Oligomers in the Early Stage of Biogenic Secondary Organic Aerosol Formation and Growth

Abstract

The formation of secondary organic aerosol (SOA) by reaction of ozone with monoterpenes (beta-pinene, delta3-carene, limonene, and sabinene) was studied on a short time scale of 3-22 s with a flow tube reactor. Online chemical analysis was performed with the Photoionization Aerosol Mass Spectrometer (PIAMS) to obtain molecular composition and the Nanoaerosol Mass Spectrometer (NAMS) to obtain elemental composition. Molecular composition data showed that dimers and higher order oligomers are formed within seconds after the onset of reaction, indicating that there is no intrinsic kinetic barrier to oligomer formation. Because oligomer formation is fast, it is unlikely that a large number of steps are involved in their formation. Therefore, ion distributions in the PIAMS spectra were interpreted through reactions of intermediates postulated in previous studies with monomer end products or other intermediates. Based on ion signal intensities in the mass spectra, organic peroxides appear to comprise a greater fraction of the aerosol than secondary ozonides. This conclusion is supported by elemental composition data from NAMS that gave C:O ratios in the 2.2-2.7 range.

Oligomer Content of α-Pinene Secondary Organic Aerosol

“…Unique molecular formulas were assigned to more than 1000 peaks for each extract obtained in this study, spanning the molecular mass range of monomers, dimers, and higher order oligomers. The distribution of assigned peaks, for example as expressed in a van Krevelen plot of oxygen to carbon (O:C) ratio vs. hydrogen to carbon (H:C) ratio, was similar to those reported in the literature for SOA samples from α-pinene ozonolysis (Heaton et al 2007(Heaton et al , 2009Reinhardt et al 2007;Walser et al 2008), and therefore will not be discussed in detail here. Table 3 provides a succinct summary of the variation of these products by comparing the assigned peaks from different measurements of the same SOA sample.…”

“…Aerosol mass concentration measurements were made sequentially with SMPS and OCEC by switching the valve back and forth. For some OCEC measurements, a flow-tube reactor (FTR), described elsewhere (Heaton et al 2007;Heaton et al 2009;Tolocka et al 2006), was placed after the three-way valve to simulate the sample stream dilution in the SMPS (0.3 LPM sample flow and 1.0 LPM sheath flow). Five L/min of clean, dry air was introduced to the FTR as a sheath (dilution) flow where it mixed with 1.5 L/min of aerosol flow from the bag.…”

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

“…Prompt formation of ELVOCs at the beginning of the ozonolysis of α-pinene is crucial to initiate organic particle growth. The observation that most ELVOC dimers grow faster than SVOCs and LVOCs indicates that there is no intrinsic kinetic barrier to the ELVOC dimer formation (16). The role of aging in the α-pinene SOA evolution is reflected, to a certain degree, by the slow but continuous growth of LVOCs over the 4-h course of an experiment.…”

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.…”