Supplementary Information for: Analysis of High Mass Resolution PTR-TOF Mass Spectra from 1,3,5-trimethylbenzene (TMB) Smog Chamber Experiments

1 Quantification PTR-TOF calibrations were performed at regular intervals using multi-component gas cylinders (Apel Riemer Environmental Inc., Denver, USA). During calibration, the sample air humidity was kept at similar levels as measured in the environmental chamber. Calibrated sensitivities for several chemical groups, including alcohols, aromatic compounds, aldehydes and ketones, were used as a basis for VOC quantification (Table S1). Proton transfer reaction rate coefficients for compounds not accessible to direct calibration were calculated from the dipole moment μD and polarizability  of the molecule using the parameterized reaction rate theory (kSC) by Su and Chesnavich (1982). Quantum chemical Gaussian 09 DFT calculations were performed to obtain the dipole moment μD and the polarizability  of the primary reaction products expected from the MCMv3.1 (Table S2). Calculated rates of the expected products were linked to experimental calibration results. The products were grouped into acetone-like compounds (kSC > 2.9 10 -9 cm 3 s -1 ), e.g. 3,5-dimethylbenzaldehyde and 3,5dimethyl-3(2H)-2-furanone, and xylene-like compounds (1.9 < kSC < 2.4 10 -9 cm 3 s -1 ), e.g. 2,4,6-trimethylphenol and 2-methyl-4-oxo-2-pentenal. These compounds were quantified using measured acetone and xylene sensitivities, respectively. In case of mass spectrometrically not separable compounds, i.e. the C6H9O2 + isomers 3,5-dimethylfuran2(5H)-one (kSC = 3.05 10 -9 cm 3 s -1 ) and (E)-2-methyl-4-oxopent-2-enal (kSC = 1.96 10 -9 cm 3 s -