Measurements of sulfur dioxide (SO2) were made at the National Oceanic and Atmospheric Administration's Mauna Loa Observatory in Hawaii, during a 12-month period beginning in December 1988. SO2 concentrations varied from background levels of less than 0.05 ppbv to a maximum of 50 ppbv, during episodes that lasted from 2 to 24 hours. Emissions from the Kilauea crater, approximately 35 km southeast of the observatory at an elevation of about 1000 m above sea level (asl), and the current eruption at Puu O'o 50 km east-southeast, are the most likely sources for the higher concentrations. These episodes occurred 10-25 times each month, mostly during the day, peak concentrations were usually recorded at mid-day. The SO2 concentrations can be grouped into three periods: low (June-September), high (October-January) and intermediate (February-May). A clear diurnal cycle of SO2 concentration exists throughout the year, although day-night changes were greatest during October-January and were barely detectable during the June-September period. The highest SO2 concentrations were recorded when the predominant wind direction was northerly to northwesterly, even though the apparent sources are in the southeastern sector. Nighttime concentrations were usually at background levels; however, many exceptions were observed. A few cases of higher than background SO2 were observed when free tropospheric (FT) conditions were identified. The possibility that long-range transport was the cause for elevated SO2 concentrations under FT conditions was examined using air mass back trajectories analyses. The highest nighttime SO2 concentrations, under FT conditions, were observed during periods with slow easterly trajectories, and the lowest concentrations were found during westerly flows. Twenty-four nighttime free tropospheric events were recorded when the SO2 concentration exceeded 0.2 ppbv. During 18 of these episodes, unusually high CO2 concentrations were observed. ¿ American Geophysical Union 1992