Atmos. Chem. Phys. Discuss., 12, 10651-10678, 2012
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Cyclobutyl methyl ketone as a model compound for pinonic acid to elucidate oxidation mechanisms
A. P. Praplan, P. Barmet, J. Dommen, and U. Baltensperger
Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Villigen PSI, Switzerland

Abstract. 3-Methyl-1,2,3-tricarboxylic acid (MBTCA), terpenylic acid and diaterpenylic acid acetate were identified in secondary organic aerosol (SOA) from α-pinene photooxidation or ozonolysis. These compounds display interesting structural features: MBTCA has a high oxygen to carbon ratio, terpenylic acid contains a lactone ring in its structure and diaterpenylic acid acetate possesses an ester functional group. The reaction mechanisms leading to these products are still unknown, but it was demonstrated experimentally in earlier studies that MBTCA is formed from pinonic acid, a primary ozonolysis product of α-pinene. Because the direct observation of pinonic acid oxidation in a smog chamber would be difficult due to its relatively low volatility, a model compound possessing the substructure of interest was used instead: cyclobutyl methyl ketone (CMK). From its oxidation, several organic acids could be measured with ion chromatography (IC) coupled to a mass spectrometer (MS). Succinic acid, the analogous product of MBTCA is formed at molar yields of 2 to 5%. Butyrolactone is detected as butanoic acid, due to hydrolysis in the sampling device. A monocarboxylic acid with nominal mass 146 was detected in the absence of nitrogen oxides (NOx) and could be the analogous product of diaterpenylic acid acetate. However, due to a lack of available standards, the exact structure of this compound remains unelucidated. Finally, 4-oxobutanoic acid could also be measured and two structures of its expected analogous compound from pinonic acid oxidation are proposed. Because these compounds are primary products of the CMK oxidation, reaction mechanisms capable of adding one or two carboxylic functional groups without formation of stable intermediate products needs to be formulated. Such a formation mechanism of MBTCA from pinonic acid was found in the literature; however, it includes a hydrogen atom migration to an acyloxy radical, which is expected to loose carbon dioxide (CO2) very rapidly. A competitive reaction pathway is expected to lead to the formation of a monocarboxylic acid with nominal mass 144, instead of this acyloxy radical; however, no such monocarboxylic acid was measured and other reaction pathways still need to be explored.

Citation: Praplan, A. P., Barmet, P., Dommen, J., and Baltensperger, U.: Cyclobutyl methyl ketone as a model compound for pinonic acid to elucidate oxidation mechanisms, Atmos. Chem. Phys. Discuss., 12, 10651-10678, doi:10.5194/acpd-12-10651-2012, 2012.
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