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(3) Paul Scherrer Institute environmental chamber under various NOx 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 similar to 200 ion signals during the TMB photo-oxidation experiments. Molecular formulas CmHnNoOp 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 (OS) over bar (C) 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 (OS) over bar (SOA)(C) were in the range of -0.34 to -0.31, in agreement with expected average carbon oxidation states of fresh SOA ((OS) over bar (C) = -0.5-0).