Analysis of secondary organic aerosols from ozonolysis of isoprene by proton transfer reaction mass spectrometry

https://doi.org/10.1016/j.atmosenv.2014年03月04日5 Get rights and content

Highlights

  • •
    Formation of oligomeric hydroperoxides involving Criegee intermediate was observed.
  • •
    Oligomeric hydroperoxides were observed in both gaseous and aerosol phases.
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    Formation of oligomers involving carbonyls was observed in aerosols by PTR-MS.
  • •
    Partitioning of products between gaseous and aerosol phases was determined.
  • •
    Saturated vapour pressures of some oligomeric hydroperoxides were estimated.

Abstract

To understand the mechanism of formation of the secondary organic aerosols (SOAs) produced by the ozonolysis of isoprene, proton transfer reaction mass spectrometry (PTR-MS) was used to identify the semi-volatile organic compounds (SVOCs) produced in both the gaseous and the aerosol phases and to estimate the gas–aerosol partitioning of each SVOC in chamber experiments. To aid in the identification of the SVOCs, the products were also studied with negative ion-chemical ionization mass spectrometry (NI-CIMS), which can selectively detect carboxylic acids and hydroperoxides. The gaseous products were observed by on-line PTR-MS and NI-CIMS, whereas the SVOCs in SOAs collected on a filter were vaporized by heating the filter and were then analysed by off-line PTR-MS and NI-CIMS. The formation of oligomeric hydroperoxides involving a Criegee intermediate as a chain unit was observed in both the gaseous and the aerosol phases by NI-CIMS. PTR-MS also detected oligomeric hydroperoxides as protonated molecules from which a H2O molecule was eliminated, [M−OH]+. In the aerosol phase, oligomers involving formaldehyde and methacrolein as chain units were observed by PTR-MS in addition to oligomeric hydroperoxides. The gas–aerosol partitioning of each component was calculated from the ion signals in the gaseous and aerosol phases measured by PTR-MS. From the gas–aerosol partitioning, the saturated vapour pressures of the oligomeric hydroperoxides were estimated. Measurements by a fast-mobility-particle-sizer spectrometer revealed that the increase of the number density of the particles was complete within a few hundred seconds from the start of the reaction.

Introduction

Organic material accounts for a substantial fraction of atmospheric fine particulate matter, which directly and indirectly affects the global climate as well as human health (Kanakidou et al., 2005). Many gas-phase organic compounds undergo oxidation in the gas phase to yield products, generally oxygenated, that have vapour pressures sufficiently low that they are partitioned between the gas and aerosol phases. Such compounds are often referred to as semi-volatile organic compounds (SVOCs) and, when present in the aerosol phase, as secondary organic aerosols (SOAs) (Seinfeld and Pandis, 1997, Kroll and Seinfeld, 2008, Hallquist et al., 2009). Quantification of the impacts of SOAs requires understanding their chemical composition and processes of formation as well as mass yields (Camredon et al., 2010). In particular, because a systematic underestimation of simulated SOA production increases with air mass ageing (Volkamer et al., 2006), speciation of the SVOCs produced by gaseous oxidation is essential. In addition, information about the partitioning of each SVOC between the gaseous and condensed phases as well as the reactions of the condensed SVOCs within the particulate phase is important (Camredon et al., 2010) for the description of SOA formation, for example, in 3-D chemical transport models (Fuzzi et al., 2006, Lane et al., 2008).
Proton transfer reaction mass spectrometry (PTR-MS) has been used as a powerful tool for measuring the concentrations of multiple SVOCs produced by gaseous oxidation of precursor VOCs in smog chamber studies (e.g., Blake et al., 2009). Proton transfer is an example of chemical ionization (CI); it enables soft ionization of chemical species that have a proton affinity higher than that of water:H3O++SVOC→SVOC⋅H++H2O
Another advantage to the use of PTR-MS is that the concentration of an SVOC can be calculated from the following kinetic relationship (Lindinger et al., 1998):[SVOC]≈i(SVOC⋅H+)k⋅t⋅i(H3O+)where i(H3O+) and i(SVOC·H+) represent the count rates of the reagent ion, H3O+, and the product ion, SVOC·H+, respectively; k represents the rate constant for the proton transfer reaction; and t represents the reaction time. This property is useful for determining SVOC concentrations, the determination of which is otherwise difficult.
With regard to analysis of the organic compounds in SOAs, off-line analyses in which SOAs are collected on a filter are typically coupled with gas chromatography/MS and liquid chromatography/MS (e.g., Hallquist et al., 2009). Recently, oligomers have been detected by direct injection of SOA samples into a mass spectrometer coupled with laser desorption ionization, electrospray ionization, and atmospheric pressure chemical ionization (e.g., Hallquist et al., 2009). Although PTR-MS is suitable for detecting oxygenated VOCs, only a few studies have been reported in which PTR-MS was used for an analysis of SOAs (Hellén et al., 2008, Holzinger et al., 2010). One of the difficulties in the application of PTR-MS to the analysis of SOAs is that aldehydes, alcohols, hydroperoxides, and carboxylic acids, which are thought to be major organic components of SOAs, fragment during PTR ionization (Warneke et al., 2003), and as a result, the identification of chemical species is difficult. Another problem that also confounds the identification of chemical species is the fact that PTR-MS is able to detect most organic compounds, and the ion signals of isobaric compounds as well as isomeric compounds overlap (Wyche et al., 2005).
In the present work, we used PTR-MS to identify SVOCs produced by the ozonolysis of isoprene in both the gaseous and the aerosol phases and to estimate the gas–aerosol partitioning of each SVOC. By using the same technique to measure SVOCs in both the gaseous and the aerosol phases, we were able to determine the partitioning of each SVOC between the gaseous and aerosol phases from the ratio of ion signals, without knowing the concentration of each SVOC. This ability to partition each SVOC between the gaseous and aerosol phases is a strong point of this approach, because most chemical species in SOAs are thought to be multifunctional, and determining their concentrations seems to be impossible. In a previous study, Hellén et al. (2008) determined the partitioning coefficient of pinonic acid by PTR-MS. To facilitate the identification of SVOCs, we also used negative ion-chemical ionization mass spectrometry (NI-CIMS) with SO2Cl− ions as the reagent ion (Hirokawa et al., 2009) in the analysis of gaseous- and aerosol-phase products. This technique has recently been demonstrated to be selective for carboxylic acids and hydroperoxides produced in the ethylene ozonolysis (Sakamoto et al., 2013).
Isoprene is the most abundant VOC emitted into the atmosphere. It originates mainly from biogenic sources (Guenther et al., 2006), and its oxidation in the atmosphere is thought to be responsible for a large amount of SOA production. Reactions involving the hydroxyl (OH) radical are a major mechanism of oxidation in the daytime. Many research groups have extensively investigated the yields of SOAs and their formation mechanisms (e.g., Carlton et al., 2009). The nitrate radical reaction occurs during the night (Ng et al., 2008). The SOA yield of the O3 reaction is known to be small compared with the yields of other reactions (Kleindienst et al., 2007, Sato et al., 2013). It should be noted that the O3 reaction occurs during both the day and the night. Therefore, because the ozone reaction can couple with other oxidation processes, we think that it is important to understand the mechanism of SOA formation in the O3 reaction. Nguyen et al. (2010) have recently used high-resolution electrospray ionization mass spectrometry (ESI-MS) to analyse the chemical compositions of SOAs produced by the ozonolysis of isoprene and have found oligomers that consist of small carbonyls with a C1–C2 skeleton. The basic chemical process of isoprene ozonolysis is summarized as Scheme 1 in Fig. A1 of the Supplementary Material.
Recently, we investigated ethylene ozonolysis and found that stabilized Criegee intermediates (SCIs), CH2OO, can produce oligomers via reactions with formic acid and hydroperoxides in the gaseous phase. Moreover, we demonstrated that these oligomers can contribute to SOA formation from the ethylene ozonolysis (Sakamoto et al., 2013). Because a variety of carboxylic acids and hydroperoxides are known to be produced in isoprene ozonolysis (see Fig. A1 of the Supplementary Material), oligomers involving SCIs are expected to participate in SOA formation from the isoprene ozonolysis. In the present study, we used mass spectrometry coupled with two types of chemical ionization to identify the SVOCs produced by the ozonolysis of isoprene and to determine the partitioning of each SVOC in the gaseous and the aerosol phases.

Section snippets

1-m3 Teflon bag experiments

On-line measurements of products of the ozonolysis of isoprene by PTR-MS and NI-CIMS were carried out using a pillow-shaped bag made of a copolymer of hexafluoropropylene and tetrafluoroethylene (FEP; bag volume 1 m3; 1800 ×ばつ 1570 mm; thickness of the FEP sheet: 50 μm, GL Sciences, Tokyo, Japan). The experimental setup was similar to that used by Sakamoto et al. (2013). Ozone was produced by irradiation of air with vacuum ultraviolet light from a low-pressure mercury lamp during the introduction

Gaseous products

Fig. 1(a) and (b) show mass spectra of gaseous products obtained by PTR-QMS and NI-CIMS, respectively, after a 2-h reaction of isoprene with ozone without an OH radical scavenger (hereafter, referred to as w/o scavenger). We shifted the horizontal axes in these figures from m/z to [m/z−1] and [m/z−35] because we expected protonated molecules and Cl−-adducts to be detected by PTR-QMS and NI-CIMS, respectively. In Fig. 1(b), the strongest peak at [m/z−35] of 64 corresponds to signals of the

Summary

We used PTR-MS to measure the SVOCs produced from the ozonolysis of isoprene in both the gaseous and the aerosol phases. The compounds were identified with the aid of NI-CIMS. The formation of oligomeric hydroperoxides involving CH2OO as a chain unit was observed by NI-CIMS in both the gaseous and the aerosol phases, and we found that PTR-MS could detect oligomeric hydroperoxides as [M–OH]+ ions. In the aerosol phase, oligomers involving H2CO and MACR as a chain unit in addition to the

Acknowledgements

This work was financially supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Sports, Science and Technology of Japan (No. 20120003, FY 2008-2012; No. 20120005, FY 2008-2012; No. 20120007, FY 2008-2012), by a grant from the Steel Industry Foundation for the Advancement of Environmental Protection Technology (05-06Taiki-157, FY2005-2006), and by a Grant for Environmental Research Projects from The Sumitomo Foundation (No. 123449,

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    Present address: Graduate School of Agriculture, Kyoto University, Kitashirakawa Oiwake-Cho, Sakyo-ku, Kyoto 606-8502, Japan.
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