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Letter
High Molecular Weight Dimer Esters are Major Products in Aerosols from #-pinene Ozonolysis and the Boreal Forest Kasper Kristensen, Ågot Kirsten Watne, Julia Hammes, Anna Lutz, Tuukka Petäjä, Mattias Hallquist, Merete Bilde, and Marianne Glasius Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.6b00152 • Publication Date (Web): 01 Jul 2016 Downloaded from http://pubs.acs.org on July 3, 2016
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High Molecular Weight Dimer Esters are Major
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Products in Aerosols from α-pinene Ozonolysis
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and the Boreal Forest
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Kasper Kristensen*,1, Ågot K. Watne2, Julia Hammes2, Anna Lutz2, Tuukka Petäjä3, Mattias
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Hallquist2, Merete Bilde1 and Marianne Glasius1
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1
Department of Chemistry and iNANO, Aarhus University, DK-8000 Aarhus C., Denmark
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2
Department of Chemistry & Molecular Biology, University of Gothenburg, Sweden
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3
Department of Physics, FI-00014 University of Helsinki, Finland
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ABSTRACT
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This study investigates the contribution of high molecular weight (HMW) dimer esters to
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laboratory-generated α-pinene gas- and particle-phase secondary organic aerosol (SOA) and
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particulate matter (PM) collected at the Nordic boreal forest site of Hyytiälä, Finland.
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Laboratory flow reactor experiments (T=25 °C) show that dimer esters from ozonolysis of α-
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pinene contribute between 5-16% of freshly formed α-pinene particle-phase SOA mass.
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Increased formation is observed at higher relative humidity (RH) of ~40% and the presence
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of a hydroxyl radical (OH) scavenger shows to affect the formation of dimer esters. Out of
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the 28 dimer esters identified in laboratory α-pinene SOA, 15 are also observed in ambient
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PM samples, contributing between 0.5 and 1.6 % of the total PM1. The observed esters show
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good correlation with known α-pinene SOA tracers in collected PM samples. This work
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reveals an, until now, unrecognized contribution of dimer esters from α-pinene oxidation to
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boreal forest PM.
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1. INTRODUCTION
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The atmospheric oxidation of α-pinene through its reaction with hydroxyl radicals (OH),
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ozone (O3) and nitrate radicals results (NO3) in a complex mixture of different products
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including carbonyls, alcohols and carboxylic acids.1 Of the identified oxidation products, low
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molecular weight (LMW; MW ̴ 100-200 Da) carboxylic acids have been shown to contribute
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significantly to both ambient and laboratory-generated α-pinene secondary organic aerosol
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(SOA) mass concentration.1-5 Although abundant in α-pinene SOA, LMW compounds, such
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as carboxylic acids, are generally considered unlikely candidates in the gas-to-particle
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conversion associated with the new particle formation observed from α-pinene oxidation.6,7
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Recently, high molecular weight (HMW, MW 200-400 Da) extremely low-volatility gas-
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phase organic compounds (ELVOCs) have been proposed as key components in new particle
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formation from α-pinene oxidation.8,9
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HMW dimer esters have been identified in the particle phase of both ambient and
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laboratory-generated α-pinene SOA.4,6,10-19 Recent studies show that these compounds only
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form through O3–initiated and not OH-initiated oxidation of α-pinene, and thus are likely to
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originate from formation of Criegee Intermediates (CI).13 This, in combination with an almost
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immediate
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experiments4,12,18,19 points towards formation through gas-phase reactions involving early
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state oxidation products of α-pinene. We believe that the fast formation and estimated low
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volatility12,19,20 of the dimer esters make these HMW compounds likely candidates involved
formation
of
dimer
esters
observed
from
α-pinene
O3
oxidation
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in new particle formation from α-pinene oxidation. Although observed in multiple laboratory
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studies, very few observations of dimer ester in ambient SOA have been reported.10,12
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We report here chemical characterization of freshly formed laboratory-generated SOA from
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both OH- and O3-initiated oxidation of α-pinene to investigate the contribution of both
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carboxylic acids and dimer esters. To our knowledge, we present here the most extensive list
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of dimer esters ever observed in both laboratory-generated and ambient SOA and furthermore
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provide information on the formation of this important class of HMW compounds.
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2. MATERIALS AND METHODS
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Field sampling
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12h PM1 samples were collected twice a day starting at 6.00 a.m. and 6.00 p.m. local time
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at the SMEAR-II (Station for Measuring Forest Ecosystem-Atmosphere Relations21) forest
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field station in Hyytiälä, Finland (June 6-19, 2012). Sampling was performed using a
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denuder/filter sampling system. The system consists of a glass tube denuder coated with
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XAD-4 attached to a low volume sampler (LVS), which enables capture and hence separation
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of gas-phase and particle-phase compounds. The system is described in details in Kristensen
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et al.22
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Flow reactor experiment
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The G-FROST (Gothenburg - Flow Reactor for Oxidation Studies at low Temperatures) is
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a flow reactor consisting of a 1.91 m long Pyrex® glass cylinder with a diameter of 10 cm. A
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constant and well-controlled laminar flow is applied through the glass cylinder into which
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volatile organic compounds (VOC, 500 ppb) and oxidant (here 2 ppm O3) are added
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continuously through separate lines and allowed to react after a well-defined mixing region of
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the glass cylinder.23 The central part of the flow containing the resulting SOA from 4 min
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oxidation of α-pinene is collected through an exit at the end of the glass cylinder. A constant
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aerosol production can be running for several days depending on the requirements, e.g., time
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needed for sampling or aerosol characterization. Due to the short residence time of the flow
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reactor (4 min), i.e. time for oxidation, the resulting SOA is henceforth described as freshly
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formed. A detailed description of the G-FROST setup is given in Jonsson et al.23
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In this study, the G-FROST flow reactor was utilized for the generation of freshly formed
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SOA from the ozonolysis of α-pinene at dry (RH < 1%) and humid conditions (RH ~ 40%).
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In addition, experiments with and without the addition of an OH-scavenger (2-butanol) were
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performed to investigate the effect of such on the formation of dimer esters. In two
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experiments the oxidation was initiated by OH radicals. This was achieved by the addition of
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a PAM (Potential Aerosol Mass) chamber after the flow reactor. The PAM chamber is
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described in previous work.24 For OH oxidation experiments, no ozone was added to the flow
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reactor. The flow reactor output, here 500 ppb unreacted α-pinene, was diluted 10 times in
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the PAM chamber and oxidized by OH radicals generated inside the chamber by photolyzing
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O3 with two UV lamps (model no. 82-9304-03, BHK Inc., λ=254nm) in humid environment
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(14% and 31% RH for dry and wet experiment, respectively). The ozone and humidity was
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set to give an OH exposure of 4.0 x 1011 molecules cm-3s-1. The amount of α-pinene reacted
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at the point of collection in O3- and OH-initiated oxidation was 1.78 mg m-3 in ozone
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oxidation experiments and 0.28 mg m-3 in the OH-initiated oxidation experiments (see
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experimental data given in supplementary material). The resulting particles were collected by
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LVS denuder/filter sampling system. All experiments were performed at 25 °C (24.73
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±0.06°C).
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Sample preparation and UHPLC/MS analysis
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All collected PM samples were extracted in a 1:1 mixture of acetonitrile and methanol with
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camphoric acid added as internal recovery standard. Details on the extraction of PM samples
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and glass denuders are described in Kristensen et al.22 After extraction, samples were
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analyzed for their content of α-pinene derived organic acids and dimer esters using an Ultra
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High Performance Liquid Chromatograph coupled to the electrospray ionization source of a
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Bruker Daltonic quadrupole time-of-flight mass spectrometer (UHPLC ESI-qTOF-MS)
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operated in the negative (-) ionization mode. UHPLC-method for the separations of organic is
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identical to that of Kristensen et al.22 The operating conditions of the MS have been described
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elsewhere.25 Procedures for the quantification of organic acids and dimer esters along with
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the description and identification of potential artefacts are also described elsewhere.12
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3. RESULTS AND DISCUSSION
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Laboratory-generated α-pinene SOA
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A total of 28 different dimer esters were observed in flow tube SOA samples. 16 of the
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identified dimer esters have previously been tentatively identified in smog chamber studies
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and flow reactor experiments.4,6,10-19 Table 1 lists the observed dimer esters along with their
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recorded m/z-values, molecular formulas, major MS/MS fragments and contributions to flow
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reactor SOA and ambient PM1. Interestingly, all dimer esters show similar fragmentation
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with m/z 185, 167, 141, 127, and 123 present in most MS/MS spectra, thus indicating
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structural similarities, i.e. similar monomeric units. Based on the recorded MS/MS spectra
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suggested molecular structures of the identified dimer esters are given in Table S2. The
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formation mechanisms involved in the formation of the identified dimer esters are still
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unclear. A recent study by Zhang et al. suggests “a combination of acetylperoxy radicals
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yielding diacyl peroxides and their subsequent decomposition in the condensed phase after
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partitioning..” as a possible formation pathway for some of the dimer esters identified in α-
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pinene SOA.19 In the current study, the formation of several of the identified dimer esters is
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suggested to occur through gas-phase reactions involving stabilized Criegee Intermediate
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(SCI) with oxygenated organics (i.e. carboxylic acids) resulting in the formation of
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hydroperoxide esters. This reaction mechanism has been suggested in previous publications.
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15,26,27
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A simplified reaction scheme is shown in Figure 1.
Of the identified dimer esters, MW 342, 358, and 368 were the most dominant ones contributing as much at 8.3% by mass to the G-FROST SOA.
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Figure 2 shows the concentrations of carboxylic acids and dimers identified in the freshly
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formed SOA particle phase and gas phase from O3- and OH-initiated oxidation of α-pinene,
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respectively. The O3-initiated oxidation yields about 900 ng m-3 of dimers, almost twice the
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amount of the identified carboxylic acid. On the contrary, the OH-initiated oxidation does not
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form dimers above the detection limit resulting in SOA consisting mostly of carboxylic acids,
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thus supporting the previous observations of Kristensen et al. (2014)13 and later Zhang et al.
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(2015)19. Dimer esters were not observed in the gas phase, most likely due to the estimated
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low vapor pressures of the dimer esters.12,19
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Figure 3 shows the chemical composition of freshly formed SOA from O3-initiated
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oxidation under dry and humid conditions and with and without the addition of an OH-
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scavenger (2-butanol). Dimer esters contribute significantly (~5-16%) to the SOA formed
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from O3 -initiated oxidation of α-pinene. Furthermore, individual dimer esters show
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comparable and even higher contributions to the particle phase than commonly abundant α-
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pinene derived first-generation carboxylic acids (i.e. pinonic acid and pinic acid), indicating
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not only a fast formation of the dimer esters, but also highlighting their potential importance
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as nucleating agents in new-particle formation from ozonolysis of α-pinene in the boreal
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forest, i.e. Hyytiälä; an area with observed high frequency of new-particle formation events.28
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Figure 3 shows that RH has a clear effect on formation of dimer esters, since significantly
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higher (2 to 3 times) concentrations of dimer esters are present at 40% RH compared to dry
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conditions ( 0.5) were found between individual dimer
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esters (Figure 4B) and between the dimer esters and the second generation organic acids 3-
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methyl-1,2,3-butanetricarboxylic acid (MBTCA) and diaterpenylic acid acetate (DTAA)
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(Figure S2), in line with previous findings from Blodgett Forest, California, USA.12 Note that
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data in Figure S2 refer to particle phase only.
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As both MBTCA and DTAA have been shown to form only in the presence of OH13,30-34
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the observed correlation of dimer esters with MBTCA and DTAA could suggest that OH-
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radicals may be involved to some extent in the formation of some dimer esters, supporting the
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flow reactor findings in this study.
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Previous studies of dimer esters in ambient SOA have shown higher concentrations of
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dimer esters during night compared to daytime.10,12 This was later suggested to arise from
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increased OH-initiated oxidation of α-pinene in the sunlit hours of the day, limiting the
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formation of α-pinene CI and thus dimer esters.13 In the current study, no clear diurnal pattern
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is observed for dimer esters in Hyytiälä. This is likely attributed to a significant contribution
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from ozonolysis of α-pinene during daytime as well as nighttime at the sampling site. This
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hypothesis is supported by recent studies showing that OH and ozone contribute almost
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equally to the oxidation of monoterpenes during day-time in the summer period (June-
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August) in Hyytiälä.35 Despite the high O3 dependence observed in this and previous studies,
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only moderate correlation (R2=0.15, Figure S1) was found between dimer ester concentration
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and O3 at the sampling site. Similarly, although laboratory studies show increased formation
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of dimer esters at higher RH, no correlation (R2=0.06, Figure S1) with RH was found for
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ambient samples during the sampling period. During sampling, the O3 mixing ratio showed
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only small changes during the course of the day with slightly higher values during daytime
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(Average O3 = 33.7 ±5.9 ppb in the period 6.00 a.m. to 6.00 p.m.) compared to night time
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(Average O3 = 28.3 ±4.7 ppb in the period 6.00 p.m. to 6.00 a.m.). The similar day and night-
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time O3 mixing ratios along with the observed week to moderate correlations indicate that the
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observed variations in the dimer esters concentration (Figure 4) may be governed by other
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factors, i.e. changes in α-pinene emissions or influence of transport.
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Interestingly, a mild negative correlation (R2=0.28 Figure 4C) is observed between dimer
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esters and nitrogen oxides (NOx), indicating that the presence of NOx may reduce the
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formation of dimer esters in SOA in ambient air. If such an effect is present, this could
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indicate that gas-phase RO2-RO2 reactions are involved in the formation of dimer esters.
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Studies show that SCI may react with NO2 in the gas phase, thus increase in the NO2 mixing
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ratios may limit the SCI available for gas-phase reactions.36 Since the dimers esters could be
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involved in new particle formation, decreased formation due to NOx could explain
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suppression of particle formation and lower yields in the presence of NOx observed in
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monoterpene oxidation experiments.37,38 Furthermore, a favorable formation in low-NOx
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condition may explain why dimer esters are found at relatively high concentration at the
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remote boreal forest site presented in this study. Based on the high content of α-pinene
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derived dimer esters found in the freshly formed flow reactor SOA and the relative high
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concentrations in the PM1 samples in Hyytiälä we believe that dimer esters may play an
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important role in the formation and growth of particles in the atmosphere.
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Figure 1. Simplified reaction scheme suggesting the formation of dimer esters from reactions
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of α-pinene derived stabilized Criegee Intermediate with carboxylic acids (pinic acid).
215 216 25000
Particle phase
Gas phase Particle phase 1400
Acids Terebic acid Terpenylic acid Pinonic acid OH-Pinonicacid Pinic acid m/z 205 m/z 271 DTAA MBTCA
1200
ng m
Acids
ng m
-3
1000
15000 10000
800
Dimers
20000
600
0
217
Acids O3
OH
200 0
O3
Acids
400
5000
Acids
Particle phase
OH
Dimers MW302 MW314 MW316 MW328 MW330 MW332 MW336 MW338 MW340 MW342 MW344a MW344b MW354 MW356 MW358 MW360 MW362 MW368 MW370 MW372 MW374 MW378 MW384 MW386 MW388 MW400 MW406
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Figure 2. Concentration (ng m-3) of carboxylic acids and dimer esters identified in freshly
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formed gas- and particle-phase SOA formed from O3-initiated oxidation of α-pinene (0.5
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ppm α-pinene, RH~40%, with OH-scavenger) in the G-FROST flow reactor and OH-initiated
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oxidation (0.05 ppm α-pinene, RH~30%) in the PAM chamber.
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a-pinene ozonolysis, Dry a-pinene ozonolysis + 2-butanol, Dry a-pinene ozonolysis, RH 40% a-pinene ozonolysis + 2-butanol, RH 40%
6
Carboxylic acids
Dimer esters
5 O
% of SOA
HO
4
OH
O O
OH
O
O
3
2
% of SOA
% of SOA
10
10
5
0
Dry + OH-Scav.
Wet
Wet + OH-Scav.
358 368 384 400
5 4
2 1
Wet
Wet + OH-Scav.
302 316 328 330 332 336 338 340
MW 406
MW 400
MW 388
MW 386
MW 384
MW 378
MW 374
MW 372
344a 386 344b 388 354 400 360 362 370 374 378
3
2
Dry + OH-Scav.
MW 370
MW
4
Dry
MW 368
7 6
6
MW 362
MW 360
MW 358
MW 356
MW 354
MW 344b
MW 342
MW 344a
MW 340
MW 338
MW 336
MW 332
MW 330
312 314 342 356
8
0
Dry
MW 328
MW 12
15
D
14
All MW dimer esters
% of SOA
C
B
MW 316
MW 314
MW 312
MBTCA
MW 302
m/z 271
DTAA acid
m/z 205
Pinic acid
Pinonic acid
OH-Pinonic acid
Terebic acid
0
Terpenylic acid
1
0
Dry
Dry + OH-Scav.
Wet
Wet + OH-Scav.
222 223
Figure 3. A) Contribution (% of PM1) of identified carboxylic acids and dimer esters to
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freshly formed SOA formed from O3-initiated oxidation of α-pinene in the G-FROST flow
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reactor. Oxidation of α-pinene was performed at dry (
