Measurements of 3-methyl furan, methyl vinyl ketone, and methacrolein at a rural forested site in the southeastern United States

Three oxygenated hydrocarbons were measured in ambient air above a rural forested site in western Alabama. Mixing ratios of methyl vinyl ketone (MVK), methacrolein (MACR), and 3-methyl furan were determined during 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 10¹² (ppt), the mean mixing ratio for methacrolein was 480 ppt. The results for methyl vinyl ketone and methacrolein are compared to previously reported measurements from this same location during the summer of 1990. Although isoprene levels were lower by ∼50% in 1992, mixing ratios of the carbonyls were reduced by only ∼25%. These differences are discussed in light of the changes that had occurred to the forest canopy in the time elapsed between the two measurement campaigns. Despite the differences observed during the 2 years, a consistent diurnal variability is observed for methyl vinyl ketone, relative to methacrolein, during both years. In addition to a weak correlation observed between the carbonyls and temperature, levels of MVK and MACR in air sampled from just above the forest canopy are influenced by the depth of the mixed layer. A third oxidation product of isoprene, 3-methyl furan, was identified and measured in ambient air. Mean daytime mixing ratios were estimated at 60 ppt. Nighttime levels were lower, averaging 40 ppt. Mixing ratios of 3-methyl furan were highly correlated with isoprene (R² = 0.82), and during the day, were typically ∼2% of levels measured for isoprene.