and were compared with the global Climate Monitoring and Diagnostics Laboratory (CMDL) data set. The background data exhibit a variable upward trend of 5.5¿2 ppbv/yr during the 4-year time period, with most of the increase observed during 1993 and 1994. The seasonal cycle for the background data set is similar to what is observed by CMDL stations and varies from 24 to 35 ppbv. The amplitude of the seasonal cycle for the full data set was larger, ranging from 35 to 44 ppbv. Differences between the full and background mixing ratios vary on a seasonal basis and are largest in the winter and smallest in the summer. These differences appear to be controlled by changes in atmospheric stability and changes in emissions from local and regional sources throughout the year. Diurnal cycles exist in the data, with the magnitude and timing of maximum and minimum values being controlled by inputs from local sources and atmospheric stability. Nearby wetlands contribute to an overnight buildup of methane in the late spring and summer. The magnitude of the daily cycle is largest in July and August (~23 ppbv), when inputs from wetlands are large and wind speed is generally low. In April, the daily cycle is smallest (~6 ppbv), when inputs from local sources are low and more vigorous atmospheric mixing limits pollution buildup. ¿ 1998 American Geophysical Union