Formation of hydrogen peroxide in the ozonolysis of isoprene and simple alkenes under humid conditions

The ozonolysis of isobutene and isoprene was performed in a 570 l static reactor at 295 K and 730 Torr synthetic air in the presence and absence of water vapour, with the reactant concentration ranges of 1-6 ppmv. Products were analysed by a combination of FTIR spectroscopy, GC-FID, and HPLC. For both alkenes, the yields of H2O2 and the primary carbonyl products (acetone for isobutene, methacrolein and methylvinyl ketone for isoprene) increased under humid conditions. In the isoprene ozonolysis, the H2O2 yields relative to the O-3 conversion were, as determined from the initial rate of the formation, 1 and 9% for dry and humid conditions, respectively. The increase in its yield under the humid conditions was correlated with the sum of the increase in the yields of methacrolein and methylvinyl ketone (similar to 13%). This was explained by rapid decomposition of the transient cc-hydroxy hydroperoxides formed in the reaction of H2O with the two stabilised C-4 Criegee intermediates. Atmospheric relevance of the results is discussed. (C) 1998 Elsevier Science Ltd. All rights reserved.