OH-Initiated Oxidation of Acetylacetone: Implications for Ozone and

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Environmental Processes

OH-initiated oxidation of acetylacetone: Implications for ozone and secondary organic aerosol formation Yuemeng Ji, Jun Zheng, Dandan Qin, Yixin Li, Yanpeng Gao, Meijing Yao, Xingyu Chen, Guiying Li, Taicheng An, and Renyi Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03972 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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Environmental Science & Technology

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OH-initiated oxidation of acetylacetone: Implications for ozone and secondary organic

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aerosol formation

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Yuemeng Jia,1, Jun Zheng,b,1 Dandan Qin,a Yixin Li,c Yanpeng Gaoa, Meijing Yaoa, Xingyu

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Chena, Guiying Lia, Taicheng Ana,*, and Renyi Zhangc,*

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a

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Environmental Science and Engineering, Institute of Environmental Health and Pollution

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Control, Guangdong University of Technology, Guangzhou 510006, China b

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Guangzhou Key Laboratory of Environmental Catalysis and Pollution Control, School of

Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, Nanjing University of Information Science & Technology, Nanjing 210044, China c

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Department of Atmospheric Sciences and Department of Chemistry, Texas A&M University, College Station, TX 77843

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1

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ABSTRACT. Acetylacetone (AcAc) is a common atmospheric oxygenated volatile organic

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compound (OVOC) due to broad industrial applications, but its atmospheric oxidation

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mechanism is not fully understood. We investigate the mechanism, kinetics, and atmospheric

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fate of the OH-initiated oxidation for the enolic and ketonic isomers of AcAc using quantum

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chemical and kinetic rate calculations. OH addition to enol-AcAc is more favorable than

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addition to keto-AcAc, with the total rate constant of 1.69 × 10-13 exp(1935/T) cm3 molecule-1

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s-1 over the temperature range of 200-310 K. For the reaction of the enol-AcAc with OH, the

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activation energies of H-abstraction are at least 4 kcal mol-1 higher than those of OH-

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addition, and the rate constants for OH-addition are by 2‒3 orders of magnitude higher than

Both authors contributed equally to this work

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those for H-abstraction. Oxidation of AcAc is predicted to yields significant amounts of

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acetic acid and methylglyoxal, larger than those are currently recognized. A lifetime of less

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than a few hours for AcAc is estimated throughout the tropospheric conditions. In addition,

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we present field measurements in Beijing and Nanjing, China, showing significant

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concentrations of AcAc in the two urban locations. Our results reveal that the OH-initiated

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oxidation of AcAc contributes importantly to ozone and SOA formation under polluted

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environments.

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KEYWORDS. Oxygenated volatile organic compounds; atmospheric chemistry; mechanism

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and kinetics; secondary organic aerosol; ozone

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INTRODUCTION

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Ketones represent an important class of oxygenated volatile organic compounds (OVOCs)

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and are emitted from natural and anthropogenic sources, e.g., from industrial activities and

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oxidation reactions of both biogenic and anthropogenic hydrocarbons.1-5 The atmospheric

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oxidation of ketones has been identified as important sources for HOx radical species (i.e.,

36

OH and HO2)6,7 and secondary organic aerosol (SOA),6,8 profoundly impacting air quality,

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human health, and climate. Acetylacetone (also known as 2,4-pentanedione or AcAc) is

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highly reactive and broadly used for various industrial applications. For example, AcAc is an

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important reagent in preparation of chelate compounds for a wide range of transition

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metals,9,10 an industrial additive,11 and a building block for synthesis of heterocyclic

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compounds and raw materials for sulfonamide drugs.12 In developed countries such as Japan,

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the United States, and Europe, the global capacity of AcAc is estimated to be approximately

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20000 t a-1,13 mainly from industrial activities, while its formation from in situ atmospheric

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photochemical production is negligible. Furthermore, emissions of AcAc in developing

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countries (such as China) are anticipated to be substantially increased because of their rapid

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industrialization and economic development.14,15

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AcAc is a prototype of β-diketone that exists in the two tautomeric forms: the enol form

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contains an intramolecular hydrogen bond and resonance stabilization through a conjugated

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π-system, whereas the diketo form contains two carbonyl groups with an ∼140° dihedral

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angle between the oxygens. The atmospheric oxidation mechanism of AcAc is complex,

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including multiple pathways and steps. Oxidation of AcAc is mainly initiated by the hydroxyl

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radical (OH).13,16-18 An earlier experimental study by Zhou et al. has evaluated the

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atmospheric chemistry for AcAc; using a relative kinetic method the authors determined the

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temperature dependent rate coefficients over the temperature range of 285-310 K, with an

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Arrhenius expression of k = 3.35 × 10-12 exp[(983 ±130)/T] cm3 molecule-1 s-1.13 That

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experimental work also identified several products from the OH-initiated oxidation of AcAc,

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including methylglyoxal (MG), acetic acid (AA), and peroxyacethyl nitrate (PAN).13 On the

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basis of their measured products, a mechanism for the reaction of AcAc with OH has been

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postulated, involving the initial OH addition to C2 and C3 positions followed by O2

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addition.13

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In contrast to investigation on the industrial applications of AcAc9-12, limited previous

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work exists on the atmospheric oxidation mechanism of AcAc, hindering accurate assessment

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of its roles in the formation of ozone and fine particulate matter (PM). For example, MG,

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organic acids, and PAN have been identified as the critical species leading to SOA

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formation.14,15,19-24 Specifically, organic acids play important roles in new particle formation

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and growth25,26 and acid-base reactions27,28, while oligomerization of small α-dicarbonyls

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represents a major source of SOA on the urban, regional, and global scales.29,30 The

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atmospheric sources of small α-dicarbonyls, organic acids, and PAN, however, remain

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poorly quantified.14 The current atmospheric chemical mechanism for the AcAc oxidation has

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been proposed on the basis of the an environmental chamber experiment,13 which has

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provided the critical information on the initial kinetic and products of the AcAc oxidation.

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However, extrapolation of the kinetics and mechanism of the AcAc reactions from the

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measured product yields is challenging, since a product formation typically involves multiple

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possible steps and pathways and the product is subject to secondary reactions or photolysis.

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In addition, there exist additional intricate difficulties using the chamber method. Noticeably,

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the limitations of the chamber method include a long reaction time, higher reactant

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concentrations, and wall loss.31,32 In particular, the significance of wall loss for reactive and

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condensable species using the chamber method has been demonstrated.33-36 Furthermore, no

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theoretical results are available on the atmospheric chemistry of AcAc.

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In this work, we have investigated the detailed oxidation mechanism of AcAc with OH

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employing quantum chemical and kinetic rate calculations within the tropospheric

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temperature range of 200-310 K. We also present field measurements of AcAc in Nanjing

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and Beijing, China using proton-transfer reaction mass spectrometry (PTR-MS). The

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atmospheric fate and oxidation products of AcAc are assessed, and the implications of our

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results for ozone and SOA formation are discussed.

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METHODS

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The electronic structures and energy calculations were carried out with the Gaussian

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09 program suite.37 Geometrical optimization of all stationary points (SPs), such as the

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reactants, transition states (TSs), complexes, intermediates, and products, was performed

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using the M06-2X level with the 6-311G(d,p) basis set denoted as the M06-2X/6-311G(d,p)

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level. The M06-2X functional is a high-nonlocality functional with double the amount of

92

nonlocal exchange (2X), with reliable performance for the thermochemistry, hydrogen

93

bonding, kinetics, and weak interactions.38 In addition, the MPWB1K/, B3LYP/, MP2/ and

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QCISD/6-311G(d,p) levels were also employed to optimize the geometry and to validate the

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convergence of the predicted geometries. The MPWB1K and B3LPY methods and the MP2

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method represent the classic density functional theory and the classic Ab Initio theory,

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respectively, while the QCISD method corresponds to a higher electronic correlation method.

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Frequency calculations were carried out at the M06-2X/6-311G(d,p) level to determine all

99

SPs as a real local minima (without any imaginary frequency) or a TS (with only one

100

imaginary frequency). The evaluation of the vibrational frequencies confirmed that all

101

optimized geometries represented the minima on the potential energy surfaces (Table S1).

102

Intrinsic reaction coordinate (IRC) calculations were performed to confirm the connection

103

between the TSs and their corresponding reactants and products.7,39 The potential energy

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surface (PES) was further refined by the M06-2X/6-311++G(3df,3pd) level to yield more

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accurate energetics. Because kinetic calculations of the organic reaction systems were highly

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sensitive to the predicted energetics, single point energy calculations were performed to

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refine the PES using the QCISD(T)/6-311+G(2df,p) and CCSD(T)/6-311+G(2df,2p) levels.

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The dual-level approach was denoted as X//Y, where a single-point energy calculation at

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level X was carried out for the geometry optimized at a lower level Y. In all cases, the

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energies were calculated relative to the corresponding reactants including ZPE corrections.

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∆Ea is defined as the activation energy (i.e., ∆Ea = ETS - Ereactants), while ∆Er is defined as the

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reaction energy (∆Er = Eproducts - Ereactants). The natural bond orbital (NBO) analysis was

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carried out to confirm the favorable reaction pathways. All barrierless processes were

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verified by performing the pointwise potential curve (PPC) scan. Furthermore, the effect of

115

the basis set superposition error on the energies was considered using the counterpoise

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method described by Boys and Bernardi40 to evaluate the stability of the complexes. All pre-

117

complexes existed, when BSSE correction was included (see Supporting Information). On

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the basis of the predicted PES, the kinetics calculations, i.e., the rate constants and product

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distributions, were performed using the Polyrate program41 with the generalized transition-

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state theory (more details in Supporting In formation).7,31,39 The most stable structure for each

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pathway was used in the kinetic study.

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In addition, we measured the concentration of AcAc in Nanjing and Beijing, China on

123

the basis of the proton-transfer reaction,42 i.e., H3O+ + AcAc H2O + AcAc• H+. The

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measurements in Nanjing were made using a high-resolution time-of-flight chemical

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ionization mass spectrometer (HR-ToFCIMS, Aerodyne Res. Inc., USA)43, while the

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measurements in Beijing were made using a quadruple mass spectrometer.30 Fig. S1 shows

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the high-resolution fit of the AcAC peak at m/z = 101.06, confirming the detection of AcAc.

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In this work, the observation sites were located on the campus of Nanjing University of

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Information Science & Technology (NUIST) in Nanjing and on the campus of Tsinghua

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University in Beijing.

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RESULTS AND DISCUSSION

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Initial reaction of AcAc with OH. There exists an equilibrium between enolic and ketonic

133

isomers of AcAc (Figure S2), with the enolic isomer representing the main form in the gas-

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phase.44 To systematically assess the OH-initial oxidation of AcAc, we considered both the

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reactions of enolic and ketonic isomers, denoted by enol-AcAc and keto-AcAc, respectively.

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The geometries of the two isomers optimized at the M06-2X/6-311G(d,p) level are displayed

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in Figure 1. For comparison, the geometries and frequencies using other level calculations,

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including MPWB1K/, B3LYP/, MP2/ and QCISD/6-311G(d,p), are included in Figure S2

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and Table S1. The structural parameters obtained by the five levels are similar, and the

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largest discrepancies are within 0.9º in the bond angles and 0.07 Å in the bond lengths. The

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calculated frequencies at the M06-2X level agree with those obtained by the B3LYP and

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MP2 levels, with the maximum error of less than 10%. Hence, the M06-2X level theory

143

accurately describes the geometry optimization and vibrational frequency calculations for the

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AcAc reaction system. Figure 1 shows that enol-AcAc exhibits a conjugated π-electron

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character that enhances the reactivity of the C atoms. As a result, the C atoms in enol-AcAc

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are more susceptible to attack by OH than those in keto-AcAc. Figures 1 and S2 depict the

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optimized geometries of SPs in the OH-AcAc reaction at the M06-2X/6-311G(d,p) level. The

148

absolute energies, ZPEs and Cartesian coordinates are also included in Table S1.

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The PESs for possible pathways of the OH-initial reaction of enol-AcAc are presented

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in Figure 2. In addition, the methods of QCISD(T) and CCSD(T) are performed. As

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discussion in Supporting Information, the M06-2X method is suitable to predict the energies

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and represents a compromise between the computational accuracy and efficiency.

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reaction for enol-AcAc with OH occurs via two distinct pathways (Figure 2): H-abstraction

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from the two methyl groups (e-Rabs1 and e-Rabs2) and OH-addition to either C2 or C3

155

position (e-Radd1 or e-Radd2). For each pathway, a pre-reactive complex is identified prior to

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the corresponding TS or products. The pre-reactive complexes are consistently more stable

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than their corresponding reactants (Figure 2). The reaction energies of e-PCadd1 and e-PCadd2

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are by -5 kcal mol-1 than those of the reactants, verifying that the enol-AcAc oxidation

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proceeds via the pre-reactive complex and TS prior to the product formation. The structures

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of the pre-reactive complexes are similar to those of the reactants, except for the forming

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bond. For instance, the forming C‒O distances are 2.78 and 2.58 Å for the e-PCadd1 and e-

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PCadd2 (Figure S4a), respectively, while the other bond distances are similar to those of the

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reactants. The existence of the pre-reactive complexes also impacts the reaction kinetics,45

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and the OH-AcAc oxidation is expected to exhibit a negative temperature effect.

The

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Figure 2 shows that the activation energies (∆Ea) of the two H-abstraction pathways (e-

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Rabs1 and e-Rabs2) are 3.04 and 0.89 kcal mol-1, respectively, which are at least 4 kcal mol-1

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higher than the two OH-addition pathways (e-Radd1 and e-Radd2). The calculated reaction

168

energy (∆Er) of -28.96 kcal mol-1 for e-Radd1 is by about 11 kcal mol-1 lower than that of e-

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Radd2. The lower exothermicity for e-Radd1 is explained by its structural characteristics

170

according to the Hammond postulate,46 since this pathway proceeds with an earlier TS due to

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an elongated C−O distance (Figure 1). However, the ∆Ea value of e-Radd2 is 1.05 kcal mol-1

172

lower than that of e-Radd1 (Figure 2), indicating that e-Radd1 is thermodynamically favored

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but e-Radd2 is kinetically favored. The natural bond orbital (NBO) charges are 0.496 and

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0.573 e for the C2 and C3 positions, respectively. Since the more positive potential bond is

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easily attacked by the nucleophiles (OH), the OH addition to the C3 position (e-Radd2) is

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more favorable than that to the C2 position (e-Radd1).

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The reaction of keto-AcAc with OH also occurs via two pathways, i.e., H-abstraction

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from the methyl and methylene groups (k-Rabs1 and k-Rabs2) and OH-addition to C4 position

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(k-Radd1). OH addition to keto-AcAc proceeds via the pre-reactive complex prior to the TS,

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with a ∆Ea value of 6.11 kcal mol-1. The OH-addition to keto-AcAc possesses a higher barrier

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(by about 10 kcal mol-1) than those of the H-abstraction pathways; the distinct behaviors

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between OH additions to enol- and keto-AcAc are explained because of the conjugative

183

effect.

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The occurrence of the H-abstraction TSs is further evaluated using the L parameter

185

(L= δ(H-O)) according to our previous study.47 This parameter not only quantifies whether the

186

TS structure exhibits a product-like (L > 1) or reactant-like (L < 1) character but also reflects

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whether the pathway is exothermic or endothermic. The L value of 0.34 for k-TSabs1 is two

188

times higher than that of k-TSabs2 (Figure 1). Hence, the k-Rabs2 pathway with an earlier TS

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corresponds to a larger ∆Er. As shown in Figure 2, the ∆Er of k-Rabs2 is -26.57 kcal mol-1,

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which is by about 4.41 kcal mol-1 more negative than that of k-Rabs1. On the other hand, the

δ(C-H)

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k-Rabs1 pathway corresponds to a ∆Ea value of 0.56 kcal mol-1, and there exists a hydrogen

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bond with the distance of 2.02 Å in the TS (Figure 1). In order to further assess the

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occurrence for H-abstraction, the dissociation energies (D0298(C‒H)) of methylene (-CH2-)

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and methyl (-CH3) groups are calculated at the M06-2X//M06-2X level. The (D0298 (C‒H))

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values are 89.77 kcal mol-1 for the -CH2- group and 94.14 kcal mol-1 for the -CH3 group,

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respectively, indicating that the H atom in the -CH2- group (k-Rabs2) are more reactive than

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that in the-CH3 group (k-Rabs1). Considering the ∆Ea and ∆Er values, the reactivity of the H

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atom in the two groups is dominantly affected by the steric effect rather than its stability,

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attributable to the presence of the -CH2- group in the middle of AcAc.

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The rate constants of the OH-initial reaction pathways of keto- and enol-AcAc are

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calculated and summarized in Table S2, and the temperature dependences of the branching

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ratios (Γ) are shown in Figure S5. For the reaction of OH with enol-AcAc, the rate constants

203

of e-Radd1 and e-Radd2 pathways are 3.78 × 10-11 and 7.46 × 10-11 cm3 molecule-1 s-1 at 298 K,

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respectively, which are by 2‒3 orders of magnitude higher than those of the two H-

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abstraction pathways (e-Rabs1 and e-Rabs2) (Table S2). The contribution of the combined H-

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abstraction pathways to the total rate constant is less than 1%, suggesting that the H-

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abstraction pathway is of minor importance. As shown in Figure S5a, the Γ of e-Radd2 is

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greater than 66% in the temperature range of 237‒298 K. Hence, OH addition at C3 position

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(e-Add-2) is more favorable, while the Γ value for OH addition at C2 position (e-Add-1) is

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33% at 298 K.

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The rate constants of the OH-addition pathway for keto-AcAc contributes negligibly to

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the total rate constants (Γ=0), and the sum Γ of the H-abstraction pathway equals to 1, in

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contrast to the case of enol-AcAc (Figure S4b). For example, the rate constants of k-Rabs1 and

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k-Rabs2 are 4.31 × 10-13 and 5.86 × 10-14 cm3 molecule-1 s-1 at 298 K, respectively,

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contributing 88% and 12% to the rate constant for keto-AcAc. Our results of dominant H-

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abstraction for k-Rabs1 are consistent with those obtained by Holloway et al16 but are in

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contrast with those by Zhou et al.13 The reactivity of the -CH2- group is likely overestimated

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using the structure-activity relationship, because the impact of the steric effect is not

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considered.13

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The calculated total rate constants (i.e., ktotal, the sum of calculated rate constants for all

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pathways) for the OH-AcAc reaction are presented in Figure 3, along with comparison with

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the available experimental data. Our derived Arrhenius expression is ktotal = 1.69 × 10-13

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exp(1935/T) cm3 molecule-1 s-1 over the temperature range of 200-310 K. The rate constant

224

for the keto-pathway (kketo) is much smaller than that of the enol-pathway (kenol).

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Furthermore, the previous studies have reported AcAc in gas phase exists predominantly in

226

the enol-form at room temperature.13,52 Hence, OH addition to enol-AcAc is favorable than

227

addition to keto-AcAc. As is evident from Figure 3, our results at the M06-2X//M06-2X level

228

also compare favorably with the available experimental data, considering the respective

229

uncertainties. For example, the total rate constant at 298 K is calculated to be 11.1 × 10-11

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cm3 molecule-1 s-1, consistent with the rate constant of (9.05 ± 1.81) × 10-11 cm3 molecule-1 s-1

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measured by Zhou et al.13 but somewhat higher than those reported by Bell et al.17 and

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Holloway et al.16 Our calculations show a strong negative temperature-dependence for the

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rate constants in the temperature range of 200-310 K, which is attributable to the presence of

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the pre-reactive complex. Also for comparison, Bell et al.17 reported report an activation

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parameter (Ea/R) of 1260 K, comparable to our theoretical value. Clearly, the M06-2X

236

method provides a reliable description for the initial kinetics of the atmospheric oxidation of

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AcAc.

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Subsequent oxidation of the OH-AcAc adduct. The attack of e-Add-1 and e-Add-2 by O2

239

yields two peroxy radicals (RO2-1 and RO2-2), which further undergo NO-addition to form

240

peroxy nitrites (RO2NO-1 and RO2NO-2) followed by NO2-elimination to form alkoxy

241

radicals (RO1 and RO2). Such multistep processes are described as the following (Figure 4), R11 or R21: + O2

R12 or R22: +NO

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e-Add-1 or e-Add-2 RO2-1 or RO2-2 RO2NO-1 or

243

RO2NO-2 RO-1 or RO-2

R13 or R23: -NO2

244

The vibrational frequencies, absolute energies, ZPEs and Cartesian coordinates of the

245

relevant species are included in Table S3. As shown in Figure 4, the ∆Ea of pathway R11 is

246

21.33 kcal mol-1, but the TS of the association of e-Add-2 with O2 (R21) is not identified.

247

The PPC was performed to confirm a barrierless process for R21 (Figure S5). The ∆Er values

248

of R11 and R21 are -26.24 and -36.98 kcal mol-1, respectively. The large ∆Ea and instability

249

of RO2-1 indicate that its formation is thermodynamically and kinetically less favorable. To

250

assess the competition between decomposition and O2 addition for e-Add-1, we calculated

251

the rate constants for both pathways, with the values of 4.80 × 10-7 s-1 and 7.75 × 10-26 cm3

252

molecule-1 s-1, respectively. Hence, decomposition of e-Add-1 to enol-AcAc and OH is more 13



253

favorable than the combination with O2 (with an equivalent first-order rate constant of 3.81 x

254

10-7 s-1). Considering the branching ratios between e-Add-1 and e-Add-2, it is estimated that

255

about 15% of e-Add-1 reacts with O2 to further propagate the oxidation (Figure 5).

256

The association reactions of RO2-1 and RO2-2 with NO (R12 and R22) are barrierless

257

and exothermic, with the ∆Er values of -22.42 and -21.12 kcal mol-1, respectively. The

258

produced peroxy nitrites (RO2NO-1 and RO2NO-2) further undergo NO-elimination via a

259

barrierless process to form the alkoxy radicals (RO-1 and RO-2), with the ∆Er values of

260

13.88 and 9.44 kcal mol-1, respectively. According to previous studies,48,49 there are three

261

primary reactions for the alkoxy radicals, i.e., dissociation, isomerization, and reaction with

262

O2. The reaction of with O2 is competitive only if isomerization is impossible and

263

dissociation forms primary alkyl radicals. The isomerization occurs only if there exists a H

264

atom located at four carbons away from the radical center (which is absent in our reaction

265

system. Hence, we focus on the subsequent dissociation of RO-1 and RO-2 involves the

266

following stepwise processes,

267

RO-1 TS12 or TS13 ER and MG or Acetyl Radical and PD

268

RO-2 TS22 or TS23 MR and AA or butane-2,4-dione-3,4-diol

269

and CH3

270

Decomposition of RO-1 via TS12 or TS13 yields (R14-1) MG and ethanediol radical (ER)

271

or (R14-2) acetyl radical (AR) and propanal-2,2-diol (PD), with the ∆Ea values of 0.40 and

272

3.73 kcal mol-1, respectively. The small ∆Ea difference between R14-1 and R14-2 indicates

273

that the products of the two pathways are both accessible. The ∆Ea values of the two

274

decomposition pathways of RO-2 are 1.48 and 8.64 kcal mol-1 to form (R24-1) AA and

275

methylglyoxal radical (MR) and (R24-2) butane-2,4-dione-3,4-diol and methyl radical (CH3), 14


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respectively. The large ∆Ea and small ∆Er for R24-2 indicate a minor importance to form

277

butane-2,4-dione-3,4-diol and CH3. Although the transition states of R14 and R24-2

278

pathways have lower energies than those of the corresponding products (Figure 4), there exist

279

the product complexes (COM12 and COM13) at the exit channels. Hence, AA and MG are the

280

major products from both R14-1 and R24-1.

281

Subsequently, ER, PD, and MR further react with O2 via the following stepwise processes, R15: + O2

R17

-HO2

R16: + O2

R18: + NO2

282

ER ER-RO2 TS15 AA

283

AR AR-RO2 PAN

284

MR TS23 MR-RO2 TS24 MG

285

The ∆Er values of R15 and R16 are -41.94 and -35.53 kcal mol-1, respectively, via the

286

barrierles processes to form two peroxy radicals (ER-RO2 and AR-RO2). The subsequent

287

reaction of AR-RO2 with NO2 is also barrierless, with a ∆Er value of 24.76 kcal mol-1 to yield

288

PAN. The ∆Ea value of ER-RO2 decomposition to form AA is 6.59 kcal mol-1. The pathway

289

from MR to MG corresponds to successively endothermic processes (R25 and R26) but

290

occurs promptly, considering the large exothermicity for the formation of e-Add-2 and

291

RO2NO-2.

R25: + O2

R26

-HO2

292

It is plausible that there are additional pathways to those investigated in our present

293

work, which require further experimental and theoretical studies. Our results indicate that

294

MG and AA represent the most favorable products from the oxidation of AcAc initiated by

295

OH, both arising from the pathways via e-Add-1 and e-Add-2. For comparison, the previous

15



Page 16 of 32

296

experimental measurements by Zhou et al. obtained the yields of (20.8 ± 4.5)% and (16.9 ±

297

3.4)% for MG and AA, respectively. In addition, we predict a minor pathway via e-Add-1 to

298

form PAN, consistent with the work by Zhao et al. for a small yield for the formation of PAN

299

(about 2%).

300

Atmospheric lifetime of AcAc. The lifetime of VOCs is a key parameter to assess their roles

301

in the formation of ozone and SOA. The OH initiated oxidation represents the dominant

302

daytime mechanism in regulating the lifetimes of VOCs.1,50 We evaluate the atmospheric

303

lifetime (τ) for AcAc at the different altitude and [OH] according to =

304

[OH] and ktotal are the OH concentration and total rate constant, respectively. Using a lapse

305

rate of 6.5 K km-1 for the typical tropospheric condition,47 we determine the lifetime of AcAc,

306

and results are presented in Table S4. At the ground level with [OH] = 1 × 106 molecules cm-

307

3

308

rapidly with increasing height and [OH]. For example, the τ value decreases to 0.16 hr as

309

[OH] increases to 1.5 × 107 molecules cm-3.

310

ATMOSPHERIC IMPLICATIONS

[]

, where

(12 hr daytime average), the lifetime of AcAc is 1.54 hr. The AcAc lifetime decreases

311

Ketones are generally less reactive because the less reactive keto-forms are generally

312

preferred than the more reactive enol-forms,1 while for AcAc the enol-form is dominant due

313

to intramolecular hydrogen bonding.44 We have assessed the mechanism, kinetics, and

314

atmospheric fate of the OH-initiated oxidation for the enolic and ketonic isomers of AcAc

315

using quantum chemical and kinetic rate calculations (Figure 5). OH addition to enol-AcAc is

16


Page 17 of 32


316

more favorable than addition to keto-AcAc, because of the conjugated π-electron effect.

317

Consequently, the rate constant for the keto-pathway is much smaller than that of the enol-

318

pathway, and the reaction of the enol-AcAc with OH represents the dominant pathway for

319

AcAc oxidation in the troposphere, largely determining the fate and impacts of its products.

320

For the reaction of the enol-AcAc with OH, the rate constants for OH addition are by 2‒3

321

orders of magnitude higher than those for H-abstraction, and OH addition to enol-AcAc

322

occurs via a prereactive complex. Our derived total rate constant for the reaction of AcAc

323

with OH is ktotal = 1.69 × 10-13 exp(1935/T) cm3 molecule-1 s-1 over the temperature range of

324

200-310 K and has a value of 11.1 × 10-11 cm3 molecule-1 s-1 at 298 K, consistent with the

325

previous experimental measurements.13,16,17

326

For the subsequent reactions of the OH-enol-AcAc adduct, O2 addition to the C3-adduct

327

proceeds barrierlessly with a large negative reaction energy (e-add-2), while O2 addition to

328

the C2-adduct occurs with a high activation barrier (about 21 kcal mol-1) and a smaller

329

exothermicity (e-add-1). AA and MG are the most favorable products from the two peroxy

330

radical pathways (RO2-1 and RO2-2), with a comparable yield. The branching ratio of about

331

81% leading to the formation of the two peroxy radicals likely corresponds to the upper limit

332

for the AA and MG yields, indicating the dominant production of the two species from the

333

OH-initiated oxidation of AcAc. The previous experimental study for the OH-initiated

334

oxidation of AcAc by Zhou et al. obtained the yields for AA (about 17%) and MG (about

335

21%).13 Considering possible wall loss and secondary reactions (including photolysis for

17



336

MG) of these species in the chamber work,33-36 those measured yields likely correspond to

337

the lower experimental limits. On the other hand, our results indicate a minor pathway

338

leading to PAN, consistent with the small experimental yield of PAN (2%).13 Other plausible

339

products from the OH-initiated oxidation of AcAc include organic nitrates (RONO2) directly

340

arising from the peroxy nitrites (RO2NO-1 and RO2NO-2) and minor butane-2,4-dione-3,4-

341

diol. However, direct formation of organic nitrates from peroxy nitrites typically constitutes a

342

minor pathway, in contrast to the dominant formation for alkoxy radicals.51 Using the

343

predicted temperature-dependence kinetic data, we estimate a lifetime of less than a few

344

hours due the OH-initiated oxidation of AcAc throughout the entire tropospheric conditions.

345

Figure 6 shows a time series of AcAc measurements in Nanjing and Beijing, China. The

346

measured AcAc concentration varies from ppt to ppb levels and exhibits a diurnal variation in

347

both locations, which is likely regulated by the emission, planetary boundary layer height,

348

and photochemical activity. Such atmospheric concentrations of AcAc are clearly significant,

349

considering its high reactivity with OH and short atmospheric lifetimes under tropospheric

350

conditions. Since AA and MG are important SOA precursors,14,15,19-24,52 accurate

351

determination of their yields from AcAc oxidation by OH is critical to assess SOA formation

352

under polluted environments. Furthermore, organic peroxy radical (RO2) and hydroperoxy

353

radical (HO2) are produced from the OH-initiated oxidation of enol-AcAc (Figure 4),

354

facilitating the cycling between NO and NO2 and producing tropospheric ozone.51 Hence,

355

because its high reactivity and dominant yields for MG and AA, the photochemical oxidation

18


Page 18 of 32

Page 19 of 32


356

of AcAc may contribute importantly to ozone and SOA formation under polluted

357

environments, with implications for air quality, human health, and climate. Our results

358

provide the kinetic and mechanistic data for inclusion of the oxidation of AcAc in

359

atmospheric models. Future studies are necessary to assess the impacts of AcAc on ozone

360

and SOA formation using chemical transport models, with the consideration of its emission

361

inventory, chemistry, and transport.

362

ASSOCIATED CONTENT

363

Supporting Information

364

The structures, Cartesian coordinates, frequencies, zero-point energies, and absolute energies

365

of all relevant species involved in the title reaction, along with the branching ratios, the rate

366

constants, and the lifetimes are listed in the Supporting Information. This material is

367

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

368

AUTHOR INFORMATION

369

Corresponding Authors

370

* Phone: 86-20-23883536; Fax: 86-20-23883536; E-mail address: [email protected]

371

* Phone: 979-845-7656; Fax: 979-862-4466; E-mail address: [email protected]

372

Notes

373

The authors declare no competing financial interest.

374

ACKNOWLEDGMENTS

375

This work was financially supported by National Natural Science Foundation of China

376

(41675122, 41425015, U1401245 and 41373102), Science and Technology Program of

19



377

Guangzhou City (201707010188), Team Project from the Natural Science Foundation of

378

Guangdong Province, China (S2012030006604), and the Special Program for Applied

379

Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase), and

380

the National Supercomputing Centre in Guangzhou (NSCC-GZ). R.Z. acknowledged the

381

support from the Robert A. Welch foundation (A-1417).

382

REFERENCES

383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411

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541 542

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543

Figure captions

544

Figure 1. The optimized geometries of the key stationary points of OH-AcAc reaction at the

545 546 547

M06-2X/6-311G(d,p). The bond length is in Å. Figure 2. PES for the OH-initiated reactions of keto- and enol-AcAc (in the unit of kcal mol1

).

548

Figure 3. The rate constants (cm3 molecule-1 s-1) of keto- and enol-AcAc with OH radical

549

against the temperature. The experimental results (i.e., Expt. 1, 2 and 3) are from Zhou

550

et al., Bell et al., and Holloway et al., respectively.

551

Figure 4. PES for (a) the subsequent pathways of e-Add-2 and (b) the competing

552

decomposition and combination with O2 for e-Add-1. The number denotes the value of

553

∆Ea and ∆Er for each reaction step. (unit: kcal mol-1 for energies and cm3 molecule-1 s-1

554

for the rate constants).

555 556 557 558

Figure 5. Schematic representation of the preferred pathways of the OH-AcAc reactions leading to formation of methylglyoxal and acetic acid. Figure 6. Time series of AcAc measured using the PRT- MS method in Nanjing (a) and Beijing (b), China. The major tick mark on the x-axis labels the local time at midnight.

559

25



560

Figure 1

561

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Figure 2

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Figure 3

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Figure 4

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Figure 5


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Figure 6



TOC


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OH-Initiated Oxidation of Acetylacetone: Implications for Ozone and

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