Multiphase Photochemistry of Pyruvic Acid under Atmospheric

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Multiphase Photochemistry of Pyruvic Acid Under Atmospheric Conditions Allison Early Reed Harris, Aki Pajunoja, Mathieu Cazaunau, Aline Gratien, Edouard Pangui, Anne Monod, Elizabeth Campbell Griffith, Annele Virtanen, Jean-Francois Doussin, and Veronica Vaida J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b01107 • Publication Date (Web): 07 Apr 2017 Downloaded from http://pubs.acs.org on April 8, 2017

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The Journal of Physical Chemistry

Multiphase Photochemistry of Pyruvic Acid Under Atmospheric Conditions

Allison E. Reed Harris,† Aki Pajunoja,‡ Mathieu Cazaunau,3 Aline Gratien,§ Edouard Pangui,§ Anne Monod,|| Elizabeth C. Griffith,†, 1 Annele Virtanen, ‡ Jean-Francois Doussin,§* and Veronica Vaida†*

† Department of Chemistry and Biochemistry, CIRES, University of Colorado, Boulder, Colorado, 80309. ‡ Department of Applied Physics, University of Eastern Finland, Kuopio Campus, P.O. Box 1627, 70211, Kuopio, Finland. § LISA, UMR CNRS 7583, Université Paris Est Cretéil (UPEC), Université Paris Diderot (UPD), Institut Pierre Simon Laplace (IPSL), 94010 Cretéil, France || Aix Marseille Université, CNRS, LCE, Marseille, France

*Corresponding Author Emails: [email protected], [email protected]

1 Current Affiliation: Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland, 20742.

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Abstract Aerosol and molecular processing in the atmosphere occurs in a complex and variable environment consisting of multiple phases and interfacial regions. To explore the effects of such conditions on the reactivity of chemical systems, we employ an environmental simulation chamber to investigate the multiphase photolysis of pyruvic acid, which photo-reacts in the troposphere in aqueous particles and in the gas phase. Upon irradiation of nebulized pyruvic acid, acetic acid and carbon dioxide are rapidly generated, which is consistent with previous literature on the bulk phase photolysis reactions.

Additionally, we identify a new C6 product,

zymonic acid, a species that has not previously been reported from pyruvic acid photolysis under any conditions. Its observation here, and corresponding spectroscopic signatures, indicates it could be formed by heterogeneous reactions at the droplet surface. Prior studies of the aqueous photolysis of pyruvic acid have shown that high-molecular-weight compounds are formed via radical reactions; however, they are inhibited by the presence of oxygen, leading to doubt as to whether the chemistry would occur in the atmosphere. Identification of dimethyltartaric acid from the photolysis of multiphase pyruvic acid in air confirms radical polymerization chemistry can compete with oxygen reactions to some extent under aerobic conditions. Evidence of additional polymerization within the particles during irradiation is suggested by the increasing viscosity and organic content of the particles. The implications of multiphase specific processes are then discussed within the broader scope of atmospheric science.


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I. Introduction I.A. Atmospheric and Aerosol Chemistry Sunlight drives the complex physical and chemical cycles of Earth’s atmosphere, creating a delicate energy balance that defines our climate.1, 2 Atmospheric constituents filter incoming solar and outgoing terrestrial radiation and therefore play a central role in determining the global energy budget.3, 4 While many of these impacts are reasonably well quantified, such as that from abundant well-mixed gas-phase compounds,3 the contribution from particulate matter in the atmosphere remains dramatically less well-understood.5, 6 Aerosols, stable suspensions of condensed matter in air, influence the planet’s radiative balance directly by scattering sunlight and indirectly by initiating cloud formation. The physical and chemical characteristics of aerosol are key factors in determining the total aerosol optical depth and their efficiency as cloud condensation nuclei.7-10 However, with combined uncertainties near ± 1.0 W m-2, the effects of atmospheric particles account for the largest source of error in climate forcing calculations.3 They present a complex problem to model, as aerosol properties are controlled by the particle’s composition, morphology, and size, which are themselves governed by the chemistry of the greater atmospheric system.11-17 While the importance of inorganic components has been recognized for decades, it was only more recently documented that organic molecules comprise a large portion of aerosol mass on a global scale. Organic aerosols are directly emitted from biomass burning and anthropogenic energy combustion; however, they can also be produced in the atmosphere via the oxidation of volatile organic compounds (VOC).18-20 These aerosols, termed secondary organic aerosol (SOA), are a current focus of climate science, as their formation and aging pathways are not well constrained in global models.12, 21, 22


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Recently, aqueous organic chemistry within existing tropospheric particles has been identified as an important source of SOA,18, 23-29 and its incorporation into regional models greatly improves local estimates of secondary aerosol mass.30-34 Specifically, products from the oxidation of VOCs partition into the condensed phase of aerosol, where they are further processed into low-volatility or high-molecular-weight compounds.35, 36 Such species remain particulate upon droplet evaporation, resulting in additional condensed matter that is SOA.18, 29, 37-40 Isoprene, which encompasses approximately one third of global non-methane VOC emissions each year,41 and other terpenes are the precursors for a substantial portion of SOA resulting from condensed phase chemistry.18, 40, 42 Another recent advance in our understanding of SOA is the recognition of the importance of their surface composition and heterogeneous reactions.43-49 Fatty acids and other naturally occurring surfactants (surface active agents) partition to the air-water interface of an aerosol, affecting gas-particle mass transfer and further chemistry.50-54 Organic compounds are concentrated at the aerosol surface and oriented such that hydrophobic moieties are directed away from the aerosol and hydrophilic functionalities persist within. This, combined with the relatively water restricted nature of the interface, promotes heterogeneous reactions that are not always favorable in bulk systems.55-58 At present, these processes and their ramifications on aerosol are largely omitted from global climate forecasting models.59-61 In the quest to rigorously characterize properties of SOA, there has been a recent focus on fundamental organic chemistry under tropospheric conditions. There exists a significant body of literature focused on the photooxidation of VOC’s, which yields highly oxidized and/or high molecular weight products that are known SOA precursors.42,

62-64

Aqueous acid catalyzed

mechanisms have also been recognized as high yield pathways to SOA and have been linked to


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polymers and oligomers that further increase secondary organic mass.65, 66 Though less studied, direct photolysis is another possible sink for organic compounds that have a chromophore in the actinic range. Pyruvic acid, a key intermediate in the oxidative channels of isoprene and SOA formation,67, 68 is processed primarily through photodecomposition and displays interesting and intricate chemistry in the atmosphere.69-74 Though the literature has elucidated many nuances and complexities of isolated bulk and heterogeneous reactions, the atmosphere contains particulate matter that introduces surfaces and condensed matter, making the multiphase system fundamentally different from single-phase experiments.75-80 Each species will have some equilibrium between particulate matter and the gas phase, which will be determined by its own intrinsic chemical properties, as well as external conditions such as the relative humidity, pressure, temperature, and aerosol phase.14, 15, 81 Perhaps more importantly, there exists a high surface area of interfacial region on aerosols, which increases the importance of this auspicious environment in the atmosphere.52 Here, we discuss the photochemistry of nebulized aqueous solutions of pyruvic acid to highlight new multiphase specific chemistry and its effects on particle properties. I.B. Pyruvic Acid in the Atmosphere The primary source of pyruvic acid in the atmosphere is the particle-phase aqueous reaction between the hydroxyl radical and hydrated methylglyoxal,82 an abundant oxidation product of precursors such as isoprene and aromatic compounds.83-85 Due to its moderate vapor pressure, numerous field campaigns have detected gaseous pyruvic acid, in addition to that in clouds, fogs, and aerosols.86-88 Further condensed phase oxidation of pyruvic acid by OH has been linked to the formation of SOA in the atmosphere through the synthesis of oligomeric species and the generation of highly soluble organic acids, such as glyoxylic and oxalic acids.67, 68 However,


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pyruvic acid absorbs sunlight in the actinic range between 300 and 380 nm,89 and, because its reaction with OH is relatively slow,90, 91 its major tropospheric sink is direct photolysis.91, 92 The significance of the oxidative products must therefore be weighed against its fast photolytic decomposition. Nevertheless, the aqueous photochemistry may also be responsible for the creation of SOA because it forms high-molecular-weight products and acetic acid,69 which is one of the oxidative intermediates between pyruvic acid and oxalic and glyoxylic acids.67 Numerous computational,93-97 laboratory,69-71,

98-104

and chamber studies105-108 have characterized pyruvic

acid’s photo-induced reaction pathways in aqueous solutions and in the gas-phase, revealing rates and branching ratios that are extremely dependent on the environment, further complicating predictions of resultant SOA.71,

100

Given the sensitivity to conditions, the multiphase

photochemistry of pyruvic acid must also be explored in order to elucidate its likely primary atmospheric removal pathways and the implications for SOA. Previous literature on the photolysis of approximately 1 Torr of pyruvic acid, its vapor pressure at room temperature, under low buffer gas pressures concluded that the mechanism proceeds though simultaneous concerted hydrogen atom transfer and decarboxylation, a process that occurs with a quantum yield of unity on the singlet electronic potential energy surface.101-103 The immediate result from this reaction is methylhydroxycarbene; however, it isomerizes to acetaldehyde, the final observed species.102, 103 In contrast, experiments performed with much lower concentrations of pyruvic acid and close to ambient pressure of air and have detected many additional products, including acetic acid and CO.100, 105-108 Reed Harris et. al. (2016)100 executed a systematic study of the gas-phase photolysis of pyruvic acid as a function of buffer gas pressure and concluded that, while acetaldehyde and CO2 remain the major products under their conditions, the percent yield of acetic acid increases with increasing pressure of air,


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reaching 18% near 600 Torr.100 Additionally, reported photolysis quantum yields vary dramatically from the reactions performed with buffer gasses near 1 atm, from 0.85 to 0.24.100, 106108

Energetic and conformational changes to pyruvic acid in aqueous solution result in a dramatically different photolysis mechanism from that in the gas phase. First, when dissolved in water, approximately 60% of pyruvic acid hydrates to its geminal-diol, 2,2-dihydroxypropanoic acid (DHPA), which is not photoactive at wavelengths available near the Earth’s surface.109-111 Secondly, after photon absorption by the keto form in water, pyruvic acid undergoes intersystem crossing from the S1 (n, π*) state followed by internal conversion into T1 (n, π*),69, 98, 99, 112 an effect likely due to stabilization of electronic surfaces through interactions with water. Though all mechanisms invoke excitation and intersystem crossing into the T1 state of pyruvic acid, there are multiple proposed pathways for how the following aqueous phase chemistry proceeds. Early work by Davidson and Goodwin (1981)112 suggested an electron transfer mechanism. Guzman et al. (2006)70 also relied on electron transfer reactions, now employed to explain high molecular weight compounds including dimethyltartartic acid (DMTA). Griffith et. al. (2013)69 proposed a mechanism, in which the T1 (n, π*) state of pyruvic acid acts as a radical initiator by abstracting the acidic hydrogen atom from 2,2dihydroxypropanoic acid (Figure 1).69 This chemistry creates two radicals in the same solvent cage, resulting in a high probability for their recombination.69 Major products include CO2, acetic acid, and dimethyltartaric acid but the individual yields in solution are entirely dependent on the initial concentration of pyruvic acid and the presence of oxygen.71 Reed Harris et al. (2014)71 describes the kinetics for this mechanism, demonstrating that, in an aerobic environment, O2


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quickly quenches the radicals, favoring the formation of acetic acid and inhibiting, though not eliminating, the radical recombination reactions that create dimethyltartaric acid.71

Figure 1: Summary of gas and aqueous phase photolysis mechanisms for pyruvic acid (pyruvic acid), with selected products relevant to this work.71,

100

Blue

indicates pathways enhanced by oxygen and red those inhibited by oxygen. Figure 1 depicts a summary of the gas and aqueous photolysis products as described by the Vaida group.69, 71, 100 This figure also underscores the coupling between the different phases that will occur in a multiphase environment, hinting at the complexity of the system. Given the variability of pyruvic acid photochemistry under different conditions in the bulk aqueous and gas phases, the present study will consider the possibility of effects from mixed systems of particulate and gas phase matter. We perform the experiments in the atmospheric simulation chamber, CESAM,113 where pyruvic acid can be nebulized into a highly controlled environment,


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and for both the gas and particulate phases to be monitored in situ. We highlight important differences from and similarities to the bulk photolysis mechanisms and demonstrate the effect that photochemistry can have on aerosol properties including size, viscosity, and composition. The results emphasize the importance of integrating data from fundamental laboratory work with observations of multiphase systems in order to create a more robust understanding of chemistry as it likely fluctuates under different atmospheric conditions. II. Methods: CESAM Environmental Simulation Chamber and Experimental Procedure Here we impart a brief description of a few unique but essential details of CESAM (French for Experimental Multiphasic Atmospheric Simulation Chamber), which allow for chemical systems to be observed under conditions similar to the environment. In the supporting information we provide a full methods section (SI Section 1), and Wang et al. (2011)113 and Brégonzio-Rozier et al. (2015)39 offer a comprehensive description of the CESAM facility and analysis techniques. CESAM is a 4.2 m3 stainless-steel chamber, with a low surface area-tovolume ratio (minimizing wall processes), and full temperature and pressure control (Figure S1).113 A solar simulator is constructed with three 4000 W high pressure xenon arc lamps (MHDiffusion ®, MacBeamTM 4000) and 6.5 mm thick Pyrex windows to filter wavelengths below 300 nm. The result is an irradiance spectrum very similar to the actinic flux in the region responsible for pyruvic acid photolysis, 300-380 nm (Figure S2).89, 113 114 The chamber is equipped with a number of instruments to monitor experiments in situ. The chemical composition of the gas phase is analyzed with a Fourier Transform Infra-Red spectrometer (FTIR Bruker Tensor 37) coupled to a multipath path cell (total path length = 192 ± 4 m),113 and a proton-transfer reaction time of flight mass spectrometer (PTR-ToF-MS Series II, Kore Technology) that samples the air in counter-flow, preventing aerosol and droplet


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sampling.39 The number size distribution of dried particles is monitored by a Scanning Mobility Particle Sizer (SMPS).113 To deduce some information about the chemical composition of the aerosol, the chamber can be evacuated through a filter to collect a sample of the post-photolysis particles. Such samples were later extracted in methanol and analyzed with high-resolution electrospray ionization mass spectrometry operated in the negative ion mode. Lastly, for these experiments we also utilized an Aerosol Bounce Instrument (ABI) to measure the particle bounced fraction (BF), which indicates the viscosity of the humidified aerosols. This is a relatively new technique that has been employed in various chamber and field studies,115-120 and is described in more detail by Pajunoja et. al. 2015.115 Essentially, an ABI detects the fraction of size-selected particles bouncing off a single stage impactor by measuring the number of particles to and from its inlet and outlet, respectively. This reveals the amount of sampled particles that are viscous enough to bounce off the impaction plates, and gives some information of particle hygroscopicity.115-117 Aqueous pyruvic acid (0.1-0.4 M) was nebulized into CESAM at a high relative humidity (~90%) and ambient pressure (~1012 mbar) of air or nitrogen. Unless otherwise specified, here we focus on the photolysis of nebulized 0.1 M pyruvic acid because it results in a Henry’s law constant of ~4 × 107 M atm-1, which is between the standard Henry’s law constant for pyruvic acid (3.1 × 105 M atm-1) and one that reflects partitioning as observed in the atmosphere (2 × 109 M atm-1).71 Upon entering the chamber, some pyruvic acid partitions into the gas-phase, ensuring a true multiphase reaction where photolysis can occur in three distinct environments: as a vapor, near the aerosol surface, or inside the aqueous solution of the droplets. Nebulization of pyruvic acid was deemed complete when the organic mass in the aerosol phase reached 20 μg/m3, which generally corresponded to a total particle number concentration close to 1 × 105 particles/cm3 and


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an average dried particles size of 60 nm. For this study, we collected two filter samples, one following irradiation of nebulized 0.4 M pyruvic acid in air and another after the multiphase photolysis of nebulized 0.1 M pyruvic acid in nitrogen. These experiments were chosen to maximize the mass of sample collected on the filter. III. Results and Discussion III. A. Product Analysis of the Multiphase Photochemistry of Pyruvic Acid We first emphasize the agreement between the irradiation of nebulized pyruvic acid in air and the existing bulk photochemical mechanisms by highlighting common products. For all reactions performed with oxygen present, acetic acid and carbon dioxide appear in the FTIR spectra immediately after illumination. This is consistent with previous literature, as acetic acid and CO2 are known products from the photolysis pathways in both the gas and condensed phases (Figure 1).69, 71, 100, 106 Figure 2a shows the concentrations of these compounds as determined by FTIR, and clearly defines acetic acid and carbon dioxide as the major species residing in the gasphase. PTRMS data verifies the presence of acetic acid and also identifies acetaldehyde, a welldocumented product from the photolysis of pyruvic acid vapor (Figure 2b).100, 102, 106 Further, dimethyltartaric acid (Figure 3), a dimer of pyruvic acid that is known to form during its aqueous phase photolysis,69-71 was observed in the ESI(-)MS analysis of the filter sample from the photolysis of nebulized 0.4 M pyruvic acid in air (m/z 177.040).


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Figure 2: Gas-phase concentrations of pyruvic acid and photochemical products as detected by a) FTIR and b) PTRMS during the irradiation of nebulized 0.1 M pyruvic acid in air. We note that neither instrument samples the aerosol composition. The background colors indicate nebulization (white), dark decay (grey), and photolysis (yellow). The zymonic acid structure is shown in panel b.

Figure 3: ESI(-)MS from the filter sample taken after 1.3 hours of irradiation of nebulized 0.4 M pyruvic acid in air. Blue, positive facing peaks arise from the sample taken after photolysis, while the green, negative facing peaks are from the


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chamber blank. Signals from dimethyltartaric acid (177.040 m/z, *) and zymonic acid (157.014 m/z, §) are highlighted in red.

For comparison, we observed the gas-phase photolysis of pyruvic acid in humid air in CESAM (i.e. no particles were present), which produces primarily carbon dioxide and acetic acid. While the observation of a higher concentration of acetic acid than acetaldehyde in this control experiment is a deviation from previous literature on the gas-phase photolysis of pyruvic acid, it is not unreasonable, especially given that the percent yield of acetic acid from the gas phase photolysis is documented to increase as the pressure of dry air is increased.100 Changes in product ratios observed here may be further explained by the ability of water to influence reaction pathways by hydrogen bonding with pyruvic acid or by reaction with photolysis intermediates. Based on acetic acid’s rapid production while some pyruvic acid remains in the gas phase, and its subsequent decrease after the pyruvic acid vapor is depleted (Figure 2), we speculate that the gas-phase photolysis of pyruvic acid is the primary source of the observed acetic acid. However, laboratory studies of the photochemistry of aqueous pyruvic acid have concluded that, in an aerobic environment, the major products are also carbon dioxide and acetic acid.71 Therefore, since we cannot detect the aerosol composition in situ, and since species will have an equilibrium between the particles and the gas phase, it is not straightforward to gauge which reaction (vapor or condensed photolysis) acts as the primary sink for pyruvic acid under these conditions. We can, however, conclusively state that photolysis in each phase is active to some extent, given the identification of dimethyltartaric acid and acetaldehyde, as they are products distinct to the photochemistry in the aqueous and gas phase, respectively.71, 100


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Some information regarding the specific pathways active in the multiphase photochemistry can be deduced by examining the irradiation of nebulized pyruvic acid in nitrogen, which does not generate acetic acid (Figure S3, S4). This indicates that its multiphase formation from irradiation of pyruvic acid in air requires reaction of photolysis intermediates with O2, a condition that is consistent with the known mechanisms for the bulk photochemical reactions.71, 100

Within pyruvic acid’s aqueous photochemistry, it has been shown that O2 quenches radicals

(specifically AA in Figure 1) leading to the production of acetic acid.71 Similarly, Reed Harris et al. (2016)100 documented an increase in the acetic acid percent yield from irradiation of gas-phase pyruvic acid with increasing partial pressure of oxygen. They proposed a reaction between O2 and methylhydroxycarbene or radicals resulting from the direct bond cleavage of pyruvic acid.100 The above processes are likely also responsible for the formation of acetic acid from the multiphase photolysis of pyruvic acid in air, given that the system demonstrates sensitivity to the presence of oxygen similar to that of the bulk photochemistry. In addition to those expected from the known bulk processes, we also detect a series of new products from the multiphase photolysis of pyruvic acid in air. PTRMS peaks at m/z 62.032, 72.051, and 101.055 remain unidentified (Figure S5); however, a significant peak, at m/z 113.018, was characterized as arising from the fragmentation of zymonic acid (Figure 2b), a compound that has been sporadically investigated as a contaminant in pyruvic acid since 1835.121 Here we observed zymonic acid as a closed enol ring dimer of pyruvic acid (see Figure 2b for structure); however, in aqueous solution it has a number of tautomers that are explored in depth in the recent work of Perkins et al. (2016).122 This form of zymonic acid, which has an exact mass of 158.014 amu (C6O5H6), is actually observed at m/z 113.018 in the PTRMS because protonation of the acid group triggers decarbonylation and dehydration (Figure S6). To verify the


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peak at m/z 113.018 is due to this chemistry, pure zymonic acid, isolated from pyruvic acid (see SI Section 1.1 for details), was analyzed by PTRMS, and indeed produced a strong signal at m/z 113.018 (Figure S7). We also identify zymonic acid on the filter sample from the irradiation of nebulized 0.4 M pyruvic acid in air at m/z 157.014, corroborating its formation from the irradiation of this multiphase system (Figure 3). Zymonic acid’s evolution in the chamber during the reaction is similar to that of acetic acid, increasing in concentration for the first hour and subsequently decreasing. Unlike acetic acid, which is not photoactive and is likely removed from the gas phase due to its partitioning into particles, the conjugated ketone functional group on zymonic acid will likely absorb UVradiation available from our solar simulators, allowing for its photodecomposition. The loss of gaseous zymonic acid in these experiments, therefore, could be due to both partitioning into particles and to direct photolysis of the compound itself. Another notable property of zymonic acid, though not explored here, is its degree of unsaturation, making it susceptible to attack by ozone or OH radical in the atmosphere, which would trigger further chemistry. We note that zymonic acid only accumulates during nebulized experiments in air (Figure 2b, S3), and does not amass from irradiation of nebulized pyruvic acid in nitrogen, nor from the gas-phase photolysis of pyruvic acid, suggesting that H2O and O2 participate in its synthesis. Further, zymonic acid is not observed after irradiation of aqueous pyruvic acid, suggesting the air-water interface of the particles may play a crucial role in its formation.71 Figure 4 contains some evidence for a surface process in the pyruvic acid multiphase photochemistry. It shows the first FTIR spectrum of pyruvic acid in the dark compared with the post-photolysis spectrum and relevant reference spectra. The pink stars mark the C-O acid stretching mode (1134 cm-1) and the C-O-H bending mode (1218 cm-1) of pyruvic acid.123 In the


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pyruvic acid reference spectrum, the bend has about twice the oscillator strength as the stretch; however, in the FTIR spectrum of pyruvic acid prior to irradiation in the multiphase system, the two peaks are approximately equal in intensity and the stretch at 1134 cm-1 is slightly broadened. This indicates a change in the vibrational behavior of the stretch and/or the bend, likely due to hydrogen bonding of the acid group with water molecules.58, 124 Spectroscopic effects due to hydrogen bonding have been observed in vibrational spectra of water clusters as well as vibrational spectra of organic surfactants at the water-air interface.125, 126 The modification to the spectrum observed here, however, is not reproduced when pyruvic acid is in humid air, which implies it may be a result of pyruvic acid at or near the interface of the aqueous aerosol in the chamber (Figure S8). An infrared reflection absorption spectrum (IRRAS, see Griffith et al. (2012)55 for experimental description) of pyruvic acid at the surface of water shows a broad peak at 1134 cm-1 that corresponds well with the C-O acid stretch in the FTIR spectrum of multiphase pyruvic acid before photolysis (Figure S8). This indicates some pyruvic acid likely partitions to the surface of the aerosol in CESAM, with consequences for reaction pathways, kinetics, and product yields, possibly enhancing channels not previously identified, such as that leading to to zymonic acid. While we cannot confirm zymonic acid signal in the FTIR spectra, likely due to low gasphase concentrations that are obscured by pyruvic acid and water stretches (Figure 4), there are other residual spectroscopic features, only formed from the multiphase photolysis of pyruvic acid in air (see Figure S9 for final spectrum from a reaction in nitrogen). These species, each marked by a purple § in Figure 4, may be related to the uncharacterized compounds detected by PTRMS. While we have not been able to determine the species responsible for these features, the frequencies at which these absorb are appropriate for O-O stretches in peroxides or ring stretches


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in epoxides,127 either of which could be intermediates in pyruvic acid radical chemistry or new oxidized products formed via heterogeneous processes.

Figure 4: FTIR spectrum before (blue) and after (green) photolysis of nebulized 0.1 M pyruvic acid in air. Also shown are reference spectra for pyruvic acid (red), acetic acid (black), and zymonic acid122 (grey). The pink stars indicate pyruvic acid’s C-O acid stretch and C-O-H bend, highlighting the change in ratio of the two peaks between the dark spectrum and the reference spectrum. Unidentified peaks in the post-photolysis spectrum are labeled with purple §.

We also document polymerization within particles during the multiphase photolysis in both air and nitrogen. In addition to the formation of zymonic acid and dimethyltartaric acid from the irradiation of nebulized pyruvic acid in air (Figure 3), the filter sample collected after the multiphase photolysis of pyruvic acid in nitrogen shows significantly more high molecular weight compounds (HMWC) than the chamber blank, further evidence that polymers are created in this system (Figure S10). Currently, the HMWC that have been detected in atmospheric aerosol cannot be fully explained with mechanisms that are favorable in air. Among the photochemical studies, proposed pathways progress through radical polymerization reactions that are known to be inhibited by O2, similar to the formation of dimethyltartaric acid in Figure 1.35, 71,


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128-130

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In contrast to these pathways, the production of zymonic acid is an example of chemistry

that increases the complexity of the solution only when oxygen is present. Further, detection of dimethyltartaric acid implies that radical polymerization can occur to some extent under oxygenated atmospheric conditions. Such processes are important to understand because HMWC like these may partition to the surface of aerosol, alter particle optical and mass-transfer properties, increase the hydrophobicity and viscosity of the particle, or contribute to the creation of new SOA due to their low vapor pressures. III.B. Effect of Irradiation on Particle Properties Impacts of the multiphase photolysis of pyruvic acid on the particles in the system were tracked here by measuring droplet size, particle number size distribution, and the particle bounced fraction. A combined view of the organic mass, number concentration, and mean size of the particles can be seen in contour plots included in the supplemental information (Figure S11). We first observe a dark decay before irradiation to allow for light specific processes to be highlighted through comparison. During this period, collisions in the chamber lead to coagulation of particles and a corresponding decrease in number concentration and increase in size. The total dry aerosol loading, however, remains constant, indicating a stable equilibrium between pyruvic acid in the gas and condensed phase before photolysis. Upon irradiation, the organic mass increases dramatically for approximately an hour, when it reaches a level that persists for the duration of the experiment (Figure 5a). Decomposition of pyruvic and the corresponding increase of products will disrupt the equilibria between the particles and the gas phase, in this case causing uptake of organic molecules by the particle. We have seen chemical evidence for such partitioning in Figure 2, which shows decreasing gaseous acetic acid, a compound without chemical sinks in this system. Further, heterogeneous processes


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between organic gas-phase reactants and molecules at the aerosol surface can yield low-volatility compounds that remain condensed, increasing the organic mass in particles. Conversely, there is no discontinuity in the decrease in particle number concentration when the lamps are switched on, demonstrating steady particle coalescence throughout the reaction (Figure 5b). However, when photolysis begins, there is a well-defined increase in the rate at which particles are growing, which, given the above, must be explained by something other than an increase in particle coagulation (Figure 5c, see Supporting Information, Table S1 for determined linear equations demonstrating statistically different slopes prior to irradiation and during photolysis for reactions in air and nitrogen). While the partitioning of organic content influences aerosol size, other factors, such as water uptake or formation of surface films may also play a role. These changes to particle size and organic content will in turn influence other aerosol properties, such as overall hygroscopicity and aerosol phase.


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Figure 5: (a) Total organic mass calculated assuming spherical aerosol with a density of 1 g/ml, (b) particle number concentration, and (c) mean particle diameter during the photolysis of nebulized 0.1 M pyruvic acid in air (yellow traces) and in nitrogen (blue traces). The background colors indicate nebulization (white), dark decay (grey), or photolysis (yellow). The grey striped background shows times for which nebulization is occurring for the reaction in nitrogen and dark decay for the reaction in air. In part c, best-fit lines for the data just before and just after the lights are turned on are shown to highlight the discontinuity in


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particle size upon irradiation. Determined linear equations and related uncertainties are listed in the Supporting Information (Table S1) and clearly demonstrate statistical difference between the slopes prior to and during irradiation.

Figure 6: Particle bounced fraction (BF) with RH prior to and during the photolysis of nebulized 0.1 M pyruvic acid in air (hollow triangles) and in nitrogen (filled dots). The time scale marked in the legends is set so that the photolysis for each experiment begins at t=0 minutes. In addition, (RH, BF) data for atomized sucrose particles measured at the same day are shown in black hollow squares. In all cases, selected dry particle size (ddry) was 150nm. To assess the effect that multiphase photochemistry of pyruvic acid has on the overall phase of the particles as a function of relative humidity (RH), the particle bounced fraction (BF)


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was measured with an aerosol bounce instrument (ABI). Particles with BF=0 are considered to be liquid, with an increased bounced fraction corresponding to a more semisolid particle. Figure 6 shows the bounced fraction of particles increasing throughout photolysis for both nebulized pyruvic acid in air and in nitrogen, compared with that of atomized sucrose particles (150nm). Initially the nebulized pyruvic acid particles have a BF = 0.80±0.05 at RH = 20±3% and BF > 0 until about 45% relative humidity, above which all particles are liquid. As the system is irradiated, the bounce curve shifts to higher relative humidities and the dry bounced fraction (RH < 25%) slightly increases, up to around BF = 0.9. The instantaneous response of particle viscosity to irradiation was captured with ABI data when the lights were switched on in the middle of ABI humidogram; in Figure 6 the bounced fraction prior to the photolysis (in air) is shown in blue triangles whereas the triangles for the bounced fraction from the same scan, but during irradiation, are red and are shifted somewhat toward higher relative humidity. Regardless of the background gas, the nebulized pyruvic acid particles exhibit an increase in the relative humidity at which the phase transition from solid to liquid occurs; though, the increase is slightly larger in nitrogen than in air. Since phase state and viscosity are inherently linked to both the composition and the microphysical state of the particle, they can be used as extremely sensitive probes to monitor changes that would remain invisible otherwise. The tendency to remain solid at high relative humidities is because of the increase in organic mass in the aerosol, coupled with the production of HMWC from the aqueous photolysis of pyruvic acid (Figure 1). Both of these processes cause a decrease in a particles water uptake propensity from the decreasing solubility of molecular components. Chemical evidence for such polymerization is also documented here from in the mass spectra data from the post-photolysis filter samples (Figure 3 and S8). Given the


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comparison in the increase in bounced fraction between the experiments in air and in nitrogen, there may be a slightly lower degree of polymerization following irradiation of nebulized pyruvic acid in air.

This is consistent with the known mechanism for the formation of

dimethyltartaric acid from the photodecomposition of aqueous pyruvic acid, which is partially inhibited by the presence of O2. These physical changes to the aerosols are all fundamentally tied to the multiphase photochemistry of the system, but have further implications for the climate forcing of atmospheric aerosols. The sharp increase in condensed organics will impact the viscosity and hygroscopicity of aerosol, as documented by the aerosol bounce data. Further, aerosol growth upon irradiation, which is in part linked to the hygroscopicity, is directly related to aerosol optical depth calculations because particle size affects the scattering efficiency of an atmospheric aerosol. IV. Conclusions: Atmospheric Implications and Future Directions The above analysis reveals a multiphase photochemistry of pyruvic acid whose major products are consistent with the literature on the bulk photolysis reactions, but clarifies the importance of the previously proposed mechanisms under multiphase conditions that are more relevant to the atmosphere. We show that acetic acid and CO2 are the major products, but that dimethyltartaric acid is also formed under conditions relevant to the troposphere. The combined analyses show that formation of HMWC occurs efficiently and on a short timescale. Although it has been previously described that O2 inhibits this process, we show here that, while polymerization reactions are slowed down when oxygen is present, they are not completely inhibited. Furthermore, the competition between radical quenching by O2 and propagation may be somewhat dependent on the photon flux into the system. It may be worthy to note here that,


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while the solar simulators used in CESAM are realistic in terms of their spectrum, the total irradiation flux is approximately one-fourth of the maximum solar flux available at Earth’s surface in the mid-latitudes (Figure S2).113 This type of reaction is hence expected to be even more efficient in the atmospheric environment. The importance of such polymerization reactions is highlighted in the changes detected in particle phase and size, traits that are essential to calculating the aerosol optical depth and the overall impact on climate. In addition to alterations to particle properties, some of the products identified here could contribute to SOA loading in the atmosphere. Dimethyltartaric acid may itself remain condensed upon droplet evaporation, and a portion of the resultant acetic acid residing in the aqueous particles will be further oxidized in aqueous solution to glyoxylic and oxalic acids, which are known SOA precursors.25, 67, 131 However, while some model studies have included pyruvic acid and its oxidation products in global estimates of SOA,132, 133 its aqueous photochemistry, which is expected to account for more pyruvic acid loss than reaction with OH,71 has yet to be considered. In order to fully quantify the photochemistry for such a model investigation, a thorough kinetic analysis of the products from the aqueous photolysis of pyruvic acid as a function of the oxygen concentration and photon flux must be completed. We also highlight the complex nature of multiphase systems by identifying compounds that are not produced from the single-phase gas and aqueous photochemical studies. The formation of zymonic acid and other new, yet unidentified, products underscores a class of reactions, such as heterogeneous processes, specific to multiphase chemistry that may be important to aerosol properties in the atmosphere. The existing literature on the photolysis of pyruvic acid enabled this study to differentiate between bulk processes and those specific to the multiphase reaction. The fundamental laboratory work develops the essential groundwork for


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understanding atmospheric chemistry, but does not provide a robust framework for analyzing a multiphase system. While multiphase and heterogeneous experiments have gained recent popularity, many such studies investigate multifaceted systems of chemical mixtures to elucidate the effect of chemistry on particle populations or SOA formation. Here we show that the chemistry of a single component system can itself be complex in nature and may have additional chemical channels when in a multiphase environment. Using the CESAM simulation chamber has been crucial to extending the laboratory results to more environmentally relevant conditions. Increased investigation of such systems, based on chemistry that is well characterized in the bulk phases, is essential in order to fully understand how different aspects of a multiphase system may alter chemical and physical processes. The final, overall goal of fundamental atmospheric aerosol science would be to improve calculations of radiative forcing from particles to better predict future climate. At present, atmospheric models that incorporate organic chemistry generally use bulk laboratory results or reactive uptake coefficients to define reasonable input, with little inclusion of interfacial chemistry. This is in part due to the immense number of reaction pathways to consider in the atmosphere. Investigation of multiphase systems in simulation chambers should help streamline and focus on only chemistry truly important in the natural environment. As shown here, processes in multiphase systems may be different from laboratory studies, illustrating the significance of considering such experiments in model implementations, and highlighting the need for future atmospheric research to explore more of these systems.

Supplemental Information:


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A full experimental methods section is available in the supplemental information (Section 1), as are figures depicting the relative humidity and temperature for key experiments, the CESAM irradiation spectrum, identified PTRMS products from the photolysis of nebulized pyruvic acid in nitrogen, unidentified PTRMS products from the photolysis of nebulized pyruvic acid in air, the mechanism for zymonic acid dehydration and decarbonylation to form a compound with m/z 113.018, confirmation that zymonic acid produces the signal at m/z 113.018 in the PTRMS, FTIR spectra from the photolysis of nebulized pyruvic acid in nitrogen, ESI-MS of the filter sample from the photolysis of nebulized pyruvic acid in nitrogen, contour plots of the particle behavior for the multiphase photolysis of pyruvic acid in air and nitrogen, and the linear equations corresponding to the lines in Figure 5c. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements: The authors wish to thank Dr. Mila Ródenas García for the development of Main Polwin and for sharing her expertise in using it. The EUROCHAMP-2 project (EC contract no.228335) is acknowledged for allowing the distribution of this software. We are very grateful to Prof. Barry Carpenter, who identified zymonic acid as the compound responsible for the signal at m/z 113.018 in the PTRMS. This work was supported by grants from the National Science Foundation (CHE 1306386 and CHE 1611107), the European Research Council (ERC-StGQAPPA, grant 335478), and the Academy of Finland (grants 259005, 272041). A.E.R.H. also acknowledges funding from the National Science Foundation Graduate Research Fellowship under Grant No. DGE 1144083. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors(s) and do not necessarily reflect the views of


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the National Science Foundation. We also wish to recognize CNRS-INSU for supporting the CESAM chamber as a national instrument.

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TOC Graphic


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Author Biographies Allison E. Reed Harris is a PhD candidate in Prof. Veronica Vaida's group at the University of Colorado, Boulder and a National Science Foundation Graduate Research Fellow. She received her B.S. in chemistry from Macalester College in 2010. Her dissertation is focused on the multiphase photochemistry of environmental keto-acids and their importance to atmospheric and aerosol chemistry. Aki Pajunoja, a PhD student from Annele Virtanen’s research group at the University of Eastern Finland, received his MSc in Technical Physics in Tampere University of Technology in 2012. His PhD work is concentrated on secondary organic aerosol studies with a special focus on physical phase state of particles and their interaction with water vapor. Currently he is working for Agco Power Ltd. Mathieu Cazaunau is a research engineer in the Inter-university Laboratory of Atmospheric Systems (LISA) at the University of Paris East, Créteil. He is the technical project manager of the Multiphase Atmospheric Experimental Simulation Chamber (CESAM) and the instrumentation manager for the LISA laboratory equipment. His research concentrates on the development and use of specific instruments dedicated to gas phase and particle measurements in the atmosphere. Aline Gratien completed her PhD at University Paris East, Créteil and is currently an Assistant Professor of Chemistry at the University Paris, Diderot and a scientist in the Inter-university Laboratory of Atmospheric Systems (LISA). Her research activities surround the fate of organic carbon in the troposphere, specifically probing the reactivity of trace gases and particles using mass spectrometry, chromatography, spectroscopy, and atmospheric simulation chambers (CESAM facility).


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Edouard Pangui is an engineer in the Inter-university Laboratory of Atmospheric Systems (LISA) at the University of Paris East, Créteil. He oversees the maintenance and upkeep of the Multiphase Atmospheric Experimental Simulation Chamber (CESAM) and the accompanying spectroscopic and spectrometric tools. His specializations include instrument development and interfacing new techniques with the chamber. Anne Monod completed her PhD at the University of Paris 7, and her habilitation diploma at the University of Provence, Marseille. She is currently a Professor of atmospheric chemistry at the Aix-Marseille University. Her research is focused on atmospheric multiphase photochemistry of organic compounds, in an effort to understand the impact of these processes on the oxidizing capacity of the atmosphere and their ability to form organic aerosol. Elizabeth C. Griffith completed her Ph.D. at the University of Colorado, Boulder as a NASA Earth and Space Science Graduate Fellow and has a B.S. in Chemistry from the University of Maryland, Baltimore County. Her PhD work concentrated on unique chemistry and morphology at water-air interfaces in both the prebiotic and modern atmospheres. She is currently a lecturer in the Department of Chemistry and Biochemistry at the University of Maryland, College Park. Annele Virtanen completed her PhD in Tampere University of Technology. Currently she is a Professor of Aerosol Physics and the head of the Aerosol Physics Group at the University of Eastern Finland. Her research is focused on secondary organic aerosol formation and characterization, as well as aerosol-cloud interaction studies, implementing experimental laboratory studies, field work, and method development. Jean-Francois Doussin is a Professor at the University of Paris East, Créteil (UPEC) and leader of the "MEREIA group" at the Inter-university Laboratory of Atmospheric Systems (LISA). He


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specializes in both field and airborne measurements of VOCs and NOy and in using laboratory and smog chamber experiments to investigate secondary aerosol, mineral dust, and cloud droplets chemistry. His interests are related to the understanding of multiphase chemical transformations in the troposphere and their impacts on air quality and climate forcing. Veronica Vaida completed her Ph.D. at Yale University and is currently a Professor of Chemistry at the University of Colorado and a fellow of the Cooperative Institute for Research in Environmental Sciences. Her interests have followed an interdisciplinary path at the forefront of physical chemistry and atmospheric science. Her research is focused on light-initiated reactions of molecules, radicals, water complexes and aerosols of interest in planetary atmospheres including the contemporary and prebiotic Earth.


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Figure 1: Summary of gas and aqueous phase photolysis mechanisms for pyruvic acid (pyruvic acid), with selected products relevant to this work.69, 92 Blue indicates pathways enhanced by oxygen and red pathways inhibited by oxygen. 152x103mm (300 x 300 DPI)


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Figure 2: Gas-phase concentrations of pyruvic acid and photochemical products as detected by a) FTIR and b) PTRMS during the irradiation of nebulized 0.1 M pyruvic acid in air. We note that neither instrument samples the aerosol composition. The background colors indicate nebulization (white), dark decay (grey), and photolysis (yellow). The zymonic acid structure is shown in panel b. 82x87mm (300 x 300 DPI)



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Figure 3: ESI(-)MS from the filter sample taken after 1.3 hours of irradiation of nebulized 0.4 M pyruvic acid in air. Blue, positive facing peaks arise from the sample taken after photolysis, while the green, negative facing peaks are from the chamber blank. Signals from dimethyltartaric acid (177.040 m/z,*) and zymonic acid (157.014 m/z,§) are highlighted in red. 82x44mm (300 x 300 DPI)


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Figure 4: FTIR spectrum before (blue) and after (green) photolysis of nebulized 0.1 M pyruvic acid in air. Also shown are reference spectra for pyruvic acid (red), acetic acid (black), and zymonic acid114 (grey). The pink stars indicate pyruvic acid’s C-O acid stretch and C-O-H bend, highlighting the change in ratio of the two peaks between the dark spectrum and the reference spectrum. Unidentified peaks in the post-photolysis spectrum are labeled with purple §. 82x43mm (300 x 300 DPI)



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Figure 5: (a) Total organic mass calculated assuming spherical aerosol with a density of 1 g/ml, (b) particle number concentration, and (c) mean particle diameter during the photolysis of nebulized 0.1 M pyruvic acid in air (yellow traces) and in nitrogen (blue traces). The background colors indicate nebulization (white), dark decay (grey), or photolysis (yellow). The grey striped background shows times for which nebulization is occurring for the reaction in nitrogen and dark decay for the reaction in air. In part c, best-fit lines for the data just before and just after the lights are turned on are shown to highlight the discontinuity in particle size upon irradiation. Determined linear equations and related uncertainties are listed in the Supporting Information (Table S1) and clearly demonstrate statistical difference between the slopes prior to and during irradiation. 82x142mm (300 x 300 DPI)


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Figure 6: Particle bounced fraction (BF) with RH prior to and during the photolysis of nebulized 0.1 M pyruvic acid in air (hollow triangles) and in nitrogen (filled dots). The time scale marked in the legends is set so that the photolysis for each experiment begins at t=0 minutes. In addition, (RH, BF) data for atomized sucrose particles measured at the same day are shown in black hollow squares. In all cases, selected dry particle size (ddry) was 150nm. 198x112mm (300 x 300 DPI)



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TOC Graphic 82x49mm (300 x 300 DPI)


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Multiphase Photochemistry of Pyruvic Acid under Atmospheric

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