Mechanistic study of the formation of ring-retaining and ring-opening products from the oxidation of aromatic compounds under urban atmospheric conditions The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Zaytsev, Alexander, et al. "Mechanistic study of the formation of ring-retaining and ring-opening products from the oxidation of aromatic compounds under urban atmospheric conditions." Atmospheric Chemistry and Physics, 19, 23 (December 2019): 1511-15129. © 2019 Author(s). As Published http://dx.doi.org/10.5194/ACP-19-15117-2019 Publisher Copernicus GmbH Version Final published version Citable link https://hdl.handle.net/1721.1/125755 Terms of Use Creative Commons Attribution 4.0 International license Detailed Terms https://creativecommons.org/licenses/by/4.0/ Atmos. Chem. Phys., 19, 15117–15129, 2019 https://doi.org/10.5194/acp-19-15117-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. Mechanistic study of the formation of ring-retaining and ring-opening products from the oxidation of aromatic compounds under urban atmospheric conditions Alexander Zaytsev1, Abigail R. Koss2,a, Martin Breitenlechner1, Jordan E. Krechmer3, Kevin J. Nihill2, Christopher Y. Lim2, James C. Rowe2, Joshua L. Cox4, Joshua Moss2, Joseph R. Roscioli3, Manjula R. Canagaratna3, Douglas R. Worsnop3, Jesse H. Kroll2, and Frank N. Keutsch1,4,5 1John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA 2Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA 3Aerodyne Research Inc., Billerica, MA 01821, USA 4Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA 5Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA anow at: TOFWERK USA, Boulder, CO 80301, USA Correspondence: Alexander Zaytsev ([email protected]) and Frank N. Keutsch ([email protected]) Received: 23 July 2019 – Discussion started: 7 August 2019 Revised: 5 November 2019 – Accepted: 14 November 2019 – Published: 13 December 2019 Abstract. Aromatic hydrocarbons make up a large frac- retaining products for these two precursors are more di- tion of anthropogenic volatile organic compounds and con- verse and abundant than predicted by current mechanisms. tribute significantly to the production of tropospheric ozone We present the speciated elemental composition of SOA for and secondary organic aerosol (SOA). Four toluene and four both precursors and confirm that highly oxygenated products 1,2,4-trimethylbenzene (1,2,4-TMB) photooxidation exper- make up a significant fraction of SOA. Ring-scission prod- iments were performed in an environmental chamber un- ucts are also detected in both the gas and particle phases, and der relevant polluted conditions (NOx ∼ 10 ppb). An exten- their yields and speciation generally agree with the kinetic sive suite of instrumentation including two proton-transfer- model prediction. reaction mass spectrometers (PTR-MS) and two chemical C − ionisation mass spectrometers (NH4 CIMS and I CIMS) allowed for quantification of reactive carbon in multiple gen- erations of hydroxyl radical (OH)-initiated oxidation. Oxida- 1 Introduction tion of both species produces ring-retaining products such as cresols, benzaldehydes, and bicyclic intermediate com- Aromatic compounds represent a significant fraction of pounds, as well as ring-scission products such as epoxides volatile organic compounds (VOCs) in the urban atmosphere and dicarbonyls. We show that the oxidation of bicyclic in- and play a substantial role in the formation of tropospheric termediate products leads to the formation of compounds ozone and secondary organic aerosol (SOA) (Calvert et al., with high oxygen content (an O V C ratio of up to 1.1). 2002). Typical anthropogenic sources include vehicle ex- These compounds, previously identified as highly oxy- haust, solvent use, and evaporation of gasoline and diesel genated molecules (HOMs), are produced by more than one fuels. Toluene, the most abundant alkylbenzene in the atmo- pathway with differing numbers of reaction steps with OH, sphere, is primarily emitted by the aforementioned anthro- including both auto-oxidation and phenolic pathways. We re- pogenic processes (Wu et al., 2014). Toluene-derived SOA port the elemental composition of these compounds formed is estimated to contribute approximately 17 %–29 % of the under relevant urban high-NO conditions. We show that ring- total SOA produced in urban areas (Hu et al., 2008). More highly substituted aromatic compounds make up another im- Published by Copernicus Publications on behalf of the European Geosciences Union. 15118 A. Zaytsev et al.: Mechanistic study of the formation of ring-retaining and ring-opening products with NO to form bicyclic oxy radicals that decompose to ring-scission carbonylic products such as (methyl) glyoxal and biacetyl. Recent theoretical studies have predicted new epoxy-dicarbonyl products that have not been reported in previous studies (Li and Wang, 2014, Wu et al., 2014). Reaction of BPRs with NO can also result in the forma- tion of bicyclic organonitrates. In addition, BPRs react with HO2 and RO2, forming bicyclic hydroperoxides and bi- cyclic carbonyls respectively (Fig. 2). Finally, BPRs can un- dergo unimolecular H migration followed by O2 addition (auto-oxidation), leading to the formation of non-aromatic ring-retaining highly oxygenated organic molecules (HOMs) (Bianchi et al., 2019). Molteni et al. (2018) reported the ele- mental composition of the HOMs from a series of aromatic compounds produced under low-NO conditions. The auto- oxidation pathway might be more important for the substi- tuted aromatics because of the higher yield of BPR forma- tion and the larger number of relatively weak C–H bonds Figure 1. Major gas-phase oxidation pathways for aromatic hydro- (Wang et al., 2017). Both ring-retaining and ring-scission carbons, using toluene as an example. Reaction yields for the oxida- compounds are expected to be low in volatility and contribute tion pathways of toluene recommended by MCM v3.3.1 are shown in black (Bloss et al., 2005). The proposed yields from the present significantly to SOA (Schwantes et al., 2017). A number of study are shown in blue. The yield of the peroxide-bicyclic pathway major uncertainties remain in the model representation of the is calculated based on the yields of ring-scission products. oxidation of aromatic compounds, including the overpredic- tion of ozone concentration, the underprediction of OH pro- duction and the lack of detailed description of SOA forma- tion (Birdsall and Elrod, 2011; Wyche et al., 2009). portant group of aromatic compounds as they tend to have In the present work, we investigate detailed mecha- high SOA yields (Li et al., 2016) and account for a significant nisms of hydroxyl radical multigenerational oxidation chem- fraction of non-methane hydrocarbons in the industrialised istry of two aromatic hydrocarbons – toluene and 1,2,4- regions of China (Tang et al., 2007; Zheng et al., 2009). trimethylbenzene – under moderately high, urban-relevant 1,2,4-Trimethylbenzene (1,2,4-TMB) is chosen as a model NOx levels (∼ 10 ppbv). Laboratory experiments were con- molecule to study the oxidation of more substituted aromatic ducted in an environmental chamber over approximately 1 d compounds (i.e. trimethylbenzenes). of atmospheric-equivalent oxidation. We use three chemical In the atmosphere, oxidation of aromatic hydrocarbons ionisation mass spectrometry (CIMS) techniques (I− reagent C C is primarily initiated by their reactions with hydroxyl rad- ion, NH reagent ion, and H3O reagent ion) to characterise 4 C icals (OH) via H abstraction from the alkyl groups or OH and quantify gas-phase oxidation products. In addition, NH4 addition to the aromatic ring (Fig. 1) (Calvert et al., 2002). CIMS and I− CIMS were used to detect particle-phase prod- The abstraction channels are relatively minor, leading to the ucts. The goal of this work is to identify gas-phase pathways formation of benzyl radicals and benzaldehyde with yields leading to the production of low-volatility compounds, which of ∼ 0:07 in the case of toluene (Wu et al., 2014) and ∼ 0:06 are important for SOA formation and support these identi- in the case of 1,2,4-TMB (Li and Wang, 2014). The OH fications with CIMS data and a method to characterise the adducts can react with atmospheric O2 via H abstraction kinetics of an oxidation system. to form ring-retaining phenolic compounds (i.e. cresols and trimethylphenols) (Jang and Kamens, 2001; Kleindienst et al., 2004). The phenol formation yield decreases for the more 2 Methods substituted aromatics: in case of toluene, the cresol yield is ∼ 0:18 (Klotz et al., 1998; Smith et al., 1998), whereas 2.1 Experimental design for 1,2,4-TMB, the trimethylphenol yield is ∼ 0:03 (Smith et al., 1999; Bloss et al., 2005). Both abstraction and pheno- All experiments were performed in a 7.5 m3 Teflon environ- lic channels lead to the formation of products retaining the mental chamber (Hunter et al., 2014). Prior to experiments, aromatic ring. the chamber was flushed and filled with purified air. Dur- The OH adducts can also react with O2 through re- ing experiments the environmental chamber was operated in combination. In this case they lose aromaticity and form the constant-volume (“semi-batch”) mode, in which clean air non-aromatic ring-retaining bicyclic peroxy radicals (BPRs). (11–14 L min−1) was constantly added to make up for instru- Under urban-relevant high-NO conditions BPRs also react ment sample flow. The temperature of the chamber was con- Atmos. Chem. Phys., 19, 15117–15129, 2019 www.atmos-chem-phys.net/19/15117/2019/ A. Zaytsev et al.: Mechanistic study of the formation of ring-retaining and ring-opening products 15119 Figure 2. Oxidation pathways of bicyclic peroxy radicals in the OH-initiated oxidation of 1,2,4-trimethylbenzene. The starting radical is shown in blue.