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

Abstract

Abstract. Stable high-molecular-weight esters are present in α-pinene ozonolysis secondary organic aerosol (SOA) with the two most abundant ones corresponding to a hydroxypinonyl ester of cis-pinic acid with a molecular weight (MW) of 368 (C 19 H 28 O 7 ) and a diaterpenylic ester of cis-pinic acid with a MW of 358 (C 17 H 26 O 8 ). However, their molecular structures are not completely elucidated and their relationship with highly oxygenated molecules (HOMs) in the gas phase is still unclear. In this study, liquid chromatography in combination with positive ion electrospray ionization mass spectrometry has been performed on highmolecular-weight esters present in α-pinene ozonolysis SOA with and without derivatization into methyl esters. 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. It is suggested that an acyl peroxy radical related to cis-pinic acid (RO 2 q ) and an alkoxy radical related to 7-or 5-hydroxypinonic acid (R O q ) serve as key gas-phase radicals and combine according to a RO 2 + R O q → RO 3 R radical termination reaction. Subsequently, the unstable C 19 H 28 O 11 HOM species decompose through the loss of oxygen or ketene from the inner part containing a labile trioxide function and the conversion of the unstable acyl hydroperoxide groups to carboxyl groups, resulting in stable esters with a molecular composition of C 19 H 28 O 7 (MW 368) and C 17 H 26 O 8 (MW 358), respectively. The proposed mechanism is supported by several observations reported in the literature. On the basis of the indirect evidence presented in this study, we hypothesize that RO 2 + R O q → RO 3 R chemistry is at the underlying molecular basis of high-molecular-weight ester formation upon α-pinene ozonolysis and may thus be of importance for new particle formation and growth in pristine forested environments.

Quantifying organic matter and functional groups in particulate matter filter samples from the southeastern United States – Part 1: Methods

“…The median concentrations of OM estimated as the sum of functional groups were within the range of those measured previously using aerosol mass spectrometry and FT-IR spectrometry (∼ 1-10 µg m −3 ; Kamruzzaman et al, 2018;Ruthenburg et al, 2014;Sun et al, 2011;Xu et al, 2015.). The absolute median OM concentrations were greater at urban sites (JST, BHM) than at rural sites (CTR, YRK, OLF), as anticipated.…”

“…S10, and Naes et al, 2002). Examples of PLS calibration of functional groups in atmospheric OM include the work of Reff et al (2007), Coury and Dillner (2009), Ruthenburg et al (2014), and Kamruzzaman et al (2018).…”

“…The concentrations of these functional groups (and OM concentrations as weighted sums of these functional groups) were predicted in ambient filter samples collected from seven IMPROVE network sites in 2011. The same measurements were made, adding an amine functional group model, for a larger group of IMPROVE network sites from 2013 (Kamruzzaman et al, 2018). However, the relatively short list of chemicals to represent atmospheric composition likely limited the ability of these models to characterize the aerosol composition fully.…”

“…Amines and amino acids have been studied in functional group calibrations (Kamruzzaman et al, 2018;Liu et al, 2009). Amines and amino acids could contribute ∼ 5 %-10 % of organic aerosol concentrations and come from a variety of anthropogenic and biogenic sources (Russell et al, 2011).…”

“…Nonacid carbonyls also contributed 5 %-20 %, and oxOCO contributed 5 %-10 % of OM by mass; oxOCO concentrations were equivalent to ∼ 40 % of COOH concentrations (interquartile range 36 %-54 %). Data from all analyzed SEARCH years will be further detailed in a forthcoming paper on trends in the southeastern US, but data from 2013 are discussed briefly here to demonstrate model improvements, since there are FT-IR spectrometry measurements using previous FT-IR functional group models (Kamruzzaman et al, 2018) in that year.…”