2‑Hydroxyterpenylic
Acid: An Oxygenated Marker
Compound for α‑Pinene Secondary Organic Aerosol in Ambient
Fine Aerosol
- Publication date
- Publisher
Abstract
An oxygenated MW 188 compound is
commonly observed in substantial
abundance in atmospheric aerosol samples and was proposed in previous
studies as an α-pinene-related marker compound that is associated
with aging processes. Owing to difficulties in producing this compound
in sufficient amounts in laboratory studies and the occurrence of
isobaric isomers, a complete assignment for individual MW 188 compounds
could not be achieved in these studies. Results from a comprehensive
mass spectrometric analysis are presented here to corroborate the
proposed structure of the most abundant MW 188 compound as a 2-hydroxyterpenylic
acid diastereoisomer with 2<i>R</i>,3<i>R</i> configuration.
The application of collision-induced dissociation with liquid chromatography/electrospray
ionization-ion trap mass spectrometry in both negative and positive
ion modes, as well as chemical derivatization to methyl ester derivatives
and analysis by the latter technique and gas chromatography/electron
ionization mass spectrometry, enabled a comprehensive characterization
of MW 188 isomers, including a detailed study of the fragmentation
behavior using both mass spectrometric techniques. Furthermore, a
MW 188 positional isomer, 4-hydroxyterpenylic acid, was tentatively
identified, which also is of atmospheric relevance as it could be
detected in ambient fine aerosol. Quantum chemical calculations were
performed to support the diastereoisomeric assignment of the 2-hydroxyterpenylic
acid isomers. Results from a time-resolved α-pinene photooxidation
experiment show that the 2-hydroxyterpenylic acid 2<i>R</i>,3<i>R</i> diastereoisomer has a time profile distinctly
different from that of 3-methyl-1,2,3-butanetricarboxylic acid, a
marker for oxygenated (aged) secondary organic aerosol. This study
presents a comprehensive chemical data set for a more complete structural
characterization of hydroxyterpenylic acids in ambient fine aerosol,
which sets the foundation to better understand the atmospheric fate
of α-pinene in future studies