MEASUREMENTS OF 3-METHYL FURAN, METHYL VINYL KETONE, AND METHACROLEINAT A RURAL FORESTED SITE IN THE SOUTHEASTERN UNITED-STATES

Three oxygenated hydrocarbons were measured in ambient air above a rur al forested site in western Alabama. Mixing ratios of methyl vinyl ket one (MVK), methacrolein (MACR), and 3-methyl furan were determined dur ing a 3-week period in the summer of 1992. While the mean mixing ratio for methyl vinyl ketone was determined to be 740 parts per 10(12) (pp t), the mean mixing ratio for methacrolein was 480 ppt. The results fo r methyl vinyl ketone and methacrolein are compared to previously repo rted measurements from this same location during the summer of 1990. A lthough isoprene levels were lower by similar to 50% in 1992, mixing r atios of the carbonyls were reduced by only similar to 25%. These diff erences are discussed in light of the changes that had occurred to the forest canopy in the time elapsed between the two measurement campaig ns. Despite the differences observed during the 2 years, a consistent diurnal variability is observed for methyl vinyl ketone, relative to m ethacrolein, during both years. In addition to a weak correlation obse rved between the carbonyls and temperature, levels of MVK and MACR in air sampled from just above the forest canopy are influenced by the de pth of the mixed layer. A third oxidation product of isoprene, 3-methy l furan, was identified and measured in ambient air. Mean daytime mixi ng ratios were estimated at 60 ppt. Nighttime levels were lower, avera ging 40 ppt. Mixing ratios of 3-methyl furan were highly correlated wi th isoprene (R(2) = 0.82), and during the day, were typically similar to 2% of levels measured for isoprene.