Isomerization of Second-Generation Isoprene Peroxy Radicals

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Isomerization of second generation isoprene peroxy radicals: epoxide formation and implications for secondary organic aerosol yields Emma L. D'Ambro, Kristian H. Møller, Felipe D. Lopez-Hilfiker, Siegfried Schobesberger, Jiumeng Liu, John E. Shilling, Ben Hwan Lee, Henrik G. Kjaergaard, and Joel A. Thornton Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00460 • Publication Date (Web): 07 Apr 2017 Downloaded from http://pubs.acs.org on April 9, 2017

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Environmental Science & Technology

Isomerization of second generation isoprene peroxy radicals: epoxide formation and implications for secondary organic aerosol yields Emma L. D’Ambro1,2, Kristian H. Møller3, Felipe D. Lopez-Hilfiker1,a , Siegfried Schobesberger1,b , Jiumeng Liu4, John E. Shilling4,5, Ben Hwan Lee1, Henrik G. Kjaergaard3, Joel A. Thornton*,1,2 1

Department of Atmospheric Sciences, University of Washington, Seattle, Washington 98195, United States 2 Department of Chemistry, University of Washington, Seattle, Washington 98195, United States 3 Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark 4 Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, WA, 99352, USA 5 Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA, 99352, USA a Now at: Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Zurich, Switzerland b Now at: Department of Applied Physics, University of Eastern Finland, Kuopio, Finland Author Information Corresponding Author * Phone: 206-543-4010; Email: [email protected]; Mail: 408 ATG Building, Box 351640, University of Washington, Seattle, WA 98195

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Abstract

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We report chamber measurements of secondary organic aerosol (SOA) formation from

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isoprene photochemical oxidation, where radical concentrations were systematically varied and

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the molecular composition of semi to low volatility gases and SOA were measured online. Using

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a detailed chemical kinetics box model, we find that to explain the behavior of low volatility

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products and SOA mass yields relative to input H2O2 concentrations, the second generation

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dihydroxy hydroperoxy peroxy radical (C5H11O6•) must undergo an intra-molecular H-shift with

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a net forward rate constant of order 0.1 s-1 or higher. This finding is consistent with quantum

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chemical calculations which suggest a net forward rate constant of 0.3-0.9 s-1. Furthermore, these

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calculations suggest the dominant product of this isomerization is a dihydroxy hydroperoxy

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epoxide (C5H10O5) which is expected to have a saturation vapor pressure ~2 orders of magnitude

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higher, as determined by group-contribution calculations, than the dihydroxy dihydroperoxide,

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ISOP(OOH)2 (C5H12O6), a major product of the peroxy radical reacting with HO2. These results

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provide strong constraints on the likely volatility distribution of isoprene oxidation products

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under atmospheric conditions and thus on the importance of non-reactive gas-particle

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partitioning of isoprene oxidation products as an SOA source.

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1. Introduction Atmospheric submicron aerosol particles impact climate, human health, and visibility.1 A

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large portion of submicron aerosol mass (~20-90%) is secondary organic aerosol (SOA),2,

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which arises from chemical processes in the atmosphere that convert volatile organic compounds

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(VOC) into condensed-phase organic material. A significant fraction of SOA is thought to be

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formed from biogenic rather than anthropogenic VOC.4,

5

3

The annual emissions of isoprene


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(C5H8), primarily from deciduous vegetation, represent nearly half the total of all non-methane

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hydrocarbons.6 Thus, isoprene has the potential to generate large quantities of SOA, even if its

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overall conversion is inefficient.

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The formation of SOA from the free radical photochemical oxidation of biogenic VOC

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(BVOC) is governed in part by the mass concentration (c) weighted volatility distribution of the

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resulting products. For example, BVOC oxidation products having equilibrium saturation vapor

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concentrations (c*) lower than ~100 µg m-3 can be expected to partition into an existing organic

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condensed-phase and thus contribute to SOA, as typical organic aerosol mass concentrations

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( ) are in the range of 1-10 µg m-3.7, 8 The volatility distribution of BVOC oxidation products

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is governed by the ratio of the BVOC that undergoes functionalization relative to fragmentation

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during atmospheric oxidation. Oxidation of BVOC is frequently initiated by the formation of a

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carbon-centered radical primarily due to reaction with a hydroxyl radical (OH). In this initial

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stage, atmospheric oxygen (O2) can add to the carbon-centered radical to form an organic peroxy

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radical (RO2). RO2 can then undergo bimolecular reactions with typically HO2, NO, or RO2. All

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of these bimolecular reactions can produce more functionalized products, while the latter two can

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also form alkoxy radicals (RO) which can undergo H-shift isomerization9 or fragment into

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smaller, more volatile products. Thus, the volatility distribution is largely determined by the fate

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of RO2 intermediates.

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In the specific case of isoprene (scheme 1), OH addition to a double bond followed by O2

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addition yields a hydroxy RO2. Some isomers of the first-generation RO2 can undergo

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unimolecular isomerization (not shown), with a forward rate constant of order 0.01 - 1 s-1,10-12

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followed by HO2 loss to produce a hydroperoxyenal (HPALD),10, 11 or dihydroperoxy-carbonyl

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peroxy radicals (di-HPCARP).12 The net isomerization rate constant weighted by the isomer



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distribution was calculated to be ~0.001 s-1.11,

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bimolecular reactions with HO2 to form a hydroxy hydroperoxide (ISOPOOH), NO to form an

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alkyl nitrate,13 or NO or RO2 to form an alkoxy radical. The latter tends to decompose by carbon-

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carbon bond scission to form products such as formaldehyde, methacrolein, or methyl vinyl

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ketone.14-17 These first generation closed-shell products are rather volatile, and though they may

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undergo multiphase reactions, their contribution to SOA by direct partitioning is likely small.

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Given that a large fraction of isoprene reacts to form ISOPOOH under low NOx conditions,

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possibly up to or greater than 70%,18 and that it likely has the lowest c* of the first generation

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products, its oxidation products are the subject of continued focus.

The first generation RO2 can undergo

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ISOPOOH reacts rapidly via OH addition to the remaining double bond producing a

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carbon-centered radical often α to the hydroperoxide moiety (Scheme 1). This alkyl radical has

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been shown to form an epoxide bridge with the hydroperoxide group. Consecutive loss of OH

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from that hydroperoxide group forms an epoxydiol (IEPOX)18 ~70-80% of the time19 (Scheme

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1). A significant fraction of isoprene-derived SOA is now understood to originate from acid

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catalyzed aqueous phase chemistry of IEPOX,18, 20-27 although SOA also forms from isoprene

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oxidation in the absence of an acidic aqueous phase.28-31 Recent gas-29 and particle-phase30-32

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composition measurements of low-NOx OH oxidation of both isoprene and ISOPOOH reveal

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highly oxygenated second generation products. A major particle-phase component has the

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composition C5H12O6,30-32 likely a dihydroxy dihydroperoxide, which we refer to as

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ISOP(OOH)2 (see Scheme 2, top reaction, for structure). This product is believed to form from

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O2 addition to the carbon-centered radical from ISOPOOH + OH to yield a second generation

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RO2 in competition with IEPOX formation (Scheme 1), followed by reaction of the RO2 with

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HO2 (Scheme 2, top reaction). A fraction of ISOPOOH + OH is also expected to react via H-


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abstraction from an alkyl group or the –OOH group, although these channels are relatively small

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contributions to net ISOPOOH reactivity.19

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In previous work, we showed that steady-state ISOP(OOH)2 and SOA mass yields

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increase together as the OH and HO2 source,30 H2O2, is increased relative to isoprene. Reasons

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for this behavior have yet to be determined. Mechanistic explanations for prominent products

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from this system other than ISOP(OOH)2, such as C5H12O5 and C5H10O5-7, are also lacking.

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Herein, we compare an expanded set of chamber experiments with novel predictions from a

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detailed multiphase chemical model, and quantum chemical calculations of RO2 radical fates, to

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improve our understanding of the isoprene oxidation mechanism and thus the formation of

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isoprene SOA from non-IEPOX oxidation products. Specifically, we test different hypotheses for

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the fate of the C5H11O6• peroxy radical so that we can understand the dependence of non-IEPOX

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isoprene SOA formation on chamber oxidation conditions. We conclude with a discussion of the

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atmospheric implications of our findings.

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2. Methods

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2.1 Laboratory Experiments

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A suite of online instruments were employed to monitor the gas- and particle-phase chemical

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composition. Ozone (Thermo Environmental Instruments model 49C), NO/NO2/NOx (Thermo

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Environmental Instruments model 42C), and isoprene (Ionicon proton-transfer-reaction mass

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spectrometer (PTR-MS)) concentrations were measured in the chamber outflow. Bulk submicron

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organic and inorganic particle-phase composition and mass loading were measured with an

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Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HRToF-AMS). A high-

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resolution time-of-flight chemical ionization mass spectrometer (HRToF-CIMS) with iodide



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ionization was coupled to a Filter Inlet for Gases and AEROsols (FIGAERO).33 The HRToF-

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CIMS FIGAERO coupling has been described in detail previously in the configuration used in

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these experiments.30, 32, 34 Briefly, the FIGAERO is an inlet manifold that allows for sampling

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gas-phase composition at 1 Hz through an independent inlet while collecting aerosol particles on

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a Teflon filter through another inlet. Particle-phase composition is assessed at approximately

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hourly resolution by actuating the filter to be in front of the HRToF-CIMS entrance orifice and

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performing temperature-programmed thermal desorption of particles. Vaporized particle-phase

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compounds are then detected by the HRToF-CIMS.

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Experiments presented here have been discussed previously,32 but the observed

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relationships of SOA yields to radical sources in these specific experiments have not been

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presented. In brief, experiments were performed in 2015 in the Pacific Northwest National

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Laboratory’s 10.6 m3 fluorinated ethylene polypropylene (FEP) environmental chamber, which

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has been described before.32, 35 The chamber was controlled to a constant 50% relative humidity

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and operated in continuous-flow mode with a residence time of 5.2 hours where reactants are

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continuously delivered at a constant flow rate while products are continuously monitored with a

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suite of gas- and particle-phase instrumentation. OH and HO2 radicals were generated from the

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photolysis of excess H2O2 (Sigma-Aldrich, 50% in water), with chamber concentrations ranging

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from 0.1 s-1.

, which is slower than other recent determinations in chambers operated in batch-mode.45-48, 50, 51

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The important role of isomerization is demonstrated further in Figure 2, where we

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compare model predicted and observed ISOP(OOH)2 mass yields as a function of H2O2 in the

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chamber, used as a proxy for [HO2] (Fig. S2). Model runs were conducted at isomerization rate

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constants, kisom, of 0 (i.e. the base-case), 0.005, 0.3, and 0.8 s-1 and all other parameters the same

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as the base-case (IEPOX yield of 82%, kwall =10-5 s-1). The runs at the two slowest isomerization

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rates (0 and 0.005 s-1) are statistically the same and significantly over-predict the ISOP(OOH)2

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mass yield relative to the measurements by up to a factor of 30. The runs with kisom of 0.3 and 0.8

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s-1 are both much closer to the data, with the run at kisom = 0.8 s-1 predicting ISOP(OOH)2 mass

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yields closest to the observations (Fig. 2, inset), and have the approximate linear increase of yield

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with H2O2. We demonstrate in the SI with a steady-state chemical analysis that the dependence


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of the ISOP(OOH)2 and SOA mass yields on changing input H2O2 concentrations requires the

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existence of a pathway such as isomerization that competes with the RO2 + HO2 reaction. Simply

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increasing kwall or ϕ moves the model curve down, but does not change the shape (Fig. S4), nor

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does [OH] change enough to explain the observations. We can also rule out significant nitric

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oxide chemistry using our observations of organic nitrates (and NOx).32 Increasing the RO2 +

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RO2 reaction rate constant to the kinetic limit also did not improve the agreement. Thus, we

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conclude that isomerization of the C5H11O6• peroxy radical competes with the reaction of

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C5H11O6• with HO2 and thereby causes the observed trend with H2O2 concentration.

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While Crounse et al.11 reported experimentally measured net isomer-weighted

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isomerization rate of the first generation isoprene hydroxy peroxy radical, C5H9O3 (ISOPO2), of

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order 0.001 s-1, we note specific isomers had larger isomerization rates,73 and other studies have

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found the H-shift kisom of methacrolein and a peroxy radical of -OOH substituted 3-pentanone to

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be 0.5 s-1 74 and >0.1 s-1,73 respectively. Computational studies have also calculated rates similar

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to these for peroxy radical hydrogen shift reactions.54, 75 Thus, the kisom required for agreement

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with the observations we present is consistent with kisom for other peroxy radicals.

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Our independently performed quantum chemical calculations examined the isomerization

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rate of C5H11O6• and possible products for the two most prominent ISOPOOH isomers, 1,2-

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ISOPOOH (shown in Scheme 1) and 3,4-ISOPOOH. In Scheme 3, we show the C5H11O6•

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isomers formed from 1,2-ISOPOOH, however, the same is obtained for 3,4-ISOPOOH due to the

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rapid interconversion between the two peroxy radicals, which has also been previously

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demonstrated for another hydroperoxy-peroxy system.76 The forward rates of the 1,5 H-shift

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reactions were found to be 0.3 s-1 and 0.9 s-1 for the two isomers, respectively (Scheme 3). The

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reverse rates in both cases were too slow at room temperature to compete with epoxide formation



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or the addition of O2 to the resulting alkyl radical, which presumably occurs near the kinetic limit

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(~108 s-1 at 1 atm total pressure) as previously shown.77 Thus, the ISOPOOH-derived RO2 will

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convert into other products due to intramolecular H-shifts at a net forward rate of order 0.3 s-1 or

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faster according to the quantum chemical calculations, consistent with the observations and box

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modeling described above. The fact that the box model and quantum chemical calculations

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independently arrive at a similar value for the rate of C5H11O6• isomerization supports the

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hypothesis that such a pathway competes with HO2, not only in the chamber, but also the

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atmosphere where HO2 concentrations are significantly lower.

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For isomerization of the ISOPOOH-derived RO2 to explain a decreasing SOA mass yield,

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the products that result from the isomerization must be of higher volatility than ISOP(OOH)2.

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The quantum chemical calculations predict three possible products: a hydroxy dihydroperoxy

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aldehyde or ketone (C5H10O6 or C5H10O7, respectively) and a dihydroxy hydroperoxy epoxide

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(C5H10O5), (Schemes 2 and 3). Additionally, C5H10O5 could be a dihydroxy hydroperoxy

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aldehyde formed from addition of OH to the internal carbon of the remaining ISOPOOH double

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bond (Scheme S1). Previous studies suggest branching to this pathway is a few percent,13, 78, 79

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and so was not included. As it is also a product of isomerization, it does not affect our

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determination of kisom or conclusions (see SI). The calculations suggest that formation of the

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epoxide, analogous mechanistically to the formation of IEPOX,18 is one to two orders of

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magnitude faster than O2 addition to form the aldehyde/ketone, where O2 addition occurs at a

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rate of ~107 s-1 for both 1,2- and 3,4-ISOPOOH and epoxide formation occurs at calculated rates

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of 9 × 109 or 3 × 108 s-1 for 1,2- and 3,4-ISOPOOH, respectively. That an epoxide is formed is

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intriguing, suggesting that epoxide formation may be a more general characteristic of isoprene

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oxidation rather than specific to IEPOX, and that epoxide formation when a hydroperoxide is α


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to a carbon-centered radical is more common than our current mechanisms of VOC oxidation

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suggests.

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Although the FIGAERO HRToF-CIMS does not provide structural information, we can

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assess whether the products of C5H11O6• peroxy radical isomerization are consistent with the

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observed compositions of measured oxidation products. Figure 3 shows the behavior of the sum

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of isomerization products relative to the HO2 bimolecular reaction product, where we assume

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ions with the compositions C5H10O5, C5H10O6, and C5H10O7 correspond to the predicted epoxide,

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aldehyde, and ketone products, respectively. The model predicts the relative shape of the

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relationship with respect to H2O2 correctly, but the absolute agreement in the ratio of these

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products is better for kisom of ~0.1 s-1, as opposed to the 0.3 – 0.8 s-1 suggested by the comparison

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to the concentration of ISOP(OOH)2. Uncertainties in the relative loss rates assumed in the

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model and in the relative instrument response to these various products likely explain such

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discrepancies. The results shown in Figures 2 and 3, combined with the quantum calculations, all

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point to a kisom of 0.1-0.8 s-1. That independent approaches all arrive at kisom > 0.1 s-1 therefore

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has important implications for SOA formation from isoprene in the atmosphere.

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The SOA mass concentration and the distribution of compounds in the particle-phase

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provide an additional test of the model. Figure 4, top, shows that a C5H11O6• kisom of 0.3 s-1

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provides good model/measurement agreement of the SOA abundance (R2=0.98). The base-case

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model over-predicts SOA, while kisom of 0.8 s-1 slightly under-predicts SOA. Consistent with the

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observations, the modeled SOA contains 36% ISOP(OOH)2 (Fig. 4, bottom), with other major

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components matching compositions of major components measured by the FIGAERO HRToF-

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CIMS. The model predicts that the dihydroxy hydroperoxy epoxide formed from C5H11O6•



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isomerization makes up 48% of the aerosol composition at the highest HO2 concentration and

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kisom of 0.8 s-1, while it was observed to be 11%.

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We do not expect perfect agreement between measured and modeled SOA abundance and

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composition, in part because previous studies have shown that up to 50% of this non-IEPOX

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SOA is possibly low volatility accretion-like products32 likely formed from multiphase chemistry

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which is not included in the model. Thus, we expect to under-predict measured SOA by as much

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as a factor of two with F0AM, as well as to be unable to simulate observed products which are

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the result of thermal decomposition of low volatility oligomers during the desorption analysis.

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As such, we conclude that the overall agreement with observations in the predicted SOA mass

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abundance, dominant products, and response to changing H2O2 is reasonable, and that

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constraining kisom for the C5H11O6• peroxy radical at >0.1 s-1 is more important than kwall for

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predicting the SOA yield.

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4. Atmospheric Implications

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We utilize quantum chemical calculations and a detailed chemical kinetics box model

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compared to measurements of gas- and particle-phase oxidation products of isoprene made in an

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environmental chamber. To explain the responses of product distributions and SOA mass

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concentrations to changes in chamber radical conditions, the C5H11O6• peroxy radical formed

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from ISOPOOH + OH must undergo intra-molecular H-atom shift reactions (“RO2

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isomerization”) at rates exceeding 0.1 s-1 to form products 1-2 orders of magnitude higher

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volatility than that of ISOP(OOH)2, which forms if the peroxy radical reacts with HO2. Such a

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pathway has significant implications for how low-NO chamber studies should be conducted, and


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offers a way to reconcile differences in SOA yields measured under different chamber

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conditions.28-31

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While ISOP(OOH)2 and related highly oxygenated C5 compounds believed to come from the

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ISOP(OOH)2 pathway, have been observed in the Southeastern U.S. atmosphere during the

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SOAS 2013 field campaign, their abundance was only a few percent of the total SOA in an

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isoprene dominated region.31 Using SOAS diurnally averaged measurements of HO2 and NO,

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and our F0AM model presented here, we calculate the vast majority, >96%, of the C5H11O6• will

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undergo isomerization instead of forming the lowest volatility ISOP(OOH)2 and related products.

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The quantum calculations suggest the most likely product of the isomerization is a dihydroxy

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hydroperoxy epoxide. While we and others observe a major second generation product with the

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composition (C5H10O5) of such an epoxide,29 confirmation of the structure remains necessary.

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Epoxides in general undergo heterogeneous uptake and acid catalyzed ring opening with the

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potential to form SOA through subsequent substitution reactions.27, 80, 81 However, we speculate

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that the dominant fate in aqueous acidic particles of this specific epoxide moiety will be nearly

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instantaneous conversion to a dihydroxy hydroperoxy carbonyl, but direct experimental evidence

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is needed. That said, its lower volatility relative to IEPOX will allow it to partition more strongly

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to an organic phase where it may undergo accretion chemistry.

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The dominance of epoxide formation over O2 addition to a carbon radical α to a

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hydroperoxyl group, as calculated herein and for the case of IEPOX, suggests gas-phase epoxide

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formation may be more common than current atmospheric chemistry mechanisms allow. Such a

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situation would have implications for the propensity of acidity enhanced SOA formation from a

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variety of biogenic and anthropogenic VOC.20,

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autoxidation, where peroxy radicals form multiple hydroperoxide moieties by several intra-

26, 27, 81

For example, the peroxy radical



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molecular H-shifts could well terminate to epoxides. Epoxide formation has been commonly

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suggested, particularly for aromatic systems,82-88 supporting the idea that epoxides are potentially

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a feature of low NOx oxidation of non-aromatics when a radical center occurs α to a hydroperoxy

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group. Moreover, there is also recent evidence for epoxide formation from –ONO2 groups α to

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the carbon radical.19, 89, 90 Thus, we suggest analytical methods specific to epoxides be developed

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for assessing their ubiquity in the atmosphere and chamber studies.

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Associated Content

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Includes further detail on: chamber conditions, F0AM and the gas/particle partitioning module

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used herein, quantum chemical calculations, and the dependence of ISOP(OOH)2 and SOA mass

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yields on H2O2. Also includes a link to the gas/particle partitioning module for download.

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Acknowledgements

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This work was supported by the U.S. Department of Energy ASR grants DE-SC0011791.

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E.L.D. was supported by the National Science Foundation Graduate Research Fellowship under

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Grant No. DGE-1256082. B.H.L. was supported by the National Oceanic and Atmospheric

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Administration (NOAA) Climate and Global Change Postdoctoral Fellowship Program. PNNL

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authors were supported by the U. S. Department of Energy, Office of Biological and

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Environmental Research, as part of the ASR program. Copenhagen authors were supported by

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the University of Copenhagen. Pacific Northwest National Laboratory is operated for the DOE

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by Battelle Memorial Institute under contract DE-AC05-76RL01830. We thank G.M. Wolfe

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(NASA/UMBC), T.A. Spencer (Dartmouth College), R. Ditchfield (Dartmouth College), and


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Rasmus V. Otkjær (University of Copenhagen) for helpful discussions, and the Danish Center

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for Scientific Computing for computational time.

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Figures


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Scheme 1. Isoprene photochemical oxidation mechanism for one isomer of ISOPOOH under low-NOx conditions. ϕ and 1-ϕ represent the branching ratio between IEPOX and the peroxy radical, respectively.

Scheme 2. Proposed mechanism for one isomer of C5H11O6, the peroxy radical formed from ISOPOOH + OH in scheme 1. The formation of the three isomerization products is shown in detail in Scheme 3.



808 809 810

811

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Figure 1. Top: Model predicted (line) and measured (diamonds) isoprene remaining in the chamber at steady state. Bottom: Measured C5H12O6 (blue x’s) and SOA (black squares) mass concentration. Note the different y-axis scale.

Figure 2. Measured mass yield of C5H12O6 (black x’s) and several model results with varying isomerization rates of C5H11O6 peroxy radical precursor as a function of input H2O2 (ppm). Inset provides a zoomed-in view to better observe the behavior of the measurements and most representative model run. Measurement error bars reflect our uncertainty in the calibration factor. 28 ACS Paragon Plus Environment

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Scheme 3. Proposed detailed mechanism and calculated rates of the isomerization of one isomer of C5H11O6, the peroxy radical formed from ISOPOOH + OH in scheme 1.

Figure 3. The ratio of the sum of C5H11O6• isomerization products relative to the product of reaction with HO2 for measurements (black x’s) and various model results (lines) at varying C5H11O6• isomerization rates. Measurement error bars reflect our uncertainty in calibration factors.



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Figure 4. Top: model OA concentrations plotted versus -5 -1 measurements for model runs with 10 s wall loss, 11% -1

C5H11O6 yield, and varying isomerization rates (s ). Bottom: pie charts of the most prolific particle phase species modeled (left) for the data point in the above plot -3

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circled in yellow, and measured (right) in µg m .

TOC Graphic

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Isomerization of Second-Generation Isoprene Peroxy Radicals

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