Abstract
Abstract. Although oxidation of the atmospherically relevant compound α-pinene has been extensively studied, chemical mechanisms leading to the formation of later generation oxidation products remain poorly understood. The present work uses cyclobutyl methyl ketone (CMK) to study the oxidation mechanism of pinonic acid, an α-pinene reaction product, by hydroxyl radicals (·OH). CMK has a similar but simpler chemical structure compared to pinonic acid. Succinic acid, 4-hydroxybutanoic acid and 4-oxobutanoic acid were identified as first generation products of CMK. These observed organic acids were compared to compounds found in secondary organic aerosol formed from the oxidation of α-pinene. Results suggest that 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA), terpenylic % corrected name for MBTCA (3-methyl-1,2,3-butanetricarboxylic acid) acid and diaterpenylic acid acetate are first generation products of OH oxidation of pinonic acid. Therefore, there is strong evidence that ·OH oxidation greatly increases the oxygenation of organic compounds (e.g. monocarboxylic acid to tricarboxylic acid) through radical mechanisms, without requiring a stable intermediate. These observations cannot be explained by traditional atmospheric chemistry mechanisms.
Highlights
Monoterpenes are biogenic volatile organic compounds (VOCs) emitted into the atmosphere by vegetation
The presented analogy with pinonic acid assumes that terpenylic acid and diaterpenylic acid acetate in α-pinene secondary organic aerosol (SOA) are formed as second generation products from the oxidation of pinonic acid, to MBTCA
The experiment with NOx was performed in order to compare the results and to confirm that certain m/z of the proton-transferreaction mass spectrometer (PTR-MS) were correctly attributed to organic hydroperoxides, because organic hydroperoxides do not form at conditions with high NOx
Summary
Monoterpenes are biogenic volatile organic compounds (VOCs) emitted into the atmosphere by vegetation. The structures of further products of α-pinene SOA were identified recently: terpenylic acid and diaterpenylic acid acetate (Iinuma et al, 2008; Claeys et al, 2009). Cyclobutyl methyl ketone (CMK) was tested as a surrogate of pinonic acid to understand the oxidation mechanisms and the chemical structure of the products formed. The presented analogy with pinonic acid (grey moieties accounting for structural differences between the two systems) assumes that terpenylic acid and diaterpenylic acid acetate in α-pinene SOA are formed as second generation products from the oxidation of pinonic acid, to MBTCA. Because 4-oxobutanoic acid was observed during CMK oxidation experiments, it is included in Fig. 1 and two possible structures for analogous products from the α-pinene (or pinonic acid) oxidation are suggested
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