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

Airborne measurements of volatile organic compounds (VOC) were performed ov er the tropical rainforest in Surinam (0-12 km altitude, 2 degrees -7 degre es N, 54 degrees -58 degrees W) using the proton transfer reaction mass spe ctrometry (PTR-MS) technique, which allows online monitoring of compounds l ike isoprene, its oxidation products methyl vinyl ketone, methacrolein, ten tatively identified hydroxy-isoprene-hydroperoxides, and several other orga nic compounds. Isoprene volume mixing ratios (VMR) varied from below the de tection limit at the highest altitudes to about 7 nmol/mol in the planetary boundary layer shortly before sunset. Correlations between isoprene and it s product compounds were made for different times of day and altitudes, wit h the isoprene-hydroperoxides showing the highest correlation. Model calcul ated mixing ratios of the isoprene oxidation products using a detailed hydr ocarbon oxidation mechanism, as well as the intercomparison measurement wit h air samples collected during the flights in canisters and later analysed with a GC-FID, showed good agreement with the PTR-MS measurements, in parti cular 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 ox idation chain, thereby strongly enhancing the lifetime of gases in the fore st boundary layer.