Reassessing the atmospheric oxidation mechanism of toluene

Abstract

Photochemical oxidation of aromatic hydrocarbons leads to tropospheric ozone and secondary organic aerosol (SOA) formation, with profound implications for air quality, human health, and climate. Toluene is the most abundant aromatic compound under urban environments, but its detailed chemical oxidation mechanism remains uncertain. From combined laboratory experiments and quantum chemical calculations, we show a toluene oxidation mechanism that is different from the one adopted in current atmospheric models. Our experimental work indicates a larger-than-expected branching ratio for cresols, but a negligible formation of ring-opening products (e.g., methylglyoxal). Quantum chemical calculations also demonstrate that cresols are much more stable than their corresponding peroxy radicals, and, for the most favorable OH (ortho) addition, the pathway of H extraction by O2 to form the cresol proceeds with a smaller barrier than O2 addition to form the peroxy radical. Our results reveal that phenolic (rather than peroxy radical) formation represents the dominant pathway for toluene oxidation, highlighting the necessity to reassess its role in ozone and SOA formation in the atmosphere.

Remarkable nucleation and growth of ultrafine particles from vehicular exhaust

“…On the other hand, continuum NPF occurs at low to intermediate levels of ambient PM 2.5 without significant capture of freshly nucleated particles and at a low growth rate without rapid increase in the surface area of nucleation-mode particles. In addition, photooxidation of vehicular exhaust (dominantly aromatic VOCs) yields abundant precursors for nucleation and growth of UFPs (29,32) as demonstrated in our three different types of measurements. Our ambient and chamber measurements are representative of typical urban environments worldwide (8) since the gasoline fleet of the commonly used vehicle model in China is equivalent to those in Europe and the United States (27).…”

Enthalpies of formation of the benzyloxyl, benzylperoxyl, hydroxyphenyl radicals and related species on the potential energy surface for the reaction of toluene with the hydroxyl radical

“…Therefore, its reactions have been studied repeatedly, both experimentally and theoretically. A non-exhaustive list of recent studies can be found in references [5][6][7][8][9][10][11]. We have recently performed a detailed study of the possible routes for the reaction of toluene (T) with OH • , and further reactions of the intermediate radicals with both OH • and O2 [12,13].…”

Understanding the catalytic role of oxalic acid in the SO<sub>3</sub> hydration to form H<sub>2</sub>SO<sub>4</sub> in the atmosphere

“…et al, 2001;Cameron et al, 2002;Steckler et al, 1997;Atkinson et al, 2006). Some addition (Steudel, 1995;Williams et al, 1983;Courmier et al, 2005;Zhang and Zhang, 2002), decomposition (Rayez et al, 2002;Kumar and Francisco, 2015;Gutbrod et al, 1996), isomerization (Zheng and Truhlar, 2010;Atkinson, 2007), and abstraction reactions (Ji et al, 2013;Ji et al, 2017) also are the important HAT reaction in the atmosphere. These atmospheric HAT reactions have a main feature that two-point hydrogen bond can occur and can facilitate hydrogen atom transfer (Kumar et al, 2016).…”