Molecular-Level Study of the Photo-Oxidation of Aqueous-Phase Guaiacyl Acetone in the Presence of 3C*: Formation of Brown Carbon Products

This study examined the effects of simulated solar irradiation on a model system of atmospheric aqueous secondary organic aerosol (aqSOA) formed from oxidation of guaiacyl acetone (GA) by reactions with triplet excited carbon (3C*) generated by photoexcitation of 3,4-dimethoxybenzaldehyde (DMB). Both GA and DMB are common components of biomass burning emissions, and their mixture was selected as a case study to investigate the influence of aqueous-phase photochemistry on the light absorption properties of atmospheric brown carbon (BrC) pertinent to aqSOA formation. Irradiation was performed in a photoreactor designed to mimic sunlight irradiation, and samples were collected at different times of the experiment, which lasted for 6 h. Chemical components of the samples were separated using high-performance liquid chromatography and analyzed with a photodiode array detector and Orbitrap mass spectrometer operated simultaneously to record both UV–visible spectra and high-resolution mass spectra. This allowed us to obtain molecular characterization of the aqSOA individual components, as well as information on their light-absorbing properties and how they change over time when irradiated. The results indicate that three generations of products formed at different stages of the experiment: monomeric products, dimeric products, and less polar aromatic products similar to those formed during oxygen-deprived pyrolytic processes. The monomeric and dimeric products result from oxidation reactions initiated by 3C*, while the less polar aromatic products form as a result of radical recombination. Prior to irradiation, the BrC absorbance is dominated by monomeric species, but it is later dominated by pyrolytic-like products as the monomeric and dimeric products begin to photodegrade. Comparison with previously defined BrC classes based on optical properties suggests that the aqSOA formed in this experiment initially fall outside of these classes but then become more absorbing, consistent with the "very weak" to "weak" BrC classes.