Terpenylic acid and related compounds: precursors for dimers in secondary organic aerosol from the ozonolysis of α- and β-pinene

Abstract

Abstract. In the present study, we have characterized the structure of a higher-molecular weight (MW) 358 α-and β-pinene dimeric secondary organic aerosol (SOA) product that received ample attention in previous molecular characterization studies and has been elusive. Based on mass spectrometric evidence for deprotonated molecules formed by electrospray ionization in the negative ion mode and chemical considerations, it is suggested that diaterpenylic acid is a key monomeric intermediate for dimers of the ester type. It is proposed that cis-pinic acid is esterified with the hydroxyl-containing diaterpenylic acid, which can be explained through acid-catalyzed hydrolysis of the recently elucidated lactone-containing terpenylic acid and/or diaterpenylic acid acetate, both first-generation oxidation products. To a minor extent, higher-MW 358 and 344 diester products are formed containing other terpenoic acids as monomeric units, i.e., diaterpenylic acid instead of cis-pinic acid, and diaterebic acid instead of diaterpenylic acid. It is shown that the MW 358 diester and related MW 344 compounds, which can be regarded as processed SOA products, also occur in ambient fine (PM 2.5 ) rural aerosol collected at night during the warm period of the 2006 summer field campaignCorrespondence to: M. Claeys (magda.claeys@ua.ac.be) conducted at K-puszta, Hungary, a rural site with coniferous vegetation. This indicates that, under ambient conditions, the higher-MW diesters are formed in the particle phase over a longer time-scale than that required for gas-toparticle partitioning of their monomeric precursors in laboratory α-/β-pinene ozonolysis experiments.

High-molecular-weight esters in <i>α</i>-pinene ozonolysis secondary organic aerosol: structural characterization and mechanistic proposal for their formation from highly oxygenated molecules

“…Unambiguous evidence could be obtained for the molecular structure of the MW 368 ester in that it corresponds to an ester of cis-pinic acid where the carboxyl substituent of the dimethylcyclobutane ring and not the methylcarboxyl substituent is esterified with 7-hydroxypinonic acid. The same linkage was already proposed in previous work for the MW 358 ester (Yasmeen et al, 2010), but could be supported in the present study. Guided by the molecular structures of these stable esters, we propose a formation mechanism from gas-phase HOMs that takes into account the formation of an unstable C 19 H 28 O 11 product, which is detected as a major species in α-pinene ozonolysis experiments as well as in the pristine forest atmosphere by chemical ionization-atmospheric pressure ionization-time-of-flight mass spectrometry with nitrate clustering (Ehn et al, 2012.…”

“…Recent work by Beck and Hoffmann (2016), where use was made of derivatization to the n-butylesters and subsequent tandem MS analysis of the lithiated and ammoniated molecules, supported the structure of the MW 358 ester as a diaterpenylic ester of cis-pinic acid. Furthermore, the MW 358 ester was detected as a major tracer in β-pinene ozonolysis SOA characterization studies (Iinuma et al, 2007;Yasmeen et al, 2010).…”

“…It was shown that this compound is composed of cis-pinic and hydroxypinonic acid parts, which are linked together by an ester bridge. The structure of the MW 358 compound was also addressed by Yasmeen et al (2010), who revised the structure of this compound and presented evidence that it is a diaterpenylic ester of cis-pinic acid. The same conclusion was reached by Gao et al (2010), who also showed that the MW 358 ester is a major product in α-pinene ozonolysis experiments performed at low mass loadings.…”

“…Tolocka et al (2004) characterized highmolecular-weight compounds in α-pinene ozonolysis SOA and suggested that the products were most likely formed by aldol and/or gem-diol formation. In addition, Witkowski and Gierczak (2014) explained the formation of MW 338 and 352 compounds in α-pinene ozonolysis as aldol reaction products of α-acyloxyhydroperoxy aldehydes. All the abovementioned studies thus provide evidence that the structure elucidation of high-molecular-weight α-pinene/O 3 compounds has turned out to be very challenging.…”

“…It is noted that prior to the studies by Müller et al (2008) and Yasmeen et al (2010) several studies dealt with the molecular characterization of high-molecular-weight compounds and that very different possible structures have been advanced. Gao et al (2004) assigned the MW 358 α-pinene/O 3 compound to a dehydration product formed between the gem-diol forms of two norpinonic acid molecules.…”