Average chemical properties and potential formation pathways of highly oxidized organic aerosol

Abstract

Measurements of ambient organic aerosol indicate that a substantial fraction is highly oxidized and low in volatility, but this fraction is generally not reproduced well in either laboratory studies or models. Here we describe a new approach for constraining the viable precursors and formation pathways of highly oxidized organic aerosol, by starting with the oxidized product and considering the possible reverse reactions, using a set of simple chemical rules. The focus of this work is low-volatility oxidized organic aerosol (LV-OOA), determined from factor analysis of aerosol mass spectrometer data. The elemental composition and volatility of the aerosol enable the determination of its position in a three-dimensional chemical space (defined by H/C, O/C, and carbon number) and thus its average chemical formula. Consideration of possible back-reactions then defines the movement taken through this chemical space, constraining potential reaction pathways and precursors. This approach is taken for two highly oxidized aerosol types, an average of LV-OOA factors from ten field campaigns (average formula C10.5H13.4O7.3), and extremely oxidized LV-OOA (from Mexico City, average formula C10H12.1O8.4). Results suggest that potential formation pathways include functionalization reactions that add multiple functional groups per oxidation step, oligomerization of highly oxidized precursors, and, in some cases, fragmentation reactions that involve the loss of small, reduced fragments.

Elemental ratio measurements of organic compounds using aerosol mass spectrometry: characterization, improved calibration, and implications

“…Aerosol elemental ratios measured with the AMS have been previously used to distinguish between different types of organic aerosol Ng et al, 2010), examine the degree to which chamber SOA is able to simulate ambient SOA (Chhabra et al, 2010;Ng et al, 2010), and to constrain oxidation mechanisms used in theoretical models Kroll et al, 2011;Donahue et al, 2011;Daumit et al, 2013;Chen et al, 2014). Here we show that while the changes introduced by the Improved-Ambient method can be significant, they do not change any fundamental conclusions made from previous AMS studies.…”

“…In all of these spaces the measured bulk values of O : C, H : C, and OSc provide mechanistic insight by limiting the reaction pathways and intermediates that are potentially possible. Daumit et al (2013) have compared the difference in constraints introduced when LV-OOA elemental ratios are calculated using A-A and I-A methods (The I-A elemental ratios in Daumit et al (2013) were calculated by scaling A-A O : C and H : C ratios by 1.3 and 1.11, respectively). For the same LV-OOA volatility, elemental ratios obtained with the I-A method constrain the LV-OOA composition to contain a higher hydroxyl / carbonyl ratio than the elemental ratios obtained with the A-A method.…”

“…3.7 of this manuscript, OSc vs. carbon number, and OSc vs. saturation vapor concentration Kroll et al, 2011;Donahue et al, 2011). Daumit et al (2013) have used a three-dimensional space (carbon number, O : C, H : C) to constrain and define the chemically feasible back-reactions that could lead to the oxidized LV-OOA species observed in the atmosphere. In all of these spaces the measured bulk values of O : C, H : C, and OSc provide mechanistic insight by limiting the reaction pathways and intermediates that are potentially possible.…”

Particle size dependence of biogenic secondary organic aerosol molecular composition

“…On a molecular level, ambient organic aerosol is very complex, encompassing hundreds to thousands of individual compounds (Chan et al, 2013;Goldstein and Galbally, 2007;Hallquist et al, 2009;Mutzel et al, 2015;Putman et al, 2012). Laboratory secondary organic aerosol (SOA) is similarly complex, and a variety of oxidation and degradation pathways have been proposed to explain the product distributions (Daumit et al, 2013;Hall and Johnston, 2012;Hallquist et al, 2009;Perraud et al, 2012). Part of this complexity 7594 P. Tu and M. V. Johnston: Particle size dependence of biogenic SOA molecular composition arises from the formation of high-molecular-weight (MW) oligomeric species from two or more precursor molecules (Kalberer et al, 2004;Tolocka et al, 2004).…”

Technical Note: Development of chemoinformatic tools to enumerate functional groups in molecules for organic aerosol characterization

“…Of these components, the organic fraction can comprise as much as 80 % of the aerosol mass (Lim and Turpin, 2002;Zhang et al, 2007) and yet eludes definitive characterization due to the number and diversity of molecule types. There have been many proposals for reducing representations in which a mixture of 10 000+ different types of molecules (Hamilton et al, 2004) are represented by some combination of their molecular size, carbon number, polarity, or elemental ratios (Pankow and Barsanti, 2009;Kroll et al, 2011;Daumit et al, 2013;Donahue et al, 2012), many of which are associated with observable quantities (e.g., by aerosol mass spectrometry (AMS; Jayne et al, 2000), gas chromatography-mass spectrometry (GC-MS and GCxGC-MS; Rogge et al, 1993;Hamilton et al, 2004)). Molecular bonds or organic functional groups (FGs), which are the focus of this manuscript, can also be used to provide reduced representations for mixtures and have been shown useful for organic mass (OM) quantification, source apportionment, and prediction of hygroscopicity and volatility (e.g., Russell, 2003;Donahue, 2011;Russell et al, 2011;Suda et al, 2014).…”