Highly Oxygenated Multifunctional Compounds in Î±-Pinene

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Highly Oxygenated Multifunctional Compounds in #-pinene Secondary Organic Aerosol Xuan Zhang, Andrew T. Lambe, Mary Alice Upshur, William A. Brooks, Ariana Gray Be, Regan J. Thomson, Franz M. Geiger, Jason Douglas Surratt, Zhenfa Zhang, Avram Gold, Stephan Graf, Michael J. Cubison, Michael Groessl, John T. Jayne, Douglas R. Worsnop, and Manjula R. Canagaratna Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 26 Apr 2017 Downloaded from http://pubs.acs.org on April 26, 2017

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Environmental Science & Technology

Highly Oxygenated Multifunctional Compounds in α-pinene Secondary Organic Aerosol

Xuan Zhang,†, # Andrew T. Lambe,† Mary Alice Upshur,‡ William A. Brooks,† Ariana Gray Bé,‡ Regan J. Thomson,‡ Franz M. Geiger,‡ Jason D. Surratt,§ Zhenfa Zhang,§ Avram Gold,§ Stephan Graf,¶ Michael J. Cubison,¶ Michael Groessl,¶ John T. Jayne,† Douglas R. Worsnop†, and Manjula R. Canagaratna*,†

†

Center for Aerosol and Cloud Chemistry, Aerodyne Research Inc., Billerica, MA 01821, USA

‡

Department of Chemistry, Northwestern University, Evanston, IL 60208, USA

§

Department of Environmental Sciences and Engineering, Gillings School of Global Public

Health, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA ¶ #

TOFWERK, CH-3600 Thun, Switzerland Now at Atmospheric Chemistry Observations & Modeling Laboratory (ACOM), National

Center for Atmospheric Research (NCAR), Boulder, CO 80301, USA

1

ACS Paragon Plus Environment


1

ABSTRACT

2

Highly oxygenated multifunctional organic compounds (HOMs) originating from

3

biogenic emissions constitute a widespread source of organic aerosols in the pristine

4

atmosphere. Yet, the molecular forms in which HOMs are present in the condensed phase

5

upon gas-particle partitioning remain unclear. In this study, we show that highly

6

oxygenated molecules that contain multiple peroxide functionalities are readily

7

cationized by the attachment of Na+ during electrospray ionization operated in the

8

positive ion mode. With this method, we present the first identification of HOMs

9

characterized as C8-10H12-18O4-9 monomers and C16-20H24-36O8-14 dimers in α-pinene

10

derived secondary organic aerosol (SOA). Simultaneous detection of these molecules in

11

the gas phase provides direct evidence for their gas-to-particle conversion. Molecular

12

properties of particulate HOMs generated from ozonolysis and OH-oxidation of

13

unsubstituted (C10H16) and deuterated (C10H13D3) α-pinene are investigated using coupled

14

ion mobility spectrometry with mass spectrometry. The systematic shift in the mass of

15

monomers in the deuterated system is consistent with decomposition of isomeric

16

vinylhydroperoxides to release vinoxy radical isotopologues, the precursors to a sequence

17

of autoxidation reactions that ultimately yield HOMs in the gas phase. The remarkable

18

difference observed in the dimer abundance under O3- versus OH-dominant environments

19

underlines the competition between intramolecular hydrogen migration of peroxy radicals

20

and their bimolecular termination reactions. Our results provide new and direct

21

molecular-level information for a key component needed for achieving carbon mass

22

closure of α-pinene SOA.

23 24 25 26 27 28

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INTRODUCTION

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Atmospheric aerosols affect Earth’s energy budget by absorbing and scattering

31

radiation and by modifying microphysical and radiative properties of clouds. Globally,

32

new particle formation from the nucleation of atmospheric vapors accounts for over half

33

of the atmosphere’s cloud condensation nuclei.1 It has long been recognized that sulfuric

34

acid is essential to initiate most new particle formation in the atmosphere. Recently, a

35

family of highly oxygenated multifunctional compounds (HOMs) has been demonstrated

36

to play a major role in driving the initial particle growth in terrestrial environments that

37

are largely unaffected by anthropogenic activities.2

38

Accumulating observations suggest that HOMs are readily produced from the

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ozonolysis of endocyclic alkenes under atmospherically relevant concentrations.3-6 The

40

well-known autoxidation mechanism of inter- and intramolecular hydrogen abstraction

41

by peroxy radicals has emerged to explain the prompt HOMs production. As illustrated in

42

Scheme 1, the proxy radical (RO2×) undergoes an internal H-shift followed by sequential

43

O2 addition, leading to a hydroperoxide (–OOH) functionalized peroxy radical. The rate

44

of H-shift largely depends on the thermochemistry of the nascent alkyl radicals and can

45

be reasonably fast, on a time scale of seconds or less.7 The newly formed peroxy radical

46

could undergo successive H-shifts and O2-additions to increase the extent of oxygenation,

47

if additional acidic hydrogen atoms are present. The radical chain reaction is propagated

48

until termination occurs by either ejection of an OH or HO2 radical,8 or by bimolecular

49

reactions with NO/HO2/RO2, leading to the closed-shell products consisting of peroxides,

50

carbonyls, alcohols, and other potential functionalities.

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Owing to their (extremely) low volatilities, HOMs are expected to undergo nucleation

52

and irreversible condensation onto existing particle surfaces, producing secondary

53

organic aerosols (SOA).2, 3 Despite the well-recognized role of HOMs in initiating new

54

particle growth and contributing to SOA production, their fate upon gas-particle

55

partitioning remains elusive. Recent efforts have been devoted to characterizing a

56

particular functional group such as total carbonyl or peroxide content in HOMs and infer

57

their chemical transformation in aerosols.5, 9 To our knowledge, molecular speciation and

3



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structural analysis of HOMs in the condensed phase have not yet been performed and

59

remain a challenging task due to their highly labile and short-lived nature.

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The main objective of this study is to characterize the molecular signatures and

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chemical structures of highly oxygenated multifunctional organic compounds in α-pinene

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derived SOA. What distinguishes the present approach includes 1) the demonstrated

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electrospray ionization scheme that maintains the intact structure of HOMs by producing

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sodiated molecular ion adducts; 2) the coupled ion mobility and mass-to-charge ratio

65

measurements that provide isomeric information on the oxidation products under O3-

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versus OH-dominant environments; and 3) the mobility-selected collision-induced

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dissociation technique that facilitates the structural elucidation of the observed dimer

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components in α-pinene SOA.

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EXPERIMENTAL SECTION

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Materials.

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Synthesized 1,2-isoprene hydroxyhydroperoxides (>99% purity)10 and lauroyl

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peroxide (Sigma-Aldrich, 97% purity) dissolved in methanol were used to elucidate the

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ionization scheme of the organic peroxide functionality during electrospray ionization in

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positive mode. Dioctylphthalate (Sigma-Aldrich, >99.5% purity) and dibutyloxalate

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(Sigma-Aldrich, 99% purity) were used to examine the fragmentation pattern of the ester

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moiety upon collision induced dissociation.

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Experimental procedures.

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Natural abundance α-pinene (Sigma-Aldrich, >99% purity) and synthesized 2-methyl-

79

d3 α-pinene isotopologue (≥98% purity)11 were injected into a round-bottom flask held at

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310 K using a gas-tight syringe (Hamilton, NV, USA) held on a syringe pump (Chemyx,

81

TX, USA) at a flow rate of 5.9 nL min-1. The organic liquid coming out of the tip of the

82

syringe evaporates rapidly and the resulting organic vapors were transported into a

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Potential Aerosol Mass oxidation flow reactor (PAM)12 by a N2 flow of 1 L min-1. The

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initial α-pinene concentration in the PAM reactor is approximately 100 ppb based on a

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mass conservation calculation.13 In both ozonolysis and photooxidation experiments,

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ozone (~5 ppm) was generated upstream of the PAM reactor by passing O2 through an 4


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aluminum cylinder with irradiation generated from a mercury lamp (λ = 185 nm). In

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photooxidation experiments, hydroxyl radicals were produced via the coupled reactions

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O3 + hν → O2 + O(1D) (l = 254 nm) and O(1D) + H2O → 2OH. Two mercury lamps

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emitting 254 nm radiation were used (estimated I254 ~ 3.2´1015 photons cm-2 s-1). The

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integrated OH exposure was calculated using SO2 as a tracer species in separate

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calibration experiments.14 At the OH concentration (~1010 molec cm-3) in photooxidation

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experiments, the calculated lifetimes of a-pinene with respect to reactions with O3 and

94

OH are 97 s and 2 s, respectively. Therefore, OH oxidation should be the dominant loss

95

pathway for a-pinene. The PAM reactor was operated at a temperature of ~295 K, a

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relative humidity of ~40%, and a residence time of ~90 s. Secondary organic aerosols

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produced via nucleation of organic vapors from ozonolysis and OH-oxidation of α-pinene

98

were collected on Teflon filters (MILLIPORE, 47-mm diameter, 0.5-µm pore size)

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through active sampling at a flow rate of 9 L min-1 for ~12 hr. Filters were stored in a -20

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°C freezer for less than 24 hours prior to analysis. SOA samples were extracted in 10 mL

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HPLC-grade methanol (Sigma-Aldrich, >99.9% purity) by 60 min of sonication at 273 K

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and then concentrated to 1 mL with the assistance of a 5 L min-1 N2 stream.15

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Analytical methods.

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Mass spectra of gas-phase products from a-pinene ozonolysis were obtained with an

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Aerodyne high-resolution Time-of-Flight Chemical Ionization Mass Spectrometer using

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iodide-adducts (iodide-CIMS).16 The reagent ion iodide clusters with the analyte,

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producing molecular ions at m/z ([M+I]- ), where M is the analyte molecular weight.

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CIMS data were analyzed using the ‘Tofware’ software package (version 2.5.7b,

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www.tofwerk.com/tofware) implemented in the Igor Pro (Wavemetrics, OR, USA).

110

Off-line sample analysis was performed with Electrospray Ionization (ESI) Drift-Tube

111

Ion Mobility Spectrometer (DT-IMS) interfaced to a high-resolution Time-of-Flight Mass

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Spectrometer (TOFMS). This instrument is designed and manufactured by TOFWERK

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(Switzerland), with detailed descriptions and schematics given by a few recent studies.17,

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18

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measurement.

Here we present the instrument operation protocols pertinent to the HOMs

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Analyte solution was delivered to the ESI source via a gas-tight syringe (Hamilton,

117

NV, USA) held on a syringe pump (Harvard Apparatus, MA, USA) at a flow rate of 1 µL

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min-1. Species that do not generate stable positive ions through protonation were ionized

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by clustering with Na+ cations that are naturally present in the solvent chemicals and

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glassware.19 The optimal ESI potential for generation of sodiated ion adducts ([M+Na]+)

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was found to be +1800 V. The produced ion adducts were introduced into the drift tube

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with the assistance of 1 L min-1 nitrogen sheath gas. The drift tube was held at a constant

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temperature (340±3 K) and atmospheric pressure (~1019 mbar). A counter-flow of N2

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drift gas was introduced at the end of the drift region at a flow rate of 1.2 L min-1. Ion

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mobility separation was carried out at the field strength of 370 V cm-1. Mobility-selected

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sodiated ion adducts exit from the drift tube through a coupled ion lens and nozzle, with

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an applied potential of 200 V to ensure optimal transmission while minimizing intensive

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fragmentation of the molecular ion adducts. After exit from the drift tube, ion adducts

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were focused into a pressure-vacuum interface that includes two segmented quadrupoles.

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The frequency and amplitude were set as 1.5 ´ 106 Hz and 196 V for Q1 and 1.5 ´ 106 Hz

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and 250 V for Q2, respectively. Collision induced dissociation (CID) can be performed by

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adjusting the voltages on the ion optical elements between the two quadrupoles. Over one

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CID course, the voltage between Q1 and Q2 was programed to increase from 0 V to 60 V

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in increments of 10 V.

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The ESI-IMS-TOFMS instrument was operated over the range m/z 20 to 1500 Th with

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a total recording time of ~2 min and ~30 min for regular and CID datasets, respectively.

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Prior to each measurement, accurate calibration of the mass-to-charge ratio scale was

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completed using an equi-molar (1 µM each) mixture of tetraethyl ammonium chloride

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(Sigma-Aldrich, ≥98% purity), tetrapropyl ammonium chloride (Sigma-Aldrich, 98%

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purity), tetrabutyl ammonium iodide (Sigma-Aldrich, 98% purity), tetrapentyl

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ammonium chloride (Sigma-Aldrich, 99% purity), tetraheptyl ammonium chloride

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(Sigma-Aldrich, 98% purity), and reserpine (Sigma-Aldrich, ≥98% purity) in the positive

143

mode. Accurate ion mobility calibration was completed using 1 µM tetraethyl ammonium

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chloride (Sigma-Aldrich, ≥98% purity) in methanol as the instrument standard. Mass

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spectra and ion mobility spectra were collected by the acquisition package ‘Acquility’

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(version 2.1.0, www.tofwerk.com/acquility). Post-processing was performed using the 6


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data analysis package ‘Tofware’ (version 2.5.3, www.tofwerk.com/tofware). Both

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packages run in the Igor Pro (Wavemetrics, OR, USA) environment.

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RESULTS AND DISCUSSION

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Overview of HOMs in α-pinene SOA.

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HOMs molecules are identified in the condensed phase as their sodiated ion adducts

152

([M+Na]+) during the positive electrospray process. Here molecules with the ratio of

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oxygen to carbon atoms (O:C) exceeding 0.5 are considered as highly oxidized. This

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definition aims to distinguish these newly observed HOMs molecules from previously

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identified less oxidized species such as pinic acid in a-pinene SOA.20 As discussed

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earlier, the autoxidation of peroxy radicals in the gas phase leads to the formation of

157

HOMs molecules that contain multiple hydroperoxide functional groups. The Na+ adduct

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ionization scheme used in this study allows for the detection of organic (hydro)peroxides

159

that do not generate stable positive ions through protonation,21 as confirmed by the

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ionization patterns of isoprene hydroxyl-hydroperoxide and lauroyl peroxide (Figure S1

161

of the Supplementary Information). In addition to the peroxide functionality, previous

162

studies have shown that the HOMs dimers contain ester linkages formed during the gas-

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particle partitioning process.2, 20 Under the positive electrospray conditions employed in

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the present study, esters and polyols are ionized by the attachment of Na+ while

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molecules with carbonyl, carboxylic acid, and isolated alcohol functionalities, which are

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also potentially present in HOMS, yield weak or negligible amounts of sodiated ion

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adducts, see examples given in Figure S2 of the Supplementary Information.

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Figure 1 gives an overview of the particle-phase molecular products from ozonolysis

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of a-pinene mapped on the mass defect plot and the carbon number – oxidation state

170

(nC − OSC ) space. A spectrum of monomers and dimers, with molecular formulas of C8-

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10H12-18O4-9

172

account for ~84% of the overall signal intensity in the positive mass spectra with m/z

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ranging from 200 to 490 Th. The depicted mass defects (the deviation of the accurate

174

mass of a given molecule from its nominal mass) emphasize the high oxygen content of

175

the identified products. Following the blue dashed line, the products are distributed by a

and C16-20H24-36O8-14, respectively, is observed. These identified products

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176

progressive addition of one oxygen atom. The orange dashed line depicts the product

177

distribution pattern with one hydrogen atom as the interval. The monomers in general

178

exhibit lower mass defect yet higher oxygen to carbon ratio than the dimers, implying

179

that deoxygenation is involved during the dimer formation.

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A number of molecules observed in the particle phase (e.g., C10H14-18O6-8 and C17-

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20H26-32O5-9)

are also detected in the gas phase with the iodide-CIMS, as shown in Figure

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2. The distribution pattern of these molecules on the ESI positive mass spectra, on the

183

other hand, is distinct from that presented on the iodide-CIMS negative mass spectra.

184

First, the dominant monomers observed in the particle phase possess six or less oxygen

185

atoms whereas compounds with much higher oxygen content (nO ~ 7-9) account for a

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large fraction of the gas-phase monomeric products. This difference is likely attributed to

187

the different affinities of selected reagent ions (e.g., Na+ in ESI vs. I- in CIMS) towards

188

molecules with a wide range of polarities. Second, the relative fraction of dimers is

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notably higher in the particle phase compared to the simultaneous gas-phase

190

measurements. Enhanced irreversible partitioning of dimers onto the aerosol surface is

191

expected owing to their (extremely) low volatilities. As described below, interferences in

192

the dimer region due to electrospray induced clustering are minor and can be

193

distinguished by examining the dimer distribution patterns on the 2-D ion mobility vs.

194

mass to charge ratio space.

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Ozonolysis versus OH Oxidation.

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Figure 3 shows the 2-D ion mobility spectra (displayed as drift time) coupled with

197

positive ion mass spectra (displayed as mass-to-charge ratio) of the particle-phase

198

products generated from (A) ozonolysis and (B) OH-oxidation of α-pinene. Note that the

199

ion mobility is a quantity related to the structure and geometry of the ionized molecules.

200

Species with extended molecular geometry undergo more collisions with the buffer gas

201

and thus travel more slowly (longer drift time) than the compact ones. The IMS resolving

202

power (t/dt50 ≥ 100) in this work leads to a baseline separation of molecules that differ in

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the measured drift time by several hundreds of nanoseconds, thus capable of separating

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molecules that are not resolvable with conventional analytical methods such as liquid and

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gas chromatography.17

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One remarkable feature of the product distribution on the 2-D space is that a majority

207

of dimers generated during ozonolysis are rarely observed from the OH-initiated

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oxidation pathway, although a number of monomers with identical mass to charge ratios

209

are present under both conditions. This observation shows that the dimers observed here

210

are not artifacts from the electrospray-induced accretion reactions of the common

211

monomers from both ozonolysis and OH-oxidation routes. Instead, the less favored dimer

212

production in the OH-dominant regime implies that the potential dimer source is inhibited

213

in the presence of a large amount of OH radicals. The suppressed formation of dimers

214

under OH-dominant conditions have been observed by earlier studies,20, 22 although their

215

molecular properties are different from those identified in this work. The majority of the

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dimers revealed here (C16-20H24-36O8-14) exhibit higher oxygen content compared with

217

those reported previously (C16-19H24-28O7,8). A mechanistic understanding of the chemical

218

regime that favors dimer formation is discussed shortly.

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A number of monomers observed in both systems, while sharing the identical mass to

220

charge ratios, exhibit distinct drift times (Figure S3 of the Supplementary Information),

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thereby indicative of structurally different isomers produced from pathways that are

222

unique to O3- vs. OH-prevalent environments. Furthermore, slightly oxidized monomers

223

(nO = 3-5) predominate in the 2-D spectra of the particulate products from the OH

224

oxidation pathway, implying a rapid oxygenation process following the OH addition to

225

the endocyclic double bond of α-pinene. These observations are in line with the

226

previously suggested mechanism on the prompt opening of the strained four-membered

227

ring upon OH oxidation, as it opens up an alternative pathway for the formation of

228

organic hydroperoxides, see the outlined scheme in Figure S4 of the Supplementary

229

Information.23,

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oxidation channel is less favored compared with the ozonolysis pathway, likely due to the

231

suppression of the peroxy radical combination that ultimately yields pinonaldehyde as the

232

precursor to a sequence of further autoxidation reactions.

24

Production of highly oxidized molecules (nO = 6-9) from the OH-

233

Molecular identity of monomers.

234

The reaction of ozone with the 2-methyl-d3 α-pinene isotopologue (C10H13D3) was

235

performed to understand the molecular identity of the monomer products observed in the

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particle phase. As shown in Figure 4 A and B, ozonolysis of deuterated a-pinene results

237

in a much denser mass spectrum of the SOA molecular components. No significant

238

change in the dimer to monomer ratio is observed, implying an insignificant kinetic

239

isotope effect on the intramolecular hydrogen migration rate. A close examination of

240

selected ion clusters (m/z 232-244) colored in magenta reveals three distinct monomer

241

distribution features. First, the dominant ion at m/z 239 (C10H16O5) is shifted to m/z 242

242

(C10H13D3O5) when deuterated a-pinene is used as the precursor, suggesting that the

243

major reaction pathway yielding the C10(H/D)16O5 isotopologues favors the retention of

244

the three deuterium atoms. Second, the observed product sequence with 2 (Th) m/z

245

interval resulting from different unsaturation levels disappears in the mass spectrum of

246

the deuterated a-pinene SOA, implying that additional reaction pathways yielding the

247

C10(H/D)16O5 isotopologues likely lead to the removal of one deuterium atom. Third, the

248

relative abundance of individual ion peaks has changed in the deuterated system, with a

249

slight shift in intensity to ions at lower m/z. These three features are observed across the

250

mass spectra of ion adducts that are representative of C8-10(H/D)8-20O3-9 monomers

251

(Figure S5 of the Supplementary Information).

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The observed m/z shift patterns support gas-particle partitioning of species formed

253

from the well-established vinylhydroperoxide (VHP) reaction channel during the early

254

stage of ozonolysis, i.e., the hydrogen migration mechanism for alkyl-substituted Criegee

255

Intermediate.25 Figure 4 C outlines the 1,4-hydrogen transfer from the deuterated methyl

256

group (blue arrows) and the tertiary carbon on the four-membered ring (green arrows) to

257

the terminal oxygen of two Criegee syn-conformers, yielding OD and OH radicals,

258

respectively. The resulting vinoxy radicals rapidly combine with O2, producing the

259

C10(H/D)15O4× peroxy radicals as the precursor to a sequence of autoxidation reactions.

260

Despite the presence of the rigid four-membered ring that could limit the availability of

261

active hydrogens, the coiled structures of the syn-conformers of the C10(H/D)15O4× peroxy

262

radicals facilitate the H-abstraction from the aldehyde group. Here, the rapid 1,9-H and

263

1,7-H shift pathways, with the computed isomerization rates of 1.5×10-3 s-1 and 0.14 s-1,26

264

are proposed to rationalize the observed ion adducts at m/z 242 (C10H13D3O5) and m/z 241

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(C10H14D2O5), respectively. A unified reaction mechanism yielding a range of

266

C10(H/D)2,3O4-9 monomer isotopologues are given by Figure S6 of the Supplementary 10


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Information. Further note that the observed slight intensity shift to ions at lower m/z is

268

likely caused by reactions leading to additional deuterium losses, for example, the OD

269

ejection from a hydroperoxide radical (×COOD) yielding a carbonyl group.8

270

Toward structural analysis of dimers.

271

The IMS-MS technique used in this study enables the collision induced dissociation

272

(CID) of a given precursor ion after mobility separation but prior to mass selection. As a

273

consequence, the resulting fragment ions exhibit the identical mobility (drift time) with

274

that of the precursor ion. Here structural characterization of HOMs dimers is performed

275

by examination of the ‘mobility-selected’ mass spectra that contain a given precursor

276

dimer ion and its fragment ions upon CID. Figure 5A shows the mobility spectra for a

277

pair of precursor (m/z 409) and fragment (m/z 239) ions as a representative example of

278

the dimer series examined. A broad peak appears at a drift time window of 52-57 ms for

279

the ion at m/z 409 and is assigned to the isomeric products with a generic formula

280

C19H30O8 from a-pinene ozonolysis. Two mobility peaks are observed for the ion at m/z

281

239, with the dominant peak at 40-42 ms assigned to a monomer product (C10H16O5), and

282

the small peak at 52-57 ms assigned to the fragment ion resulting from the dissociation of

283

the precursor ion at m/z 409.

284

The intensity profiles of the mobility selected precursor-fragment ion pairs as a

285

function of the CID voltage lend further confidence to the mobility peak assignment, see

286

Figure 5B. At low CID voltage, the precursor ion at m/z 409 (52-57 ms) predominates

287

with transmission optimized at approximately 0-10 V potential gradient. As the collision

288

voltage increases, the intensity of the precursor ion at m/z 409 (52-57 ms) decreases,

289

whereas that of the corresponding fragment ion at m/z 239 (52-57 ms) increases,

290

eventually reaching a maximum level, and then decreases, likely due to the fact that

291

alternative fragmentation pathways become available at higher collisional energies. This

292

negative correlation has been observed for a series of precursor dimer and fragment

293

monomer ion pairs (Figure S7 of the Supplementary Information). The voltage at half

294

signal maximum of most precursor dimer ions is around 20 V to 30 V, consistent with

295

that for the ester standard examined, thus demonstrating that the dimer products observed

296

here are covalently bonded.

11



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Figure 5C outlines the McLafferty-type rearrangement mechanism with which the

298

C19H30O8×Na+ (m/z 409) dimer decomposes to the C10H16O5×Na+ (m/z 239) monomer.

299

Such a rearrangement involves an intramolecular g-hydrogen transfer through a six-

300

membered ring intermediate to the carbonyl moiety that serves as the nucleophile. This

301

particular charge retention fragmentation pathway, i.e., the charge of fragment ions is

302

located at the site identical to its precursor ion, is common for protonated and cationized

303

molecules.27 We show the predominance of this rearrangement reaction in the collision-

304

induced dissociation of the ester moiety by examining the fragmentation patterns of a few

305

ester standards (see examples in Figure S8 of the Supplementary Information). While the

306

McLafferty-type rearrangement of the ester functionality is a plausible mechanism to

307

rationalize the charge-retention decomposition of the C19H30O8×Na+ (m/z 409) dimer to

308

the C10H16O5×Na+ (m/z 239) monomer, it is likely that other fragmentation pathways are

309

also involved as the broad mobility peak at 52-57 ms is comprised of several peaks and

310

thus indicates the presence of multiple isomers in the dimer ion adducts at m/z 409. It is

311

noteworthy that the hemolytic cleavage of the peroxy bond (O-O) as a dimer

312

fragmentation pathway can be excluded because it leads to free radicals instead of the

313

closed-shell monomers.

314

That the ester functionality is present in the dimer components of a-pinene SOA is in

315

line with previous observations.20, 22, 28 Mechanisms that have been proposed to explain

316

the ester dimer formation include reactions of stabilized Criegee Intermediates with

317

carboxylic acids,22, 29, 30 as well as the gas-phase combination of acetyl peroxy radicals

318

yielding diacylperoxides and their subsequent decomposition upon gas-particle

319

partitioning.20 As discussed earlier, dimer production is significantly suppressed when

320

OH radicals become the primary oxidant. This could be caused by the limited supply of

321

Criegee Intermediates under OH-oxidation dominant environment, or the changing fate

322

of peroxy radicals from self-combination to reaction with HO2. Further studies are

323

necessary in order to determine the contribution of different reaction routes to the

324

abundance of ester dimers in a-pinene SOA.

12


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325

Atmospheric implications

326

We show the presence of highly oxidized multifunctional organic molecules that span

327

a wide range in volatility and carbon chain length in α-pinene SOA. The molecular

328

signature of identified particulate HOMs is largely consistent with those measured in the

329

gas phase by earlier studies.2,

330

selectively deuterating the α-pinene precursor highlights the role of intramolecular

331

hydrogen migration in increasing the oxygen content of the end products. The

332

distribution pattern of HOMs produced from α-pinene ozonolysis appears to be different

333

from that generated under OH-oxidation dominant regimes. Monomeric products from α-

334

pinene+OH reactions exhibit less extent of oxygenation compared with those produced

335

from ozonolysis of α-pinene. The observed dimer products in the ozone dominant

336

environment mostly disappear when OH becomes the primary oxidant, implying that

337

peroxy radical and stabilized Criegee Intermediate play a role in the dimer production.

3, 31

The systematic mass shift in monomers upon

338

Chemical characterization of SOA components from ozonolysis of α-pinene has

339

received much attention in the past. Identification of multifunctional products in the

340

particle phase has been reported, including monomers with carboxylic acid moieties (e.g.,

341

terpenylic acid, pinalic acid, pinic acid, and pinonic acid)32 and dimers with ester

342

moieties (e.g., pinyl-diaterebyl ester, pinyl-diaterpenyl ester, and pinonyl-pinyl ester).22, 29

343

While these identified products account for a significant fraction of the overall α-pinene

344

SOA mass, the carbon mass closure of this system has not yet been achieved.20 The role

345

of HOMs in new particle formation from α-pinene ozonolysis has been recently

346

recognized,2,

347

function of aerosol loadings remain uncertain. The present study introduces an analytical

348

method to characterize the intact molecular structures of HOMs molecules in the

349

condensed phase. Accurate characterization of their concentrations, however, is attended

350

by substantial difficulties due to the absence of authentic standards.

3

although their fate upon gas-particle conversion and abundance as a

351

The O-O bond of organic (hydro)peroxides, an important functional group in HOMs,

352

readily cleaves by photolysis, thermolysis, and solvation, yielding alkoxy radicals, esters,

353

and other moieties.33,

354

signature and chemical structures of HOMs monomers/dimers that contain multiple

34

The present study is focused on understanding the molecular

13



355

peroxide moieties in a-pinene SOA. As atmospheric aerosols are micro-reactors in which

356

chemical reactions actively alter the type and abundance of functionalities that

357

characterize the aerosol constituents, further studies on the chemical transformation of

358

particulate peroxides under a range of aerosol water contents and acidities are therefore

359

needed to establish their overall lifecycle in the atmosphere.

360

ASSOCIATED CONTENT

361

Supporting Information

362

Figure S1 shows the positive ESI mass spectra of isoprene hydroxyl-hydroperoxide

363

and lauroyl peroxide. Figure S2 gives the positive ESI mass spectra of authentic

364

standards that are representative of alcohols, carbonyls, and carboxylic acids. Figure S3

365

shows the mobility spectra of selected products with identical m/z yet produced from

366

different reaction routes (OH vs. O3). Figure S4 presents a mechanism responsible for the

367

formation of various isomeric products under O3 vs. OH dominant environments. Figure

368

S5 compares the mass spectra of monomers produced from ozonolysis of unsubstituted

369

vs. deuterated α-pinene. Figure S6 presents a mechanism explaining the observed

370

distribution pattern of the products labeled by deuterium atoms. Figure S7 shows the two-

371

dimensional ion mobility vs. m/z spectra of HOMs monomers and dimers under different

372

collision energies. Figure S8 gives the mobility selected mass spectra as well as the

373

proposed fragmentation pathways for dibutyloxalate and dioctylphthalate.

374

AUTHOR INFORMATION

375

Corresponding Author

376

Manjula R. Canagaratna (Email: [email protected])

377

Email: [email protected]

378

Address: 45 Manning Road, Billerica, MA 01821, USA

379

Phone: 978-663-9500

380

Fax: 978-663-4918

381

Notes

382

The authors declare no competing financial interest. 14


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383


ACKNOWLEDGEMENTS

384

This study was supported by the U.S. National Science Foundation (NSF)

385

Atmospheric and Geospace Sciences (AGS) grants 1537446. A. T. Lambe acknowledges

386

support from the Atmospheric Chemistry Program of the U.S. National Science

387

Foundation under grants AGS-1536939 and AGS-1537446 and by the U.S. Office of

388

Science (BER), Department of Energy (Atmospheric Systems Research) under grants

389

DE-SC0006980 and DE-SC0011935. M. A. Upshur gratefully acknowledges support

390

from a National Aeronautics and Space Administration Earth and Space (NASA ESS)

391

Fellowship, NSF Graduate Research Fellowship, and a P.E.O. Scholar Award. A. G. Bé

392

acknowledges support from a NSF Graduate Research Fellowship. F. M. Geiger and R. J.

393

Thomson acknowledge support from NSF Grant CHE-1607640. J. D. Surratt, A. Gold,

394

and Z. Zhang acknowledge support from U.S. EPA grant R835404 and NSF grant CHE-

395

1404644.

396

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15. Zhang, X.; Schwantes, R. H.; Coggon, M. M.; Loza, C. L.; Schilling, K. A.; Flagan, R. C.; Seinfeld, J. H., Role of ozone in SOA formation from alkane photooxidation. Atmos. Chem. Phys. 2014, 14, (3), 1733-1753.

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19. Kruve, A.; Kaupmees, K.; Liigand, J.; Oss, M.; Leito, I., Sodium adduct formation efficiency in ESI source. J. Mass Spectrom. 2013, 48, (6), 695-702.

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20. Zhang, X.; McVay, R. C.; Huang, D. D.; Dalleska, N. F.; Aumont, B.; Flagan, R. C.; Seinfeld, J. H., Formation and evolution of molecular products in α-pinene secondary organic aerosol. Proc. Natl. Acad. Sci. USA 2015, 112, (46), 14168-14173.

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21. Cech, N. B.; Enke, C. G., Practical implications of some recent studies in electrospray ionization fundamentals. Mass Spectrom. Rev. 2001, 20, (6), 362-387.

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22. Kristensen, K.; Cui, T.; Zhang, H.; Gold, A.; Glasius, M.; Surratt, J. D., Dimers in α-pinene secondary organic aerosol: effect of hydroxyl radical, ozone, relative humidity and aerosol acidity. Atmos. Chem. Phys. 2014, 14, (8), 4201-4218.

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23. McVay, R. C.; Zhang, X.; Aumont, B.; Valorso, R.; Camredon, M.; La, Y. S.; Wennberg, P. O.; Seinfeld, J. H., SOA formation from the photooxidation of α-pinene: systematic exploration of the simulation of chamber data. Atmos. Chem. Phys. 2016, 16, (5), 2785-2802.

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24. Vereecken, L.; Müller, J. F.; Peeters, J., Low-volatility poly-oxygenates in the OH-initiated atmospheric oxidation of α-pinene: impact of non-traditional peroxyl radical chemistry. Phys. Chem. Chem. Phys. 2007, 9, (38), 5241-5248.

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25. Docherty, K. S.; Wu, W.; Lim, Y. B.; Ziemann, P. J., Contributions of organic peroxides to secondary aerosol formed from reactions of monoterpenes with O3. Environ. Sci. Technol. 2005, 39, (11), 4049-4059.

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26. Kurtén, T.; Rissanen, M. P.; Mackeprang, K.; Thornton, J. A.; Hyttinen, N.; Jørgensen, S.; Ehn, M.; Kjaergaard, H. G., Computational study of hydrogen shifts and ring-opening mechanisms in α-pinene ozonolysis products. J. Phys. Chem. A 2015, 119, (46), 11366-11375.

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27. Demarque, D. P.; Crotti, A. E. M.; Vessecchi, R.; Lopes, J. L. C.; Lopes, N. P., Fragmentation reactions using electrospray ionization mass spectrometry: an important tool for the structural elucidation and characterization of synthetic and natural products. Nat. Prod. Rep. 2016, 33, (3), 432-455.

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28. Müller, L.; Reinnig, M. C.; Warnke, J.; Hoffmann, T., Unambiguous identification of esters as oligomers in secondary organic aerosol formed from cyclohexene and cyclohexene/α-pinene ozonolysis. Atmos. Chem. Phys. 2008, 8, (5), 1423-1433.

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29. Kristensen, K.; Watne, Å. K.; Hammes, J.; Lutz, A.; Petäjä, T.; Hallquist, M.; Bilde, M.; Glasius, M., High-molecular weight dimer esters are major products in

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aerosols from α-pinene ozonolysis and the boreal forest. Environ. Sci. Technol. Lett. 2016, 3, (8), 280-285.

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30. Riva, M.; Budisulistiorini, S. H.; Zhang, Z.; Gold, A.; Thornton, J. A.; Turpin, B. J.; Surratt, J. D., Multiphase reactivity of gaseous hydroperoxide oligomers produced from isoprene ozonolysis in the presence of acidified aerosols. Atmos. Environ. 2017, 152, 314-322.

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31. Ehn, M.; Kleist, E.; Junninen, H.; Petäjä, T.; Lönn, G.; Schobesberger, S.; Maso, M. D.; Trimborn, A.; Kulmala, M.; Worsnop, D. R., Gas phase formation of extremely oxidized pinene reaction products in chamber and ambient air. Atmos. Chem. Phys. 2012, 12, (11), 5113-5127.

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32. Claeys, M.; Iinuma, Y.; Szmigielski, R.; Surratt, J. D.; Blockhuys, F.; Van Alsenoy, C.; Böge, O.; Sierau, B.; Gómez-González, Y.; Vermeylen, R., Terpenylic acid and related compounds from the oxidation of α-pinene: Implications for new particle formation and growth above forests. Environ. Sci. Technol. 2009, 43, (18), 6976-6982.

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33. Zhang, X.; Chen, Z. M.; He, S. Z.; Hua, W.; Zhao, Y.; Li, J. L., Peroxyacetic acid in urban and rural atmosphere: concentration, feedback on PAN-NO x cycle and implication on radical chemistry. Atmos. Chem. Phys. 2010, 10, (2), 737-748.

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34. Zhang, X.; He, S. Z.; Chen, Z. M.; Zhao, Y.; Hua, W., Methyl hydroperoxide (CH 3 OOH) in urban, suburban and rural atmosphere: ambient concentration, budget, and contribution to the atmospheric oxidizing capacity. Atmos. Chem. Phys. 2012, 12, (19), 8951-8962.

521 522 523

35. Donahue, N. M.; Epstein, S. A.; Pandis, S. N.; Robinson, A. L., A twodimensional volatility basis set: 1. organic-aerosol mixing thermodynamics. Atmos. Chem. Phys. 2011, 11, (7), 3303-3318.

524 525 526 527 528 529

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530 531

Scheme 1. Schematic illustrations of multifunctional organic peroxide formation via the

532

intramolecular H-abstraction followed by O2 addition at the alkyl radical site.

533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 19



551 552

Figure 1. (Upper panel) Mass defect versus mass to charge ratio of the sodiated ion

553

adducts of multifunctional organic compounds ([M+Na]+) in secondary organic aerosols

554

derived from α-pinene ozonolysis. An ensemble of monomers and dimers, separated from

555

each other by the mass of one oxygen atom and/or two hydrogen atoms, is observed. The

556

mass bands are labeled by different oxygen numbers and colored by different carbon

557

numbers. Each circle represents a distinct oxidized molecule and the circle size represents

558

the signal of the corresponding sodiated ion adduct. The two dashed trend lines represent

559

the progressive addition/removal of one oxygen and hydrogen atom, respectively, on the

560

plot. (Lower panel) Distribution of condensed-phase monomer and dimer products on the

561

oxidation state (OSC) vs. carbon number (nC) space. Each triangle represents a group of

562

molecules with the same OSC and nC, and the triangle size represents the mass weighted

563

sodiated ion adduct signal. The volatility of each molecule is calculated according to

564

Donahue et al. (2011).35

565 566 567

20


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568 569

Figure 2. Mass spectra of a-pinene ozonolysis products detected (A) in the gas phase as

570

iodide clusters ([M+I]-) by the iodide-CIMS and (B) in the particle phase as sodiated ion

571

adducts ([M+Na]+) by the IMS-MS.

572 573 574 575 576 577 578 579 580 581

21



582 583

Figure 3. The measured drift time versus mass to charge ratio of particulate products

584

from (A) a-pinene ozonolysis and (B) OH-oxidation. The marker color represents the

585

normalized intensity of the corresponding sodiated ion adduct of each product

586

([M+Na]+).

587 588 589 590 591 592 593 594 595 596 597 598 599 600 22


Page 22 of 25

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601 602

Figure 4. Positive ion mass spectra of a series of particle-phase monomers and dimers

603

produced from the ozonolysis of (A) α-pinene and (B) deuterated α-pinene under

604

identical experimental conditions. The subset figures illustrate the systematic m/z shifts

605

and production of new ion adducts upon deuterating the methyl group attached to the

606

endocyclic double bond. (C) Proposed mechanisms for the formation of the C10(H/D)16O5

607

product isotopologues from ozonolysis of α-pinene. Reaction pathways involving the

608

removal of one deuterium atom are highlighted in blue and those retaining the deuterium

609

labels are highlighted in green.

610 611 612 613 614 615 616 617 23



618 619

Figure 5. (A) Ion mobility spectra for one pair of precursor-fragment ion adducts as a

620

representative example of the dimer series examined. Here the ion adduct at m/z 239 with

621

a drift time of 40-42 ms is assigned to a monomer product (C10H16O5) and the ion adduct

622

at m/z 409 with a drift time of 52-57 ms is assigned to a dimer product (C19H30O8) from

623

α-pinene ozonolysis. The ion adduct at m/z 239 with a drift time of 52-57 ms is assigned

624

to the fragment ion from the collision-induced dissociation of its precursor ion at m/z 409.

625

(B) Intensity profiles of these three ion adducts as a function of the collision energy as

626

characterized by the CID voltage. (C) Proposed McLafferty-type rearrangement for the

627

collision-induced dissociation of the C19H30O8×Na+ dimer to the C10H16O5×Na+ monomer.

628

(D) Positive ion mass spectra of SOA from ozonolysis of α-pinene.

629 630 631 632 633

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Table of Contents

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Highly Oxygenated Multifunctional Compounds in Î±-Pinene

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