LETTER

LETTER Elucidating the fate of the OH-adduct in toluene oxidation under tropospheric boundary layer conditions Mike J. Newlanda,1, Michael E. Jenkinb, and Andrew R. Rickarda,c

5 Ji et al. (1) published a study examining the mechanism OH-adduct with NO2 is a factor of 10 greater than with of the initial stages of OH-initiated oxidation of toluene. O2 (3, 8). Hence, for the present experimental setup, the Their results challenge the mechanisms used in atmo- NO2 reaction will become the dominant fate of the OH- 10 −3 spheric chemistry models [e.g., Master Chemical Mech- adduct at [NO2] > 1 × 10 cm . This reaction yields anism (MCM) (2)] derived from chamber experiments at cresol (Y[NO2]) (8) but not dicarbonyls (9). atmospheric conditions, and previous experimental (3, From a simple model based on Scheme 1 [see the 4) and theoretical studies (5, 6). In these mechanisms, recent theoretical work of Wu et al. (6) and references

the main product (55–65%) of the O2 reaction with the therein for a more detailed description], at [O2] = 5 × − OH-adduct is a peroxy-peroxide bridged radical (cyc- 1018 cm 3 the yield of products via pathway II is cal- RO2, Scheme 1), the subsequent chemistry of which culated to be 70%, while the yield of cresol is 23%, in goes on to yield dienals and the dicarbonyls: glyoxal good agreement with the MCM (2) and previous ex- 15 −3 and methylglyoxal. The present study detected a neg- perimental studies (10). Reducing [O2]to1× 10 cm ligible yield of these dicarbonyl products. From this, the (not including the OH-adduct + NO2 reaction), the authors conclude that the cyc-RO2 pathway is insignif- cresol yield is 3% and products via pathway II yield is icant under atmospheric conditions. 6%, with 91% of the oxidized toluene present as the 15 −3 The experiments use [O2] = 1 × 10 cm , roughly OH-adduct on the timescale of the experiments 5,000 times lower than in the atmospheric boundary (∼50 ms). Including the NO2 reaction in the model, 18 −3 11 −3 layer ([O2] = 5 × 10 cm ). The authors state that assuming [NO2] = 5 × 10 cm and Y[NO2] = 0.5, “the absolute O2 concentration had no effect for the we calculate a cresol yield of 32%. competing reactions between the cresol and primary Hence, while the present study provides an in-

RO2 pathways (i.e., pathway I vs. pathway II in Fig. teresting exploration of the toluene system, its rele- 1).” However, we believe this not to be the case, vance to understanding the oxidation of toluene in the and their figure 1 to be an incomplete representation atmospheric boundary layer appears limited. We do of the chemistry pertinent to the study. Crucially, it is not consider that the experimental results contradict missing the decomposition of RO2 back to the OH- the current understanding of the early stages of OH- adduct. initiated toluene oxidation. We would encourage the

Reducing [O2] shifts the [RO2]/[OH-adduct] equilib- authors to repeat their experiments under a range of rium toward the OH-adduct. While this alone has little [O2] and [NO2] to elucidate the driving mechanisms effect on the relative contribution of pathway I vs. path- behind their results. way II, it increases the importance of any other sinks of the OH-adduct. The authors have previously reported Acknowledgments − M.J.N. and A.R.R. acknowledge the UK Natural Environment [NO ] = 3–7 × 1011 cm 3 with a very similar experimen- 2 Research Council’s “Mechanisms for Atmospheric Chemistry: tal setup (7), presumably as a residual from the excess Generation, Interpretation, and Fidelity” Project (NE/M013448/ NO2 used in OH generation. The reaction rate of the 1) for funding.

aWolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, York, YO10 5DD, United Kingdom; bAtmospheric Chemistry Services, Okehampton, Devon, EX20 4QB, United Kingdom; and cNational Centre for Atmospheric Sciences, Department of Chemistry, University of York, York, YO10 5DD, United Kingdom Author contributions: M.J.N., M.E.J., and A.R.R. designed research; M.J.N. performed research; and M.J.N., M.E.J., and A.R.R. wrote the paper. The authors declare no conflict of interest. 1To whom correspondence should be addressed. Email: [email protected].

E7856–E7857 | PNAS | September 19, 2017 | vol. 114 | no. 38 www.pnas.org/cgi/doi/10.1073/pnas.1713678114 Downloaded by guest on September 25, 2021 Scheme 1. Simplified representation of chemical mechanism of OH-initiated toluene oxidation. For clarity, only one isomeric form of the intermediates is shown.

1 Ji Y, et al. (2017) Reassessing the atmospheric oxidation mechanism of toluene. Proc Natl Acad Sci USA 114:8169–8174. 2 Bloss C, et al. (2005) Development of a detailed chemical mechanism (MCMv3.1) for the atmospheric oxidation of aromatic hydrocarbons. Atmos Chem Phys 5:641–664.

3 Koch R, Knispel R, Elend M, Siese M, Zetzsch C (2007) Consecutive reactions of aromatic OH-adducts with NO, NO2 and O2: Benzene, naphthalene, toluene, m-andp-xylene, hexamethylbenzene, phenol, m-cresol and aniline. Atmos Chem Phys 7:2057–2071. 4 Birdsall AW, Andreoni JF, Elrod MJ (2010) Investigation of the role of bicyclic peroxy radicals in the oxidation mechanism of toluene. J Phys Chem A 114:10655–10663. 5 Suh I, Zhang R, Molina LT, Molina MJ (2003) Oxidation mechanism of aromatic peroxy and bicyclic radicals from OH-toluene reactions. J Am Chem Soc 125:12655–12665. 6 Wu R, Pan S, Li Y, Wang L (2014) Atmospheric oxidation mechanism of toluene. J Phys Chem A 118:4533–4547. 7 Zhang D, Zhang R, North SW (2003) Experimental study of NO reaction with isoprene hydroxyalkyl radicals. J Phys Chem A 107:11013–11019.

8 Atkinson R, Aschmann SM (1994) Products of the gas-phase reactions of aromatic hydrocarbons: Effect of NO2 concentration. Int J Chem Kinet 26:929–944. 9 Nishino N, Arey J, Atkinson R (2010) Formation yields of glyoxal and methylglyoxal from the gas-phase OH radical-initiated reactions of toluene, xylenes, and

trimethylbenzenes as a function of NO2 concentration. J Phys Chem A 114:10140–10147. 10 Smith DF, McIver CD, Kleindienst TE (1998) Primary product distribution from the reaction of hydroxyl radicals with toluene at ppb NOx mixing ratios. J Atmos Chem 30:209–228.

Newland et al. PNAS | September 19, 2017 | vol. 114 | no. 38 | E7857 Downloaded by guest on September 25, 2021