During recent field campaigns, measured hydroxyl radical (OH) concentrations were up to a factor of ten larger than predicted by current chemical models for conditions of high OH reactivity and low nitrogen monoxide (NO) concentrations. These discrepancies were most often observed in forests, where concentrations of biogenic volatile organic compounds (BVOCs) were large. We investigated the radical budget during oxidation of VOCs with OH including a full set of accurate and precise radical measurements in the atmosphere simulation chamber SAPHIR in Juelich, Germany. The conditions during the chamber experiments were comparable to those during field campaigns with respect to radical and trace gas concentrations. In particular, OH reactivity was high and NO mixing ratios were as low as 200pptv. VOC species included the most important single compound isoprene, its major oxidation products methacrolein and methyl vinyl ketone, monoterpenes, and aromatic compounds. Significant gaps between measured OH destruction and production pathways were found for isoprene and methacrolein, whereas the OH budget was approximately closed for aromatic compounds and monoterpenes. The additional OH needed to close the OH budget is consistent with recently proposed reaction mechanisms suggesting OH production from isomerization and decomposition reactions of organic peroxy radicals produced in the reaction of OH with VOCs.

Closing the OH budget during VOC oxidation: New insights from chamber experiments in SAPHIR at low NO concentrations

248th National Meeting (2014)

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