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Temperature effects on secondary organic aerosol (SOA) from the dark ozonolysis and photo-oxidation of isoprene Christopher Holmes Clark, Mary Kacarab, Shunsuke Nakao, Akua Asa-Awuku, Kei Sato, and David R. Cocker Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b05524 • Publication Date (Web): 13 May 2016 Downloaded from http://pubs.acs.org on May 14, 2016

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Environmental Science & Technology

Isoprene SOA Results

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SOA Density

Chamber Reaction Temperature ACS Paragon Plus Environment


1

Temperature effects on secondary organic aerosol (SOA) from the dark ozonolysis

2

and photo-oxidation of isoprene

3 4

Christopher H. Clark1,2,3, Mary Kacarab1,2, Shunsuke Nakao1,2, Akua Asa-Awuku1,2, Kei

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Sato4, and David R. Cocker, III1,2*

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1

8

Research and Technology (CE-CERT), Riverside, California, USA

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2

University of California-Riverside, College of Engineering-Center for Environmental

University of California-Riverside, Department of Chemical and Environmental

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Engineering, Riverside, California, USA

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3

Naval Surface Warfare Center, Corona Division, Norco, California, USA

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4

Asian Environment Research Group, National Institute for Environmental Studies,

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Tsukuba, Ibaraki, Japan

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* Corresponding Author, University of California–

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Riverside, College of Engineering–Center for Environmental Research

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and Technology (CE–CERT), 1084 Columbia Ave., Riverside,

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California, 92507 USA, Phone: (951) 781-5791, E-mail: [email protected]

18 19 20

Abstract Isoprene is globally the most ubiquitous non-methane hydrocarbon. The biogenic

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emission is found in abundance and has a propensity for SOA formation in diverse

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climates. It is important to characterize isoprene SOA formation with varying reaction

ACS Paragon Plus Environment

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temperature. In this work, the effect of temperature on SOA formation, physical

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properties, and chemical nature is probed. Three experimental systems are probed for

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temperature effects on SOA formation from isoprene, NO + H2O2 photo-oxidation, H2O2

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only photo-oxidation, and dark ozonolysis. These experiments show that isoprene readily

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forms SOA in unseeded chamber experiments, even during dark ozonolysis, and also

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reveals that temperature affects SOA yield, volatility and density formed from isoprene.

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As temperature increases SOA yield is shown to generally decrease, particle density is

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shown to be stable (or increase slightly), and formed SOA is shown to be less volatile.

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Chemical characterization is shown to have a complex trend with both temperature and

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oxidant, but extensive chemical speciation are provided.

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1. Introduction

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Isoprene is the most abundant non-methane hydrocarbon observed in the

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atmosphere1. Isoprene is found in the ambient in diverse meteorological, gas-phase

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oxidant concentration, and geographical environments, from boreal forests2, marine

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environments3and oil tanker emissions4. It is therefore important to study SOA

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formation from isoprene under varying oxidizing conditions, humidity, and temperatures

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to provide reaction conditions analogous to ambient conditions globally. Many have

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studied the effect of various oxidizing environments including, O3, NO, nitrate radical,

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and hydroxyl radical5-9. Most of these studies have focused on isoprene SOA formation

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from NO photo-oxidation. Though study of NO photo-oxidation provides a good analog

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for the formation of SOA from isoprene in the daytime urban environment, where NO is

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abundant, it does not provide a complete picture of SOA formation from isoprene. It is



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further of interest to study hydroxyl and ozone independently of NO for mechanistic

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insight into SOA formation; for when NO is present both O3 and hydroxyl radical will be

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present. A study spanning common atmospheric oxidant sources also gives insight into

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effects in rural environments where NO concentrations would be low and hydroxyl

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radical or O3 would be the more prevalent atmospheric oxidants. The effect of humidity

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on SOA formation from isoprene has also been probed by others, but is not studied

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here10.

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Currently, little or no literature exists on the temperature effects of isoprene SOA

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formation. Here, a temperature and humidity controlled environmental chamber is used

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to study the effect of reaction temperature on the chemical and physical properties of

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SOA formed from isoprene photo-oxidation and dark ozonolysis. Increasing the ambient

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temperature of SOA formation forms less volatile products; the aerosol contain more

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oligomer-like products of higher density and lower volatility. Further, chemical

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speciation changes of SOA under different temperature and oxidation conditions are

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reported.

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2. Materials and Methods

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2.1 Environmental chamber

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All experiments presented here were run in the UC Riverside/CE-CERT dual

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90m3 environmental chambers.11 Briefly, the reactors are constructed of Teflon® film

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and suspended and kept under constant positive differential pressure (>0.01 in.H2O). The

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insulated enclosure is temperature controlled (within ±1°C) and is continuously flushed


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with clean, dry (Tdp < −40°C) air generated by an Aadco 737 purification system. For

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photo-oxidation experiments, 276 115W Sylvania 350BL black lights are used. Due to

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the change in NO2 photolysis rate at different temperatures ( described by Qi et al.12), the

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number of black lights used are adjusted for hot and cold photo-oxidation experiments.

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Unless otherwise noted, photolysis rate was controlled at 0.29 min-1 by changing the

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number of bulbs. All isoprene (Sigma-Aldrich, 99.5%), H2O2 (50wt% in H2O, stabilized,

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Sigma-Aldrich), and perfluorohexane dilution tracer(Sigma-Aldrich, 99%) injections are

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performed via a a heated glass manifold system pure N2. Precise concentrations of NO

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(ultra high purity, Matheson) were generated in a vacuum controlled manifold into a glass

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bulb of known volume and then injected into the chambers. O3 was generated from clean

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air by a Horiba OZG-UV-01 Ozone Generator Unit and injected into the chambers by a

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controlled flowrate.

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2.2 Gas and particle analysis

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Hydrocarbon precursors and products were measured with an Agilent 6890 GC-

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FID. NOx, CO, and O3 concentrations were monitored using a Thermo Environmental

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Instruments Inc. model 42C trace level NO-NO2-NOy analyzer, Thermo Environmental

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Instruments Inc. model 48C trace level CO analyzer and Dasibi O3 analyzer, respectively.

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Before the start of each experiment, all gas-phase instruments are checked with a

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calibration standard.

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Particle size distribution between 27 nm and 686 nm was monitored by a custom

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built Scanning Mobility Particle Sizer (SMPS) similar to that described by Cocker et

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al.13. Particle volatility was monitored with a house-built volatility tandem differential



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mobility analyzer (VTDMA). The first DMA column size-selects (Dmi) the aerosol based

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off of the peak diameter seen by the SMPS and sends it through a Dekati thermodenuder

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(TD) at 100°C. The particle size after the TD (Dmf) is then measured by the second DMA.

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Volume fraction remaining (VFR) is then calculated as VFR = (Dmf/Dmi)3.

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Particle effective density was measured with a Kanomax Aerosol Particle Mass

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Analyzer (APM)14 and in series with an SMPS. The APM is located upstream of the

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SMPS, providing improved time resolution and sensitivity (S/N), with a detailed

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description of the system its data algorithms described elsewhere15. Briefly, the aerosol is

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size selected by the APM based on its mass and the resulting mobility diameter of the

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aerosol is then measured by the SMPS.

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An Aerodyne high resolution time-of-flight aerosol mass spectrometer (HR-ToFAMS)16 was operated in high resolution, or “W”, mode throughout all experiments.

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Elemental analysis (EA) of the resulting data was used to determine the bulk chemical

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ratios (H/C, O/C, and N/C) of non-refractory organic aerosols17.

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A Particle-Into-Liquid-Sampler (PILS)18, 19 was interfaced with an Agilent 6210

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Time-of-Flight Mass Spectrometer (TOFMS), hereafter referred to as the PILS-ToF. The

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instrument was equipped with a multimode ionization source for both electrospray and

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atmospheric pressure chemical ionization (ESI/APCI), providing accurate online mass

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analysis of water soluble organic compounds9. HPLC pumps were used in addition to a

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common peristaltic pump to couple the PILS (Brechtel Manufacturing Inc.) to the

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TOFMS. This coupling was critical in order to overcome the backpressure of the TOFMS

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inlet and to supply steady flow of water (18.2MΩ, Milli-Q, Millipore) to the boiler. The


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PILS-ToF system is described in detail elsewhere9. For all TOFMS data reported in this

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work, the electrospray ionization source (ESI) was operated in the negative ion mode

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with a vaporizer temperature of 200 °C, nebulizer pressure of 40 psig, corona current of 2

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µA, and fragmentor voltage of 100V.

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Occasionally during the sample analysis, higher mass errors (up to 100 ppm) were

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observed, resulting in a consistent shift of mass throughout the range of the instrument.

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Since the extent of the shift can be inferred from repeatedly observed ions (e.g., methyl

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vinyl ketone), formulas were carefully assigned based on tendency of shift and repeat

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experiments.

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3. Results and Discussions

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3.1 SOA formation

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Wall-loss corrected aerosol mass concentration was measured for 16 isoprene

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experiments. All experiments reported here have the same initial isoprene concentration,

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697 µg m-3 (250ppb). SOA yield (Y) is calculated as the mass of aerosol formed (wall-

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loss-corrected) (M0) divided by mass of hydrocarbon reacted (∆HC)

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Y=

M0 ∆HC

(1)

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Table 1 presents environmental chamber conditions for the experiments presented here.

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In the yield data reported from SMPS measurements in Table 1 and Figure 1, size

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distributions of all experiments were reviewed with no significant or patterned variations

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in size distribution with temperature observed.



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In Figure 1 the effect of environmental chamber reaction temperature on SOA

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yield is shown for both photo-oxidation and dark ozonolysis experiments. SOA yield

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decreases in Figure 1 by more than 2-fold between a reaction temperature of 278 K and

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300 K for NO + H2O2 photo-oxidation, H2O2 only photo-oxidation, and dark ozonolysis.

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This result supports a volatility driven process indicating the isoprene SOA at 278 K is

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made up of primarily of semi-volatile species. It should b

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For the NO photo-oxidation reactions with isoprene SOA yield is seen to decrease

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by more than half as the reaction temperature is raised from 278 K to 300 K. This

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decrease in yield is thought due to vaporization of semi-volatile components during this

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increase in reaction temperature. As reaction temperature is further increased from 300 K

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to 313 K SOA yield for the NO photo-oxidation system is seen to increase slightly,

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arguably within measurement error. This increase in yield could either be random

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measurement error or further reaction of isoprene to form less volatile components,

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potentially long chained and low volatility oligomers5, 8, 9.

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As temperature is increased in photo-oxidation reaction of isoprene with H2O2

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and during the dark ozonolysis of isoprene SOA yield is seen to continually decrease as

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temperature is raised from 278 K to 300 K and finally to 313 K. The final yields

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observed in both isoprene H2O2 photo-oxidation and dark ozonolysis at 313 K are very

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low. This large and continuous reduction of yield as temperature increases indicates that

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SOA formed through isoprene with H2O2 photo-oxidation or the reaction with O3 in the

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dark has very few non-volatile constituents. It should be noted that at 300 K two

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isoprene dark ozonolysis experiments give different values for yield, this discrepancy can


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be attributed to low SOA mass formation for these experiments. This discrepancy within

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the SOA yield measurements for isoprene dark ozonolysis leads to potentially two

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interpretations of the trend in SOA yield decrease with increasing temperature, either

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linear or non-linear (continually decreasing or asymptotic decrease).

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For SOA formation from isoprene in the presence of NO and H2O2 at high

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temperature Criegee intermediates will be stabilized, allowing further oxidation in the

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presence of OH20. Further, as Lambe et al. point out, OH concentration serves both SOA

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yield and the rate of formation of SOA21. It is assumed that OH is in abundance for NO

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and H2O2 experiments, but no measurements of hydroxyl concentration for the

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experiments reported here are available.

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In addressing SOA yield in experiments conducted in an atmospheric chamber,

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wall loss must be accounted for. Matsunaga and Ziemann as well as Zhang et al.

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conclude that vapor wall loss in SOA production in a Teflon® film chamber is

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significant; leading to underestimation of SOA yield22, 23. In the CE-CERT chamber,

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used in experiments reported here, wall loss has been considered through many

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experiments, beginning with Carter et al11. Further, the large size, 90 m3, of the CE-

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CERT chamber was intended to minimize or virtually eliminate chamber wall losses.

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With the preface on wall losses above it should be noted that wall losses for SOA

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formation from isoprene have not been investigated in the CE-CERT chamber, but wall

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loss for other SOA systems, such as α-pinene and m-xylene, have been investigated. In

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these investigations the CE-CERT chamber wall losses have been characterized with no

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indication of significant vapor wall loss has been observed, beyond that of Carter et al11.



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These observations of no significant wall loss hold even during temperature cycling of

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the chamber (hot to cold and cold to hot) when SOA formation from α-pinene is studied.

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3.2 SOA physical characterization

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Particle density and particle volatile fraction remaining were measured for a

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representative subset of the isoprene SOA experiments by APM-SMPS and VTDMA

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respectively. The physical and chemical SOA characterization data is summarized in

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Table 2. It should be noted that particle density and VFR are reported as a mean of

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measurements after particle nucleation, with error-bars indicating the observed deviation

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from this mean.

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SOA density, as measured by the APM-SMPS, is provided in Figure 2 for all

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three isoprene oxidant systems. The error bars in Figure 2 represent the maximum

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observed range of density observed in the SOA after nucleation, as measured by APM-

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SMPS. Density measurements for SOA formed from the reactions of isoprene range

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from 1.31 to 1.49 g/cm3 and are generally consistent with previous measurements of

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Kuwata et al. and Engelhart et al.24, 25. In comparing the broad observations from Kuwata

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et al and the data presented in Figure 2 the SOA density data presented here is consistent

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with dicarboxylic acids. The assertion that the SOA from isoprene is made up of

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dicarboxylic acids is further supported in the SOA chemical characterization data

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presented in section 3.3 and in Table S1.

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Within the observed measurement error, SOA density from isoprene photo-

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oxidation experiments with H2O2 alone does not vary with reaction temperature. The

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approximate mean density at all observed temperatures for isoprene reacting with H2O2 is


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1.45 g/cm3. This is comparable to isoprene photo-oxidation experiments conducted by

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Kuwata et al. where the reaction was conducted with H2O2, but in the absence of NO.

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SOA density for isoprene dark ozonolysis also does not vary with reaction temperature.

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Nevertheless, the average density for SOA produced in isoprene dark ozonolysis of 1.38

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g/cm3 is lower than the average observed for isoprene H2O2 only photo-oxidation, but

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still within the range reported by others previously24, 25.

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During isoprene photo-oxidation experiments done in the presence of NO, SOA

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density is observed to decrease slightly with increasing reaction temperature. This

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decrease is observed outside the margins of experimental error. This observation of

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decreasing density with increased temperature for the isoprene photo-oxidation

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experiments in the presence of NO indicates that the semi-volatile species observed at

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lower reaction temperature are of higher density. The densities at 278 K for the NO

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photo-oxidation are, within error, the same as those from H2O2 only photo-oxidation and

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dark ozonolysis. Furthermore densities at higher temperatures for the NO photo-

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oxidation experiments have the lowest observed densities. These lower observed

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densities could be a product of evaporated dicarboxylic acids leaving organic nitrate

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oligomer cores as indicated by volatility data and chemical characterization data reported

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here and previously8, 9, 26.

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Figure 3 plots SOA volatile fraction remaining (VFR) for all three isoprene

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oxidation systems. Less semi-volatile products are formed at higher temperatures; VFR

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increases with temperature for all three oxidation systems. In Figure 3, isoprene NO +

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H2O2 photo-oxidation VFR is the same value at 300 K and 313 K, indicating little change



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in particle volatility at higher temperature. Both isoprene NO + H2O2 and H2O2 only

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photo-oxidation show non-linear VFR trends, they plateau at 300 K. However, dark

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ozonolysis VFR does not plateau at the two higher temperatures but behaves linearly

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continuing to increase throughout the temperature range tested. In all three scenarios

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VFR is increasing with temperature, indicating a non-volatile SOA core. Overall, the

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high particle volatilities at low temperature indicate that the SOA is made up of semi-

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volatile species. The trend of the SOA VFR results agrees with the SOA yield and

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density. It is hypothesized that upon heating the SOA reaction, less semi-volatile species

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are present in the particle phase and reactions are occurring faster leading to an aerosol

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made of more oligomer products of much lower volatility. This hypothesis will be

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further supported by PILS-ToF chemical characterization.

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3.3 SOA chemical characterization

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Two mass spectral particle characterization data sets are compared from the AMS

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and the PILS-ToF. The HR-AMS gives quantitative chemical description through the

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elemental ratios, O:C, H:C, and N:C. The PILS-ToF provides qualitative chemical

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speciation of the particle phase compounds and elemental ratios are then interpolated

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from molecular formula. AMS and PILS-ToF calculated average particle elemental

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ratios H:C, O:C, and N:C ratios are presented in Table 2. The overall chemical

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characterization of O:C, H:C, and N:C ratios of particle composition of the two methods

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agree. PILS-ToF reports slightly higher O:C ratios for all of the isoprene SOA

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experiments. Another discrepancy between AMS and PILS-ToF is isoprene NO + H2O2

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where an average of data for photo-oxidation experiments (EPA1559A, EPA1353A, and


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EPA1288A) shows that regardless of reaction temperature, the PILS-ToF observed H:C

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ratio is approximately 0.5 less than that observed by AMS. Table 2 shows that AMS

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elemental ratios are insensitive (within the error of the data) to changes in reaction

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temperature.

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Figure 4 shows the PILS-ToF mass spectra for isoprene NO + H2O2 photo-

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oxidation experiments at two reaction temperatures, 300 K and 278 K. For NO + H2O2

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isoprene photo-oxidation performed at 300K (Fig 4a), the PILS-ToF mass spectra have a

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clear repeating ion peak pattern indicative of oligomerization. It should be noted that the

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data reported in Figure 4a has been presented and discussed elsewhere and used here as a

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base of comparison9.

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In contrast, PILS-ToF mass spectra of an isoprene NO + H2O2 photo-oxidation

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experiment EPA1559A performed at 278 K, does not have an evident repeating pattern of

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mass spectral peaks indicative of oligomerization (Fig. 4b). Yet oligomers could be

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present. Detailed PILS-ToF molecular formula matching provided in Figure 7 and Table

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S1 show that there are oligomers observed in Figure 4b, but clearly not as many. Instead

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the PILS-ToF mass spectra in Figure 4b is shifted to a lower molecular weight with a

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broad diverse “hump” of peaks observed between m/z 200 and m/z 600. It is thought that

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these smaller molecular weight species correlate to semi-volatiles. SOA formation at 278

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K may be dominated by the condensation of semi-volatiles with reaction rates of

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oligomerization slowed by kinetic effects of decreased temperature and decreased

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concentrations of oligomer precursors.



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In Figure 5a, isoprene H2O2 photo-oxidation experiment EPA1566A performed

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at 313 K, mass spectral peak intensity lies below m/z 400, with most mass spectral peak

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intensity falling below m/z 200. In Figure 5b, photo-oxidation experiment EPA1467A

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performed at 300 K in the absence of initial NO, larger m/z are observed with the mass

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spectral intensity existing in another broad “hump” of mass spectral peaks between m/z

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100 and m/z 400. The PILS-ToF mass spectra of the H2O2 photo-oxidation experiment

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EPA1556A performed at 278 K is shown in Figure 5c. Figure 5c shows a further mass

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spectral intensity increase at higher molecular weights, greater than m/z 200, in

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comparison to Figures 5a and 5b. Furthermore, the mass spectral “hump” observed in

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Figure 5b has decreased between m/z 200-300 with discrete high intensity and higher m/z

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peaks in its place at above m/z 300. In contrast to the PILS-ToF mass spectral results for

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the NO + H2O2 photo-oxidation, PILS-ToF mass spectral peak intensity for the H2O2

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only system increases at all m/z with increasing temperature, including high m/z; a result

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indicative of a volatility driven process. PILS-ToF results in Figure 5a low molecular

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weight and high volatility SOA.

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Figure 6 shows PILS-ToF mass spectral results for isoprene dark ozonolysis

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experiments at 3 temperatures, 313 K, 300 K, and 278 K. In Figure 6a, isoprene dark

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ozonolysis experiment EPA1563A performed at 313K, nearly all mass spectral intensity

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is observed below 400 m/z. Although in Figure 6b, the PILS-ToF mass spectra for

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isoprene dark ozonolysis experiment EPA1563A performed at 300K, mass spectral

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intensity is below m/z 400 with two sets of peaks at higher m/z ranges observed between

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m/z 400-500 and m/z 600-700. For Figure 6c, the PILS-ToF mass spectra for isoprene


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dark ozonolysis experiment EPA1563A performed at 278K, overall mass spectral

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intensity increase below m/z 400 while the mass spectral peaks observed in Figure 6b

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above m/z 400 are present but far less intense. Like the case of H2O2 photo-oxidation,

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the change in PILS-ToF mass spectra with temperature observed in Figure 6 for isoprene

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dark ozonolysis support a SOA formation process driven by chemical species volatility,

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and not by reactions forming high molecular weight chemical species.

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Figure 7 shows Van Krevelen diagrams of molecular matches for PILS-ToF mass

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spectral ions 27 observed to have intensity greater than 1000 ion counts in Figure 4, the

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PILS-ToF mass spectra for isoprene NO + H2O2 photo-oxidation experiments. The

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observed intensity in Figure 7 is provided by a grey scale, where a black open circle

293

represents a molecular match at 5000 ion counts and above and no circle (white)

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represents zero ion counts. Details of the mass spectral peaks observed in Figure 7 are

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provided in Table S1. For isoprene NO + H2O2 photo-oxidation experiments EPA1353A

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and EPA1559A performed at 300 K and 278 K respective, show the same general focal

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point at O:C=1 and H:C=1.5 (Figure 7a and 7b). However Figure 7a and 7b differ in the

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quadrant below O:C=1 and H:C=1.5; many more points are observed in this quadrant in

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Figure 7b. It is hypothesized that these points with lower O:C and H:C in Fig. 7b are less

300

oxidized hydrocarbons species of higher volatility.

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Van Krevelen diagrams of molecular matches to PILS-ToF mass spectral ions

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observed at intensities above 1000 ion counts observed in Figure 5, for isoprene H2O2

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photo-oxidation experiments done in the absence of initial NO, are provided in Figure 8.

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The detailed molecular matches in Figure 8 are presented in Table S1. In Figures 8a-c,



305

for H2O2 photo-oxidation experiments EPA1566A, EPA1467A, and EPA1556 performed

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at 313 K, 300 K, and 278 K respectively, some trends can be observed. First, though the

307

eye is drawn to the focal point in all three Van Krevelen diagrams at O:C=1.0 and

308

H:C=2.0 matched to the molecule C5H10O5, most of the molecules and mass spectral

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intensity lies in the quadrant below O:C=1.0 and H:C=2.0. The molecular abundance and

310

intensities grow in the lower left quadrant of Figure 8a-c as temperature decreases.

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Indicating, as was hypothesized for NO + H2O2 photo-oxidation, that semi-volatile

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species occur at lower O:C and H:C. These molecules below an O:C=1.0 and H:C=2.0

313

are shown in Table S1 to be small less oxygenated hydrocarbons. Nevertheless, three

314

clear lines also grow in intensity and length in Figure 8a-c as temperature decreases,

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these lines occur at O:C=1, H:C=2 and H:C = -(O:C) + 3.

316

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Figure 9 shows Van Krevelen diagrams of molecular matches for PILS-ToF mass

317

spectral ions observed to have intensity greater than 1000 ion counts in Figure 4, the

318

PILS-ToF mass spectra for isoprene dark ozonolysis experiments. The observed intensity

319

in Figure 9 is provided by a grey scale as in Figures 7 and 8. The details of the mass

320

spectral peaks observed in Figure 9 are provided in Table S1. In comparing Figure 9a-c,

321

for isoprene dark ozonolysis experiments EPA1563A, EPA1445A, and EPA1483A

322

performed at 313 K, 300 K and 278 K respectively, show a focal point, as observed in

323

Figure 8, at O:C=1 and H:C=2. The molecular abundance and intensities grow in the

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lower left quadrant of Figure 9a-c, below O:C=1 and H:C=2, as temperature decreases.

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This result indicates, as was hypothesis for the photo-oxidation systems, that semi-

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volatile species occur at lower O:C and H:C.


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5. Acknowledgements We gratefully acknowledge funding support from University of California

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Transportation Center, W. M. Keck Foundation, National Science Foundation (ATM-

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0449778 and ATM-0901282), the Naval Surface Warfare Center, Corona Division, and

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University of California, Riverside, Department of Chemical and Environmental

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Engineering. We also acknowledge Kurt Bumiller and Charles Bufalino for experimental

333

setup, William P. L. Carter and Gookyoung Heo for helpful discussions.

334 335

Supporting Information

336

The supporting information for this manuscript, Table S1, gives a detailed table of PILS-

337

ToF observed ions and corresponding ion formula matches. This information is available

338

free of charge via the Internet at http://pubs.acs.org/

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340

6. Reference

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7. Tables

430 431

Table 1. Experimental conditions and results

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433 434 435 436 437

Run

Temperature (K)

∆HC (µg m-3)

[NO]0 (ppb)

[H2O2]0a (ppm)

[O3]0 (ppb)

EPA1483A EPA1739A EPA1556A EPA1559A EPA1445A EPA1440A EPA1736A EPA1353Ac, d EPA1288A d EPA1567A EPA1467Ad EPA1563A EPA1505A EPA1566A EPA1803A EPA1508A

278 278 278 278 300 300 300 300 300 300 300 313 313 313 313 313

278 301 669 697 394 334 401 697 697 585 697 418 446 557 660 362

0 0 0 500 0 0 0 500 500 0 0 0 0 0 0 500

0 0 3 3 0 0 0 3 3 3 3 0 0 3 3 3

125 135 0 0 125 125 150 0 0 0 0 125 125 0 0 0

a

Final SOAb (µg m-3) 25 27 196 288 7 6 17 68 68 15 55 10 5 3 17 42

SOA Yield

Reaction Notes

0.09 0.09 0.29 0.41 0.02 0.02 0.04 0.10 0.10 0.03 0.08 0.02 0.01 0.01 0.02 0.12

dark ozonolysis dark ozonolysis photo-oxidation photo-oxidation dark ozonolysis dark ozonolysis dark ozonolysis photo-oxidation photo-oxidation photo-oxidation photo-oxidation dark ozonolysis dark ozonolysis photo-oxidation photo-oxidation photo-oxidation

Initial H2O2 concentration is not measured, but estimated by isoprene decay. Assumed average measured density of 1.34 g cm-3. c Data previously published in 9. d Light intensity was higher in these experiment with an estimated NO2 photolysis rate of 0.51 min-1 b


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Table 2. Summary of SOA chemical and physical characterization data

Run

Density (g cm-3)

VFR

EPA1483A EPA1739A EPA1556A EPA1559A EPA1445A EPA1440A EPA1736A EPA1353A EPA1288A EPA1567A EPA1467A EPA1505A EPA1563A EPA1566A EPA1803A EPA1508A

1.40 1.49 1.46 1.36 1.33 1.32 1.45 1.34 1.36 1.38 1.45 1.31

0.25 0.12 0.31 0.39 0.71 0.48 0.63 0.47 0.73

AMS H:C 1.40 ±0.10 1.55 ±0.20 1.52 ±0.10 1.52 ±0.15 1.50 ±0.10 1.48 ±0.20 1.72 ±0.20 1.46 ±0.20 1.45 ±0.18 -

AMS O:C 0.20 ±0.06 0.30 ±0.08 0.51 ±0.10 0.25 ±0.08 0.55 ±0.05 0.31 ±0.12 0.45 ±0.05 0.29 ±0.12 0.19 ±0.10 -

AMS N:C 0.00 0.00 0.05 ±0.04 0.00 0.09 ±0.04 0.00 0.00 0.00 0.00 -

PILS-ToF H:C 1.44 1.63 0.96 1.33 0.81 1.62 1.53 1.42 -

a

-no APM-SMPS data available, -no VTDMA data available, c -no AMS data available, d -no PILS-ToF data available b


PILS-ToF O:C 0.53 0.85 0.81 0.66 0.82 0.80 0.76 0.74 -

PILS-ToF N:C 0.00 0.00 0.03 0.00 0.02 0.00 0.00 0.00 -

PILSToF Percent Ions Matched 83 71 68 64 78 84 75 89 -

Notes

a, b c, d

c a, c, d c, d a, c b, d b, d c, d a, b b c, d c, d


8. Figures

Figure 1: Experimental SOA yield as a function of environmental chamber reaction temperature for a) NO and H2O2 photo-oxidation experiments and b) dark ozonolysis experiments


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1 2 3 4

Figure 2: SOA density as a function of environmental chamber reaction temperature for isoprene H2O2 only photo-oxidation, NO + H2O2 photo-oxidation, and dark ozonolysis.

5



6 7 8 9

Figure 3: Volatile fraction remaining as a function environmental chamber reaction temperature for isoprene H2O2 only photo-oxidation, NO + H2O2 photo-oxidation, and dark ozonolysis

10


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11 12 13 14 15 16 17 18 19

Figure 4: PILS-ToF mass spectra for isoprene NO photo-oxidation experiments (a) EPA1353A and (b) EPA1559A performed at environmental chamber temperatures of 300 K and 278 K respectively. Figure 4a Reprinted from Clark, C. H.; Nakao, S.; Asa-Awuku, A.; Sato, K.; Cocker, D. R., Real-Time Study of Particle-Phase Products from α-Pinene Ozonolysis and Isoprene Photooxidation Using Particle into Liquid Sampling Directly Coupled to a Time-of-Flight Mass Spectrometer (PILSToF). Aerosol Science and Technology 2013, 47, (12), 1374-1382. 9 Copyright 2013 Taylor & Francis.

20



21 22 23 24

Figure 5: PILS-ToF mass spectra for isoprene H2O2 photo-oxidation experiments (a) EPA1566A, (b) EPA1467A, and (c) EPA1556A performed at environmental chamber temperatures of 313 K, 300 K, and 278 K respectively


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25 26 27 28

Figure 6: PILS-ToF mass spectra for isoprene dark ozonolysis experiments (a) EPA1563A, (b) EPA1445A, and (c) EPA1483A performed at environmental chamber temperatures of 313 K, 300 K, and 278 K respectively

29



30 31 32 33 34 35 36

Figure 7: PILS-ToF ion matches presented as Van Krevelen Plots for isoprene NO photo-oxidation experiments (a) EPA1353A and (b) EPA1559A performed at environmental chamber temperatures of 300 K and 278 K respectively. *Note mass spectral intensity of an individual ion is presented through a gradient gray scale, where white represents no ion intensity, black represents an ion intensity of 5000 counts or above.

37


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38 39 40 41 42 43 44

Figure 8: PILS-ToF ion matches presented as Van Krevelen for isoprene H2O2 photo-oxidation experiments (a) EPA1566A, (b) EPA1467A, and (c) EPA1556A performed at environmental chamber temperatures of 313 K, 300 K, and 278 K respectively. *Note mass spectral intensity of an individual ion is presented through a a gradient gray scale, where white represents no ion intensity, black represents an ion intensity of 5000 counts or above.



45 46 47 48 49 50 51

Figure 9: PILS-ToF ion matches presented as Van Krevelen for isoprene dark ozonolysis experiments (a) EPA1563A, (b) EPA1445A, and (c) EPA1483A performed at environmental chamber temperatures of 313 K, 300 K, and 278 K respectively. *Note mass spectral intensity of an individual ion is presented through a gradient gray scale, where white represents no ion intensity, black represents an ion intensity of 5000 counts or above.


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from the Dark Ozonolysis and Photo-Oxidation of ... - ACS Publications

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