Direct Measurements of Isoprene Autoxidation: Pinpointing Atmospheric Oxidation in Tropical Forests

Abstract

2-Methyl-1,3-butadiene (isoprene), released from biogenic sources, accounts for approximately a third of hydrocarbon emissions and is mainly removed by hydroxyl radicals, OH, the primary initiator of atmospheric oxidation. In situ measurements in clean tropical forests (high isoprene and low NOx) have measured OH concentrations up to an order of magnitude higher than model predictions, which impacts our understanding of global oxidation. In this study, direct, laser flash photolysis, laser-induced fluorescence measurements at elevated temperatures have observed OH recycling in the presence of isoprene and oxygen under conditions where interference from secondary or heterogeneous chemistry is minimal. Our results provide the first direct, time-resolved, experimental validation of the theory-based Leuven Isoprene Mechanism (LIM1), based on isomerization of isoprene-RO2radicals and OH regeneration, that partially accounts for model:measurement divergence in OH. While our data can be fit with only minor alterations in important LIM1 parameters, and the overall rate of product formation is similar to LIM1, there are differences with the recent experimental study by Teng et al. J. Am. Chem. Soc. 2017, 139, 5367-5377. In addition, our study indicates that the dihydroperoxide products are significantly enhanced over previous estimates. Dihydroperoxides are chemical and photochemical sources of OH, and the implications of enhanced hydroperoxide formation on the agreement between models and observations in tropical forests are examined.