Thermodynamics of oligomer formation: implications for secondary organic aerosol formation and reactivity

Abstract

Dimers and higher order oligomers, whether in the gas or particle phase, can affect important atmospheric processes such as new particle formation, and gas-particle partitioning. In this study, the thermodynamics of dimer formation from various oxidation products of α-pinene ozonolysis are investigated using a combination of Monte Carlo configuration sampling, semi-empirical and density functional theory (DFT) quantum mechanics, and continuum solvent modeling. Favorable dimer formation pathways are found to exist in both gas and condensed phases. The free energies of dimer formation are used to calculate equilibrium constants and expected dimer concentrations under a variety of conditions. In the gas phase, favorable pathways studied include formation of non-covalent dimers of terpenylic acid and/or cis-pinic acid and a covalently-bound peroxyhemiacetal. Under atmospherically relevant conditions, only terpenylic acid forms a dimer in sufficient quantities to contribute to new particle formation. Under conditions typically used in laboratory experiments, several dimer formation pathways may contribute to particle formation. In the condensed phase, non-covalent dimers of terpenylic acid and/or cis-pinic acid and covalently-bound dimers representing a peroxyhemiacetal and a hydrated aldol are favorably formed. Dimer formation is both solution and temperature dependent. A water-like solution appears to promote dimer formation over methanol- or acetonitrile-like solutions. Heating from 298 K to 373 K causes extensive decomposition back to monomers. Dimers that are not favorably formed in either the gas or condensed phase include hemi-acetal, ester, anhydride, and the di(α-hydroxy) ether.

Formation and evolution of molecular products in α-pinene secondary organic aerosol

“…Importantly, none of the covalent ester dimers is observed in the OH system, although the major building blocks, i.e., C 8 H 11 O 4 (m/z 171) and C 9 H 13 O 4 (m/z 185), are still present. This provides strong evidence that these dimers are not a product of particle-phase esterification (17). Growth rates of individual species in the particle phase, defined as the ratio of the mass of each species normalized by its highest mass value to the reacted mass of α-pinene normalized by its highest mass value after ∼83 min of reaction.…”

Kinetic modeling studies of SOA formation from <i>α</i>-pinene ozonolysis

“…Thermodynamic calculations indicate es-ter formation and peroxyhemiacetal formation as most likely (Barsanti and Pankow, 2006;DePalma et al, 2013) and suggest hemiacetal formation as thermodynamically unfavorable (Barsanti and Pankow, 2004;DePalma et al, 2013). Therefore, an irreversible representation of the aerosol chemistry might lead to an overprediction of the formed SOA mass, which means that it can only be considered an upper limit approach.…”

“…However, for future chamber simulations with the focus on SOA processes combined with advanced measurement data, accounting for SOA aging or oxidation state, an improved representation of particle-phase reactivity will be implemented to further develop SPACCIM. Therefore, the pseudo-first-order rate constants will be replaced, e.g., by second-order equilibrium reactions under consideration of equilibrium rates provided by Barsanti and Pankow (2004) for hydrate and hemiacetal formation or thermodynamic calculations for equilibrium constants of DePalma et al (2013) for individual dimers.…”

Identification of Organic Hydroperoxides and Peroxy Acids Using Atmospheric Pressure Chemical Ionization – Tandem Mass Spectrometry (APCI-MS/MS): Application to Secondary Organic Aerosol

“…Several mechanisms for oligomer product formation in SOA arising from VOC oxidation have been proposed: i) self-and cross-reactions of the peroxy radicals (RO 2 ) (Zhang et al, 2016), ii) reaction of ozonolysis products in the condensed-phase, such as aldol condensation, esterification, hemiacetal and peroxyhemiacetal formation (Ziemann, 2003;Tolocka et al, 2004;Kristensen et al, 2014;Docherty et al, 2005;Muller et al, 2009;Yasmeen et al, 2010;Hall and Johnston, 2012;Witkowski and Gierczak, 2012;DePalma et al, 2013;Lim and Turpin, 2015), iii) dimer cluster formation from carboxylic acids 15 (Hoffmann et al, 1998;Tobias and Ziemann, 2000;Claeys et al, 2009;Camredon et al, 2010;DePalma et al, 2013), iv) reactions of Criegee intermediates (CIs) with VOCs oxidation products (Bonn et al, 2002;Lee and Kamens, 2005;Tolocka et al, 2006;Heaton et al, 2007;Witkowski and Gierczak, 2012;Kristensen et al, 2016;Wang et al, 2016), and vi) reactions of RO 2 radicals with Cis (Sadezky et al, 2008;Zhao et al, 2015). Among them, the reactions of CIs with protic substances (water, alcohols, acids and hydroperoxides) can form ROOH.…”