Cyclobutyl methyl ketone as a model compound for pinonic acid to elucidate oxidation mechanisms

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.