Detailed organic analysis of fine (PM2.5) rural aerosol collected during summer at K-puszta, Hungary from a mixed deciduous/coniferous forest site shows the presence of polar oxygenated compounds that are also formed in laboratory irradiated α-pinene/NOx/air mixtures. In the present work, two major photooxidation products of α-pinene were characterized as the hydroxydicarboxylic acids, 3-hydroxyglutaric acid, and 2-hydroxy-4-isopropyladipic acid, based on chemical, chromatographic, and mass spectral data. Different types of volatile derivatives, including trimethylsilyl ester/ether, methyl ester trimethylsilyl ether, and ethyl ester trimethylsilyl ether derivatives were measured by gas chromatography/mass spectrometry (GC/MS), and their electron ionization (EI) spectra were interpreted in detail. The proposed structures of the hydroxydicarboxylic acids were confirmed or supported with reference compounds. 2-Hydroxy-4-isopropyladipic acid formally corresponds to a further reaction product of pinic acid involving addition of a molecule of water and opening of the dimethylcyclobutane ring; this proposal is supported by a laboratory irradiation experiment with α-pinene/NOx/air. In addition, we report the presence of a structurally related minor α-pinene photooxidation product, which was tentatively identified as the C7 homolog of 3-hydroxyglutaric acid, 3-hydroxy-4,4-dimethylglutaric acid. The detection of 2-hydroxy-4-isopropyladipic acid in ambient aerosol provides an explanation for the relatively low atmospheric concentrations of pinic acid found during daytime in forest environments.
Claeys, M., Szmigielski, R., Kourtchev, I., Van Der Veken, P., Vermeylen, R., Maenhaut, W., ... Edney, E. O. (2007). Hydroxydicarboxylic acids: Markers for secondary organic aerosol from the photooxidation of α-pinene. Environmental Science and Technology, 41(5), 1628–1634. https://doi.org/10.1021/es0620181