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 (PM2.5) rural aerosol collected at night during the warm period of the 2006 summer field campaign 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-to-particle partitioning of their monomeric precursors in laboratory α-/β-pinene ozonolysis experiments.
Highlights
The formation of higher-molecular weight (MW) products is considered as a driving force for secondary organic aerosol (SOA) formation and growth, because their formation leads to products with a substantially decreased vapor pressure that is several orders of magnitude lower than that of the precursor hydrocarbons
We deal with the structural characterization of terpenoic acids that serve as monomeric precursors to the dimeric MW 344 and 358 compounds, which will be addressed as well as with the structural characterization of some weak monomeric/isobaric compounds
It is noted that the product ions at m/z 127 and 83 are characteristic for diaterpenylic acid acetate (Claeys et al, 2009) and are expected to be formed from diaterpenylic acid, produced through hydrolysis of terpenylic acid and diaterpenylic acid acetate. (−)electrospray ionization (ESI)-MS data for diaterpenylic acid acetate are presented in Fig. 3b and Scheme 1b
Summary
The formation of higher-molecular weight (MW) products is considered as a driving force for secondary organic aerosol (SOA) formation and growth, because their formation leads to products with a substantially decreased vapor pressure that is several orders of magnitude lower than that of the precursor hydrocarbons. Considerable attention has been given in previous work to a higher-MW 358 SOA product from α-pinene ozonolysis (Hoffmann et al, 1998; Gao et al, 2004; Iinuma et al, 2004), which was first detected by direct negative ion atmospheric pressure chemical ionization – mass spectrometry and tentatively assigned to a stable adduct between cis-pinic (MW 186) and cis-norpinic acid (MW 172), both ozonolysis products (Hoffmann et al, 1998) It was shown in the latter study that the MW 358 compound remained intact upon reversed-phase liquid chromatography; a straightforward explanation for the formation of a stable dimer between cis-pinic and cis-norpinic acid was not presented. We propose that diaterpenylic acid, a hydrolysis product of terpenylic acid (MW 172) and/or diaterpenylic acid acetate (MW 232), recently reported α-pinene SOA tracers formed upon both photooxidation and ozonolysis (Claeys et al, 2009), serves together with cis-pinic acid as a monomeric building unit for the higher-MW 358 diester products
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