Analysis of high mass resolution PTR-TOF mass spectra from 1,3,5-trimethylbenzene (TMB) environmental chamber experiments

A series of 1,3,5-trimethylbenzene (TMB) photo-oxidation experiments was performed in the 27-m<sup>3</sup> Paul Scherrer Institute environmental chamber under various NO<sub>x</sub> conditions. A University of Innsbruck prototype high resolution Proton Transfer Reaction Time-of-Flight Mass Spectrometer (PTR-TOF) was used for measurements of gas and particulate phase organics. The gas phase mass spectrum displayed ~200 ion signals during the TMB photo-oxidation experiments. Molecular formulas C<sub>m</sub>H<sub>n</sub>N<sub>o</sub>O<sub>p</sub> were determined and ion signals were separated and grouped according to their C, O and N numbers. This allowed to determine the time evolution of the O:C ratio and of the average carbon oxidation state <span style="border-top: 1px solid #000; color: #000;">OS</span><sub>C</sub> of the reaction mixture. Both quantities were compared with master chemical mechanism (MCMv3.1) simulations. The O:C ratio in the particle phase was about twice the O:C ratio in the gas phase. Average carbon oxidation states of secondary organic aerosol (SOA) samples <span style="border-top: 1px solid #000; color: #000;">OS</span><sub>C</sub><sup>SOA</sup> were in the range of −0.34 to −0.31, in agreement with expected average carbon oxidation states of fresh SOA (<span style="border-top: 1px solid #000; color: #000;">OS</span><sub>C</sub> = −0.5–0)