Hydroxydicarboxylic acids: markers for secondary organic aerosol from the photooxidation of alpha-pinene

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 alpha-pinene/NOx/air mixtures. In the present work, two major photooxidation products of alpha-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 (El) 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 alpha-pinene/NOJ0 air. In addition, we report the presence of a structurally related minor alpha-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.