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Photoactivated Production of Secondary Organic Species from Isoprene in Aqueous Systems Wan-yi Li, Xia Li, Steffen Jockusch, Han Wang, Bolei Xu, Yajing Wu, William Gang Tsui, Hai-Lung Dai, V. Faye McNeill, and Yi Rao J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b07932 • Publication Date (Web): 24 Oct 2016 Downloaded from http://pubs.acs.org on October 24, 2016

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The Journal of Physical Chemistry

Photoactivated Production of Secondary Organic Species from Isoprene in Aqueous Systems Wan-Yi Li1,2, Xia Li3, Steffen Jockusch4, Han Wang1, Bolei Xu3, Yajing Wu3, William G. Tsui1, Hai-Lung Dai3, V. Faye McNeill1*, and Yi Rao3* 1

Department of Chemical Engineering, Columbia University, New York, NY 10027 2

Department of Environmental Science, Wuhan University, Hebei Province, 430072, China 3

Department of Chemistry, Temple University, Philadelphia, PA 19122

4

Department of Chemistry, Columbia University, New York, NY 10027

Abstract Photoactivated reactions of organic species in atmospheric aerosol particles are a potentially significant source of secondary organic aerosol material (SOA). Despite recent progress, the dominant chemical mechanisms and rates of these reactions remain largely unknown. In this work, we characterize the photo-physical properties and photochemical reaction mechanisms of imidazole-2-carboxaldehyde (IC) in aqueous solution, alone and in the presence of isoprene. IC has been shown previously in laboratory studies to 1

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participate in photoactivated chemistry in aerosols, and is a known in-particle reaction product of glyoxal. Our experiments confirmed that the triplet excited state of IC is an efficient triplet photo-sensitizer, leading to photosensitization of isoprene in aqueous solution

and

promoting

its

photochemical

processing

in

aqueous

solution.

Phosphorescence and transient absorption studies showed that the energy level of the triplet excited state of IC (3IC*) was approximately 289 kJ/mol, and the lifetime of 3IC* in water under ambient temperature is 7.9 μs, consistent with IC acting as an efficient triplet photosensitizer. Laser flash photolysis experiments displayed fast quenching of 3IC* by isoprene, with a rate constant of (2.7 ± 0.3)  109 M-1s-1, which is close to the diffusion-limited rate in water. Mass spectrometry analysis showed that the products formed include IC-isoprene adducts, and chemical mechanisms are discussed. Additionally, oxygen quenches 3IC* with a rate constant of (3.1 ± 0.1)  109 M-1s-1.

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Introduction

Accurately quantifying the sources and sinks of atmospheric aerosol material is of critical importance for understanding air quality and climate.1-6 Recent laboratory studies have demonstrated the formation of secondary organic aerosol (SOA) material via photoenhanced reactions between light-absorbing organic substances in the particle phase and gas-phase volatile organic compound (VOC) precursors.7-11 Photochemical reactions of organic matter in the aerosol phase may be a source of organic aerosol mass through the direct oxidation of VOCs, or via the generation of particle-phase oxidants.8-10, 12-14 Both of these processes could potentially lead to the efficient formation SOA material, however, we lack critical information including mechanisms and kinetics required to evaluate the importance of these processes on the regional and global scales via atmospheric chemistry modeling.3-6, 9, 14-18 Isoprene, the most abundant non-methane hydrocarbon,19 is believed to be a major source of SOA.20,

21

Multiple pathways for SOA formation from isoprene have been

investigated,21 including photooxidation under dry conditions and aqueous-phase SOA formation from isoprene oxidation products such as glyoxal, methylglyoxal, and isoprene epoxydiols.22 Oligomer formation, including nitrogen-containing species, has been observed for SOA formation from isoprene under irradiated conditions.23 An in-particle

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reaction product of glyoxal, imidazole-2-carboxaldehyde (IC), has recently been identified to participate in photosensitized SOA formation with VOCs including isoprene.7,

9, 11

Aregahegn et al. (2013) observed SOA formation in the laboratory from irradiated particles containing IC exposed to gas-phase isoprene,7 but the dominant chemical mechanisms and rates of this potentially important process remain unknown. In this work, we present a detailed study of the photochemical mechanisms of IC alone and in the presence of isoprene and O2. We focus in particular on the photochemistry of triplet excited state IC (3IC*), which acts as a photosensitizer in the formation of SOA.

Methods All chemicals were purchased commercially from Sigma-Aldrich. IC and isoprene were used as received without further purification. Linear optical measurements. UV-vis spectra of IC aqueous solutions were recorded in a 1 cm quartz cuvette under ambient conditions, using a UV-vis spectrophotometer (Agilent 8453). Steady-state phosphorescence spectra were taken using a Fluorolog-3 fluorometer (HORIBA Jobin Yvon). For the phosphorescence measurements, an ICethanol solution was frozen at 77 K using 3 mm quartz tubes inside a quartz liquid nitrogen dewar.

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Phosphorescence lifetime measurements. Phosphorescence lifetime measurements were performed by multichannel scaling using an OB920 spectrometer (Edinburgh Analytical Instruments) in conjunction with a pulsed Xe-lamp for excitation. Laser flash photolysis experiments. Laser flash photolysis experiments employed the pulses from a Spectra-Physics GCR-150-30 Nd:YAG laser (266 nm, ca. 5mJ/pulse, 5 ns pulse length) and a computer controlled system that has been described previously.24, 25 Aqueous solutions of IC were prepared at concentrations such that the absorbance was approximately 0.3 at the excitation wavelength of 266 nm. The IC solution was placed in a 1 × 1 cm Suprasil quartz cell and purged with argon gas for at least 20 minutes before each experiment. For determination of the oxygen quenching rate constants, IC solutions were purged with oxygen/nitrogen mixtures at different ratios of the gases. Photo-irradiation and mass spectra. Photo-reactions of IC and isoprene were carried out in a Rayonet reactor (Southern New England Ultraviolet Company) equipped with 12 UV lamps emitting at 254 nm. An aqueous solution containing 20 mM IC was saturated with isoprene and placed into a 1×1 cm quartz cuvette with cap. The solutions were continuously stirred using a magnetic stirrer during photo irradiation. For experiments performed under deoxygenated conditions, a screw-capped cuvette with Teflon lined septum was used and purged with argon for 20 minutes prior to irradiation. Atmospheric pressure chemical ionization (JEOL LCmate APCI, positive ion mode) mass spectra were taken immediately 5



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after photo-irradiation. Mass resolution was ± 0.5 amu. Additional control experiments were performed with IC and isoprene in the absence of UV. GAMMA modeling. The mechanisms and kinetics identified in this study were incorporated into the McNeill Group photochemical box model, GAMMA.26 Simulation outputs were analyzed and compared to laboratory measurements of SOA formation by irradiated particles containing IC in the presence of isoprene by Aregahegn et al. (2013).7 Details of the simulations can be found in the Supporting Information.

Results Linear optical properties and singlet excited state of IC. Figure 1(A) shows the absorption spectrum of IC in aqueous solution with an extinction coefficient of 1.28 × 104 M-1cm-1 at the maximum (288 nm). In order to characterize the singlet transition, UV

(A)

(B)

IC

Figure 1 (A) UV absorption spectrum of IC aqueous solution. (B) UV spectra of IC in different solvents. 6


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spectra of IC in several solvents were taken as displayed in Figure 1 (B). The UV spectra exhibit a pronounced redshift from non-polar to polar solvents. This solvatochromism and high extinction coefficient of the major absorbance peak at ~280 nm imply that this transition is of π-π* origin. The n-π* transition is expected to occur in the same spectral region, but with a much lower extinction coefficient.27 Triplet excited state properties of IC. We also investigated the photophysical properties of triplet states of IC using phosphorescence measurements.27 The phosphorescence emission spectrum of IC in a frozen ethanol matrix at 77 K is displayed in Figure 2 (A). A triplet energy of 289 kJ/mol was determined from the highest-energy peak in the phosphorescence spectrum (414 nm). The phosphorescence decayed with a lifetime of 59 ms at 77 K (Figure 2B). The short phosphorescence lifetime and the structured

(A)

(B)

Figure 2. Phosphorescence measurements of IC in ethanol at 77 K, excited at 300 nm. (A) Phosphorescence spectrum (B) Time trace of phosphorescence emission. 7



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phosphorescence spectrum showing vibrational patterns of carbonyl vibrational levels are typical for a triplet state of n-π* nature. Transient absorption of IC. Figure 3 (A) shows a transient absorption spectrum for IC in water at ambient temperature. The positive peak at 330 nm was assigned to the triplet state absorption of IC, and the negative change in absorbance below 310 nm was caused by ground state bleaching of IC. Figure 3 (B) shows the kinetic traces of triplet decay and recovery of the IC ground state. The lifetime of the IC triplet state in water was found to be approximately 7.9 s.

(A)

(B)

Figure 3. (A) A transient absorption spectrum for IC in water at ambient temperature at a delay time of 1 μs after pulsed laser excitation at 266 nm. (B) Time evolution of the IC T1-Tn transition at 330 nm (lower) and the IC ground state recovery at 280 nm (upper).

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Quenching of 3*IC in the presence of isoprene. In order to investigate the reaction of the triplet state 3IC* with isoprene, time-dependent changes in absorption at 330 nm were measured for IC in the presence of different concentrations of isoprene in aqueous solution. As seen in Figure 4 (A), the decay rate of the triplet state 3IC* increases as the concentration of added isoprene increases. The decay constant of 3IC* with isoprene can be given by27 kapp = k0+kq[Isoprene]

(1)

where kapp is the apparent first-order reaction rate constant in the presence of isoprene, k0 is the decay rate constant in the absence of isoprene, and kq is the second order quenching rate constant with isoprene. From Figure 4 (B), the rate constant kq of triplet state of IC for reaction with isoprene was found to be (2.7 ± 0.3) × 109 M-1s-1, which is on the same order

(A)

(B)

Figure 4. (A) Transient absorption decay of 3IC* at 330 nm by mixing with different isoprene concentrations. (B) Apparent reaction rates of 3IC* for different concentrations of isoprene in aqueous solution in the presence of 0.29 mM O2. 9



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of magnitude as the diffusion controlled bimolecular rate constant in water at room temperature (7.4 × 109 M-1s-1).27 These results are qualitatively consistent with the IClimonene laser flash photolysis experiments of Rossignol et al9, although they did not report kinetic data. Quenching of 3IC* by O2. In the presence of dissolved O2 the triplet absorbance of IC decayed faster. For example, in air-saturated water, the triplet lifetime is reduced to 0.96 μs (Figure 5A). The rate constant for bimolecular quenching of IC triplets by O2 was determined by laser flash photolysis at varying oxygen concentrations. The slope of the plot of O2 concentration vs. the apparent decay rate at different O2 concentration yielded a

(A)

(B)

Figure 5. (A) Comparison of transient absorption decays of 3IC* at 330 nm without O2 and with 0.29 mM O2 (air saturated). (B) Apparent decay rates of triplet state of IC for different concentrations of O2 in aqueous solution. The slope yields a quenching rate of (3.1 ± 0.1) x 109 M-1s-1.

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bimolecular quenching rate constant of (3.1 ± 0.1) ×109 M-1S-1 (Figure 5B). This rate constant is typical for organic compounds in the triplet state.26 Photochemical reaction products. We performed a series of mass spectrometry analyses in order to determine the main photochemical products of isoprene and IC. An aqueous solution of saturated isoprene and 20 mM IC was exposed to photo-irradiation (254 nm) under ambient conditions. Furthermore, to investigate how O2 is involved in the photochemical reaction of 3IC* and isoprene, photo-irradiation was also performed in an argon environment. Table 1 lists m/z values of major peaks (mass spectra are available in Supporting Information) for aqueous solutions of IC and isoprene under ambient conditions, under argon, and with and without photo-irradiation under ambient conditions. Table 1. Results of mass spectrometry analysis of reaction mixtures for experiments performed with mixtures of IC and isoprene, with and without photo-irradiation, in air and in argon. Major m/z signals obtained under APCI+ are listed. Experiments

Mass-to-charge ratio of observed ions in mass spectra (± 0.5 amu)

IC-isoprene

69.1

97.1

129.2

137.2

69.1

97.1

129.2

137.2

165.2

69.1

97.1

129.2

137.2

165.2

without UV in air IC-isoprene

261.3

with UV in air IC-isoprene with

UV

243.3

261.3

in

Argon

11


329.4

359.4

425.5

491.7


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Proposed peak assignments for IC and isoprene reacting under ambient conditions are discussed below. Peaks at 69.1 amu and 97.1 amu, corresponding to protonated isoprene and protonated IC, respectively, were observed under APCI+, as expected, in all samples. Additional minor signals at m/z 129.2 and 137.2 are probably created by oxidation of IC and dimerization of isoprene, respectively, in the ion source. Major reaction products under air were observed at 165.2 amu and 261.3 amu. These products likely correspond to isopreneIC adducts with molecular formulas of C9H12ON2 and C13H16O2N4, respectively, although other molecular formulas are possible. Formation of these products would involve one molecule of isoprene and one of IC (m/z 165.2) and one molecule of isoprene and two of IC (m/z 261.3). Additional products were produced under argon compared to in air, including peaks at m/z 243.3, 329.4, 359.4, 425.5 and 491.7. These peaks correspond to oligomers formed from the coupling of several molecules of IC and isoprene. Note that mass-to-charge ratios above 750 were not measured so we cannot rule out the presence of larger oligomeric species.

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Discussion The most common reactive state of the carbonyl chromophore is the 3n,π* state, which is generally more reactive than the 3π,π* and is capable of hydrogen abstraction and electron transfer reactions.27 Our phosphorescence measurements show that 3n,π* is the electron configuration of the lowest triplet state of IC, which implies the high reactivity of the 3IC*. Our experimental results are consistent with those reported by Tinel et al.11 A high rate constant of quenching of 3IC* by isoprene was observed by laser flash photolysis ((2.7 ± 0.3) × 109 M-1 s-1). The quenching should be dominated by energy transfer since isoprene has a lower triplet energy (252. 4 kJ/mol, 3π,π* configuration)28 than *IC3 (289.0 kJ/mol). The energy diagram for this triplet energy transfer is shown in Scheme 1.

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Scheme 1 Energy diagram for triplet-triplet energy transfer from IC to isoprene.

Several products consistent with addition reactions of IC and isoprene were observed, in argon and in air. This is consistent with the observations of Rossignol et al. (2014) for the limonene-IC system.9 Several possible mechanisms exist for the formation of ICisoprene adducts, including electron transfer or proton transfer to form free radicals, or photocycloaddition. Besides triplet energy transfer, another plausible mechanism for 3IC*-isoprene reaction is the Paternò-Büchi reaction,27, 29 in which the [2+2] photo-addition of alkenes to triplet excited carbonyl compounds forms oxetanes (Scheme 2). The mechanism of oxetane 14


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Scheme 2. Possible mechanism of photoreaction between IC and isoprene, leading to products with m/z 165.1 and 261.1.

formation by the interaction of the triplet n,π* state and electron-rich alkene involves the formation of biradical intermediates. The Paternò-Büchi reaction of 3IC* with isoprene would lead to an oxetane with MW=164.09 g/mol, which is consistent with the observed MS signal of m/z= 165.1 in APCI+ (Table 1). A second Paternò-Büchi reaction of 3IC* can lead to a photoproduct with MW = 260.13 g/mol (Scheme 2), which is consistent with the observed MS signal at m/z=261.1. [2+2] photocycloaddition of 3IC* with isoprene as shown in Scheme 3 or ground state IC with triplet isoprene (generated by triplet energy transfer from 3IC*) can lead to photoproducts with MW=164.21 g/mol. Subsequent Paternò-Büchi reactions, as shown in Scheme 3, can lead to photoproducts with MW= 328.19 g/mol and 424.22 g/mol, which are consistent with the observed MS signal at m/z =329.2 and 425.5, respectively. 15



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However, other mechanisms to generate adducts of IC and isoprene involving electron and proton transfer are possible as observed by Rossignol et al. (2014) for the IC-limonene system.9

Scheme 3. Possible mechanisms of photochemical reactions between IC and isoprene, leading to products with m/z 329.2 and 425.5.

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In air-saturated solutions, 3IC* is rapidly quenched by O2, with a rate constant of (3.1 ± 0.1) × 109 M-1s-1. This high quenching rate constant for O2 implies that under ambient conditions (~ 0.3 mM O2 in aqueous solution), any reactions with 3IC* slower than ~106 s1

are likely to be suppressed by O2. The reaction of 3IC* with O2 likely results in the

formation of singlet oxygen (via energy transfer) and the downstream production of other oxidants such as HO2.10,30 While the measured rate of the 3IC*- isoprene reaction was faster in isoprene-saturated solutions, and no isoprene oxidation products were observed in this study, O2 quenching will compete with the isoprene reaction. Aregahegn et al. (2013) showed limited SOA growth in pure N2 compared to in air, indicating that O2 is involved in the mechanism of SOA growth.7 Rossignol et al. (2014) also observed a number of apparent limonene oxidation products.9 This appears to be a difference in the IC-isoprene and IC-limonene systems, since isoprene dimer and IC-isoprene adducts were the main products in our experiments. We simulated the IC-isoprene SOA production experiments of Aregahegn et al. (2013)7 using the kinetic information measured in this study and the GAMMA model (see Supporting Information for details of the calculations).18, 26 We find that, due to mass transfer limitations, the reaction of 3IC* with isoprene in aerosol particles likely takes place at or near the gas-particle interface. When that is taken into account, the simulations reproduce the moderate SOA growth observed by Aregahegn et al. for the IC-isoprene 17



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system. Consistent with our mass spectrometry results, SOA generated as a result of oxidants produced via the reaction of 3IC*+O2 was predicted to contribute less than 1% of the total SOA mass generated from 3IC* and isoprene.

Conclusion and Atmospheric Implications We have investigated photochemical mechanisms of glyoxal derivative imidazole-2carboxaldehyde (IC) in aqueous solution. Our experiments confirm that the triplet excited state of IC efficiently leads to the photosensitization of isoprene in aqueous solution, promoting its photochemical processing in aqueous solution. We also report here kinetic data for the reactions of 3IC* with O2. Data of this type are critically important to enable evaluation of the atmospheric significance of photosensitizer chemistry in SOA formation via modeling. These results stress the central role of triplet excited states of organic carbonyl compounds in photochemical pathways in aqueous solution. Organic carbonyl compounds in the aqueous phase are one source of photo-sensitization transformations of volatile organic compounds, which contribute to the growth of secondary organic aerosol. Given the high density of carbonyl groups in ambient aerosol organic material, there is potential for the photosensitization of triplet excited states of organic carbonyl compounds to be significant in atmospheric aerosols. 18


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Author Information Corresponding Authors E-mail: [email protected], Phone: (215) 204-2836; [email protected], Phone: (212) 854-2869. Notes The authors declare no competing financial interest.

Acknowledgements This material is based upon work supported by the National Science Foundation under CHE–1506789. Yi Rao thanks Dr. Dezheng Sun for his discussions. Wan-Yi Li acknowledges a scholarship fund from China Scholarship Council (File No. 201306270096) for the participation in this research

Supporting Information Mass spectra and details of the GAMMA calculations.

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