Unambiguous identification of esters as oligomers in secondary organic aerosol formed from cyclohexene and cyclohexene/α-pinene ozonolysis

Abstract

Abstract. The build-up of oligomeric compounds during secondary organic aerosol (SOA) formation is subject of atmospheric research since several years. New particle formation and especially the SOA mass yield might be influenced significantly by oligomer formation. However, the chemical nature of observed oligomers and their formation pathways are still unclear. In this paper, the structural characterization of certain dimeric compounds (esters) formed during the ozonolysis of cyclohexene and cyclohexene/α-pinene mixtures are presented. The identification is based on the comparison of the mass spectra and the retention times (LC) of the oligomeric products with synthesized reference compounds. Cyclohexene is used here as a model compound for terpenes as globally most important SOA precursors, since it possesses a simpler structure than the biogenic alkenes and therefore offers the possibility to get access to reference compounds for certain of its oxidation products. In addition to cyclohexene, the formation of esters could also be observed in experiments with α-pinene as reactant.

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

“…1). In addition to the MW 358 and 368 esters, minor high-molecular-weight compounds (i.e., MWs 272,300,308,312,314,326,338,344,352,356,376,378 and 400) have also been reported in α-pinene/O 3 SOA (Müller et al, 2008;Yasmeen et al, 2010;Kourtchev et al, 2014;Witkowski and Gierczak, 2014;Zhang et al, 2015) but these products will not be addressed in the present paper. Figure 1.…”

“…Tandem MS on the deprotonated compound (m/z 357) revealed that it has a cis-pinic acid residue (m/z 185) as well as a m/z 171 residue. Later work by Müller et al (2008) focused on the structure of the MW 368 compound. It was shown that this compound is composed of cis-pinic and hydroxypinonic acid parts, which are linked together by an ester bridge.…”

“…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.…”

“…A soft methylation procedure using ethereal diazomethane was selected to avoid hydrolysis of the ester function present in the targeted hetero-dimers. The aim of the methylation was twofold: on the one hand, to confirm the number of free carboxyl functions, and on the other hand, to obtain mass spectrometric fragmentation that is different from that of intact esters in (+)ESI and to that obtained in previous studies on intact esters in (−)ESI (Müller et al, 2008;Yasmeen et al, 2010;Zhang et al, 2015). Led by the molecular structures of the MW 368 and 358 esters, we propose a formation mechanism that takes into account the detection of a C 19 HOM in the gas phase by CI-APi-TOF MS with nitrate clustering (Ehn et al, 2012Zhao et al, 2013) and involves the combination of an acyl peroxy radical related to cis-pinic acid with an alkoxy radical related to isomeric hydroxypinonic acids, which are, like cis-pinic acid, major monomers in α-pinene SOA.…”

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

“…Higher-MW products either correspond to non-covalent dimers between monomeric carboxylic acids that are held together by hydrogen bonds (e.g., Hoffmann et al, 1998;Claeys et al, 2009) or to covalent dimers between monomeric carboxylic acids that are connected through covalent bonds, such as, esters (e.g., Hamilton et al, 2006;Szmigielski et al, 2007;Müller et al, 2008) and oligomers formed through gem-diol intermediates (e.g., Gao et al, 2004;Iinuma et al, 2004;Tolocka et al, 2004;Docherty et al, 2005) or aldol condensation reactions (e.g., Tolocka et al, 2004). These heterogeneous processes are consistent 9384 F. Yasmeen et al: Terpenylic acid and related compounds: precursors for dimers in secondary organic aerosol with the observation that sulfuric acid seed aerosol catalyzes the formation of SOA (e.g., Jang et al, 2002Jang et al, , 2003Czoschke et al, 2003;Iinuma et al, 2004).…”