Isoprene and its oxidation products methyl vinyl ketone, methacrolein, and isoprene related peroxides measured online over the tropical rain forest of Surinam in March 1998

Airborne measurements of volatile organic compounds (VOC) were performed over the tropical rainforest in Surinam (0-12 km altitude, 2 degrees -7 degrees N, 54 degrees -58 degrees W) using the proton transfer reaction mass spectrometry (PTR-MS) technique, which allows online monitoring of compounds like isoprene, its oxidation products methyl vinyl ketone, methacrolein, tentatively identified hydroxy-isoprene-hydroperoxides, and several other organic compounds. Isoprene volume mixing ratios (VMR) varied from below the detection limit at the highest altitudes to about 7 nmol/mol in the planetary boundary layer shortly before sunset. Correlations between isoprene and its product compounds were made for different times of day and altitudes, with the isoprene-hydroperoxides showing the highest correlation. Model calculated mixing ratios of the isoprene oxidation products using a detailed hydrocarbon oxidation mechanism, as well as the intercomparison measurement with air samples collected during the flights in canisters and later analysed with a GC-FID, showed good agreement with the PTR-MS measurements, in particular at the higher mixing ratios. Low OH concentrations in the range of 1-3 x 10(5) molecules cm(-3) averaged over 24 hours were calculated due to loss of OH and HO2 in the isoprene oxidation chain, thereby strongly enhancing the lifetime of gases in the forest boundary layer.