Article pubs.acs.org/JPCA
Atmospheric Oxidation Mechanism of m‑Xylene Initiated by OH Radical Shanshan Pan† and Liming Wang*,†,§ †
School of Chemistry & Chemical Engineering, South China University of Technology, Guangzhou 510640, China Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, South China University of Technology, Guangzhou 510006, China
§
S Supporting Information *
ABSTRACT: The atmospheric oxidation mechanism of mxylene (mX) initiated by the OH radical is investigated at M06-2X and ROCBS-QB3 levels, coupled with reaction kinetics calculations by using transition state theory and unimolecular RRKM-ME theory. The calculations show that the reaction between OH and mX is dominated by OH addition to the C2 and C4 positions, forming adducts mX−2OH (R2) and mX−4-OH (R4). In the atmosphere, R2 and R4 react with O2 by irreversible H-abstraction to dimethylphenols or by reversible additions to bicyclic radical intermediates, which would recombine again with O2 to form bicyclic peroxy radicals, to bicyclic alkoxyl radicals by reacting with NO or HO2, and eventually to final products such as glyoxal, methylglyoxal, and their coproducts. The effects of reaction pressure and temperature are explored by RRKM-ME calculations. A mechanism at 298 K is proposed on the basis of current predictions and previous experimental and modeling results. The predicted product yields support the values in the SAPRC mechanism, even though the predicted yield of 1.0% for glyoxal is lower than the value of ∼11% from the experimental measurements. trations, for example, [NO] of ∼1.2 × 1014 molecules cm−3 by Tuazon et al.,20,21 [NO] of 2.1−24 × 1013 molecules cm−3 and [NO2] of 0−33 × 1013 molecules cm−3 by Atkinson et al.,14 [NO] of ∼2.4 × 1013 or ∼1.2 × 1014 molecules cm−3 by Kwok et al.,24 [NO] of 8.3 × 1012 molecules cm−3 by Smith et al.,15 and [NO] of ∼1.0× 1012 or 1.4 × 1013 molecules cm−3 and [NO2] of 5.0 × 1011−1.4 × 1012 molecules cm−3 by Zhao et al.16 Discrepancies between measurements are clear. The discrepancies arise from the difficulty in identifying and quantifying the multifunctional and low-volatile oxygenated products, for example, the measured product yields accounted for only 40% carbon consumed. 15 The product yields at high NO x concentrations may be different from those at ambient low NOx conditions because the reactions between mX−OH adducts and NO2 may compete with the reactions of mX−OH adducts with O2, and the reactions between peroxyl radicals and NO may compete with the isomerization of peroxyl radicals. Effects of NO2 concentration on the product yields have been observed in the oxidation of substituted benzenes including toluene, xylenes, and trimethylbenzenes.22,25−28 A few recent studies, specifically on photooxidation of mX, found the dependence of SOA formation on light intensity, light source, NOx concentration, reaction temperature, aging effects, and so forth.29−31
1. INTRODUCTION Aromatic hydrocarbons are important components of urban and regional atmospheric pollutants predominantly emitted from the anthropogenic sources such as gasoline, fossil fuel burning, vehicle emissions, and solvent use and evaporation.1−3 In the atmosphere, photooxidation of aromatic compounds contributes significantly to urban O3 production and secondary organic aerosol (SOA) formation.4−8 In the urban atmosphere of Megacities and City Clusters in China, m,p-xylene is among the largest contributors to ozone formation.9−11 In the lower troposphere, m-xylene (mX) reacts primarily with OH radical during the daytime. Kramp and Paulson12 have evaluated the early measurements and obtained the rate constant as (22.0 ± 2.7) × 10−12 cm3 molecule−1 s−1 at 296 K, and Mehta et al.13 proved that the reaction reached the high-pressure limit at the atmospheric pressure and obtained a rate constant of (21.4 ± 1.4) × 10−12 cm3 molecule−1 s−1 at 298 K. The reaction may proceed by H-abstraction from the −CH3 group, leading eventually to the formation of m-tolualdehyde and nitrates,14−16 or by OH addition to the aromatic ring, leading to the formation of mX−OH adducts (denoted as R1, R2, R4, and R5, Scheme 1).17,18 In the atmosphere, these adducts will react with O2 and NOx. Koch et al.19 have obtained the bimolecular rate constants of (18 ± 5) × 10−16 (303 K) and 140 kJ/mol (relative to R2 + O2); therefore, R25OO-a/s would simply decompose back to R2 + O2 with no net removal of R2. Similarly, ring closures from R2-1OO to R214OO and R2-15OO are negligible because their barriers are at least 40 kJ/mol higher than those to R2-13OO. Therefore, the reaction of R2 and O2 can be simplified, as shown in Scheme 2. For the “important” pathways included in the mechanism, the reaction energies and barrier heights are calculated at the ROCBS-QB3 level (Tables S1 (Supporting Information) and 3), and Figure 1 shows the Gibbs energy diagram for reaction R2 + O2 at 298 K. Table 3 also contains the high-pressure-limit rates estimated by traditional TST. In this reaction scheme, the cyclization from R2-1OO-a/s to R2-13OO-a/s is taken as being irreversible because of the high exothermicity of −47.5 and −54.5 kJ/mol (ΔrG298K⧧) for the anti and syn conformers, respectively. The high barriers of 99.0 and 92.0 kJ/mol (ΔG298K⧧) from R2-−13OO-a/s to R2-1OO-a/s lead to extremely slow rates of
