Data of chemical composition of the particles from OH-initiated oxidation of 1,3,5-trimethylbenzene

This paper presents the data of chemical composition of the particles from OH oxidation reaction of 1,3,5-trimethylbenzene (1,3,5-TMB). The particle-phase compositions are measured on-line by using a vacuum ultraviolet (VUV) photoionization aerosol mass spectrometer. The assignments of the major peaks of photoionization mass spectrum, as well as their molecular structures, are presented. The optimized structures of the reactants, intermediates and transition states involved in the reaction of the bicyclic peroxy radical with HO2 are shown. The reaction routes of the OH-initiated oxidation of the deuterated 1,3,5-TMB sample are also calculated and displayed for comparison. The data presented here is related to the paper “Direct observation of the particle-phase bicyclic products from OH-initiated oxidation of 1,3,5-trimethylbenzene under NOx-free conditions” by Lin et al. (2022).

Chemistry Specific subject area Atmospheric chemistry Type of data Table  Figure Graph How the data were acquired Mass spectra: Vacuum ultraviolet (VUV) photoionization aerosol mass spectrometer.
Structures and reaction routes: ChemBioDraw Ultra 12.0, Gaussian 16 and GaussView 6. Data format Raw Analysed Filtered Description of data collection The mass spectra of the particle-phase products from OH oxidation reaction of 1,3,5-TMB were measured by using the VUV photoionization aerosol mass spectrometer. The molecular structures were drawed manually by using Chemdraw ultra 12.0. The geometric structures were optimized by using Gaussian 16 program.

Value of the Data
• The data presented in this paper are valuable for the characterization of secondary organic aerosol (SOA) from oxidation of aromatic compounds. • The data can help researchers to better understand the reaction mechanisms of the bicyclic peroxy radical involved in the atmospheric oxidation. • The data would serve as a reference for studying or analysing the chemical compositions of SOA from OH-initiated oxidation of 1,3,5-TMB.

Data Description
The data of this paper includes one table, two figures and the mass spectrometry raw data open to readers via ProteomeXchange. Concretely, Table 1 lists the molecular structures of the major particle-phase products from OH-initiated oxidation of 1,3,5-trimethylbenzene (1,3,5-TMB). It shows the mass (m/z) and the name of these products and their corresponding molecular structures which were drawn with the ChemBioDraw Ultra program. The oxygen-containing functional groups in these molecular structures are presented in red.

Experimental design
A simulation chamber made of a Teflon bag and surrounded by six ultraviolet lamps (Philips TUV G13 36 W), providing a maximum output at 254 nm, is used to simulate the atmospheric chemical process of OH + 1,3,5-TMB in lab. A home-made vacuum ultraviolet (VUV) photoionization aerosol mass spectrometer (VUV-AMS) is installed to probe and analyze the particle-phase products inside the Teflon bag. A commercial scanning mobility particle sizer (SMPS, TSI 3936, USA) is used to measure the size distribution of the particles. The quantum chemical method using the Gaussian 16 program is adopted to calculate the parameters of the reactants, intermediates and transition states. The configuration of the VUV photoionization aerosol mass spectrometer and the theoretical methods are described in detail in Ref. [1]

Photoionization mass spectra of the particle-phase products
Raw data of the mass spectra of the particle-phase products from the OH-initiated oxidation reactions of 1,3,5-TMB and deuterated 1,3,5-TMB-d12 (C 9 D 12 ) measured by using the VUV photoionization aerosol mass spectrometer can be seen in the Supplementary data. A great deal of peaks can be observed in the mass spectrum and most of them have been assigned (see Table 1 ). The mass spectrometry data have been deposited to the ProteomeXchange Consortium via the PRIDE (Refs. [2 , 3] ) partner repository with the dataset identifier PXD030839.

The assignments of the main products
The structures of the particle-phase products from the OH-initiated oxidation of 1,3,5-TMB under NO X -free conditions are identified. The main peaks in the VUV photoionization mass spectra have been assigned and listed in Table 1 , with the aid of the literature results (Refs. [4 , 5] ). For example, the products of the O 2 -bridged bicyclic alcohol (m/z = 186) and carbonyl (m/z = 184), the peroxide (m/z = 202) and the trioxide (m/z = 218) products are clearly observed and identified in the particle-phase. In addition, the ring retaining products of 3,5-dimethyl-phenyl-hydroperoxide (m/z = 138), 3,5-dimethylbenzoic acid (m/z = 150), (3,5dimethyl-phenyl)-methyl-hydroperoxide (m/z = 152), and an array of low mass oxygenated compounds such as acetone, acetic acid and methyl glyoxal are also observed.

Geometric structures of the main species in the reaction between the BPR radical and HO 2
The optimized structures of the reactants, reaction intermediates and transition states involved in the reaction between the O 2 -bridged bicyclic peroxy radical (BPR) and HO 2 on the singlet potential energy surfaces are shown in Fig. 1 . The geometric structures were calculated at the M06-2X/MG3S level of theory by using the Gaussian 16 program. The M06-2X functional are hybrid meta density functional theory (DFT) method which has been widely utilized for applications in chemistry (Refs. [6 , 7] ). The selected bond distances such as the hydrogen bond have been given in angstroms.

Formation mechanism of the deuterated bicyclic oxygenated compounds
The reaction routes of the deuterated 1,3,5-TMB (C 9 D 12 ) with OH radical under NOx free conditions are presented in Fig. 2 . Similar to the reaction of 1,3,5-TMB with OH, the reaction occurs mainly via the OH addition to form the OH-C 9 D 12 adduct. In the presence of oxygen, the nascent OH-C 9 D 12 adduct can react with oxygen to generate the OH-C 9 D 12 -O 2 peroxy radical, which will produce the deuterated bicyclic peroxy radical after subsequent isomerization, reaction with oxygen again and cyclization. Under NOx free condition, reactions of the deuterated bicyclic peroxy radical with the HO 2 radical, other peroxy radicals, and OH radical play a major role. These reactions will produce the deuterated O 2 -bridged bicyclic alcohol and carbonyl, the peroxide and the trioxide products. In the self-reaction of the deuterated bicyclic peroxy radical, the deuterium-shift results in the products formation with an odd mass number (m/z = 199 and 195), which are observed in the mass spectrum.