A review of stereochemical implications in the generation of secondary organic aerosol from isoprene oxidation

Abstract

The atmospheric reactions leading to the generation of secondary organic aerosol (SOA) from the oxidation of isoprene are generally assumed to produce only racemic mixtures, but aspects of the chemical reactions suggest this may not be the case. In this review, the stereochemical outcomes of published isoprene-degradation mechanisms contributing to high amounts of SOA are evaluated. Despite evidence suggesting isoprene first-generation oxidation products do not contribute to SOA directly, this review suggests the stereochemistry of first-generation products may be important because their stereochemical configurations may be retained through to the second-generation products which form SOA. Specifically, due to the stereochemistry of epoxide ring-opening mechanisms, the outcome of the reactions involving epoxydiols of isoprene (IEPOX), methacrylic acid epoxide (MAE) and hydroxymethylmethyl-α-lactone (HMML) are, in principle, stereospecific which indicates the stereochemistry is predefined from first-generation precursors. The products from these three epoxide intermediates oligomerise to form macromolecules which are proposed to form chiral structures within the aerosol and are considered to be the largest contributors to SOA. If conditions in the atmosphere such as pH, aerosol water content, relative humidity, pre-existing aerosol, aerosol coatings and aerosol cation/anion content (and other) variables acting on the reactions leading to SOA affect the tacticity (arrangement of chiral centres) in the SOA then they may influence its physical properties, for example its hygroscopicity. Chamber studies of SOA formation from isoprene encompass particular sets of controlled conditions of these variables. It may therefore be important to consider stereochemistry when upscaling from chamber study data to predictions of SOA yields across the range of ambient atmospheric conditions. Experiments analysing the stereochemistry of the reactions under varying conditions of the above variables would help elucidate whether there is stereoselectivity in SOA formation from isoprene and if the rates of SOA formation are affected.

Characterizing source fingerprints and ageing processes in laboratory-generated secondary organic aerosols using proton-nuclear magnetic resonance (<sup>1</sup>H-NMR) analysis and HPLC HULIS determination

“…Methylerythritol is more abundant (60 % of the sum of the two, as an average between two samples extracted in water and three extracted in methanol) than methylthreitol. The spectra show the occurrence of only two diastereomers among the possible four ones (González et al, 2011), indicating that the formation of methyltetrols is stereoselective, as already proposed by Cash et al (2016) on the basis of a theoretical analysis of the IEPOX chemistry, and in contrast with the conclusions of González et al (2011), claiming that methyltetrols are produced in laboratory conditions only in racemic mixtures. The two methyltetrols account for 65 % of the total 1 H-NMR signal, the rest being characterized by broad background signal with very few sharp resonances, indicating that the isoprene SOA samples are composed mainly of methyltetrols together with a significant amount of mass composed of a very complex mixture of products.…”

Chamber investigation of the formation and transformation of secondary organic aerosol in mixtures of biogenic and anthropogenic volatile organic compounds

“…Against the backdrop of the near unimaginable complexity of the atmospheric mixture of SOA precursors, the basis for our understanding of SOA formation has been primarily derived from experimental investigations of single component systems (Thornton et al, 2020;Donahue et al, 2012;Jenkin et al, 2012). A wealth of literature derived from chamber experiments on biogenic (Thornton et al, 2020;Carlton et al, 2009) and anthropogenic (Schwantes et al, 2017;Nakao et al, 2012) precursors under a range of chemical environments combined with fundamental kinetic studies (Ziemann and Atkinson, 2012;Cash et al, 2016) has enabled numerous representations of atmospheric SOA at varying levels of detail (Shrivastava et al, 2017;Charan et al, 2019). There have additionally been studies of SOA formation in source-oriented mixtures from diesel (Weitkamp et al, 2007;Nakao et al, 2011) and gasoline (Nordin et al, 2013;Platt et al, 2013) exhaust, woodburning (Tiitta et al, 2016), cooking (Reyes-Villegas et al, 2018;Kaltsonoudis et al, 2017) and from macroalgal (McFiggans et al, 2004) and plant (Joutsensaari et al, 2005;VanReken et al, 2006;Pinto et al, 2007;Mentel et al, 2009;Hao et al, 2009;Wyche et al, 2014) emissions.…”

Varying chiral ratio of Pinic acid enantiomers above the Amazon rainforest

“…However, when oligomerization based on chiral building blocks occurs, a process also relevant to secondary organic aerosol formation (Tolocka et al, 2004;Hallquist et al, 2009), changes in physicochemical properties may well occur. It has been shown that physical and chemical properties, such as melting point and water solubility, can then be determined by stereochemistry (Katsumoto et al, 2010;Baker et al, 2015;Cash et al, 2016). Thus, chirality might affect the ability of SOA formation and, consequently, influence for example the radiative forcing and cloud processing of aerosol particles.…”