Though it has been widely recognized that the aqueous-phase chemistry of carbonyl compounds in the atmosphere (e.g., cloud droplets, fog, aerosols) is dominated by photochemical processes driven by hydroxyl radical (OH), increasing evidence suggests that photosensitized chemistry also have a significant impact on the transformation of carbonyl compounds, especially into secondary organic aerosols (SOA) through photo-induced generation of high molecular weight compounds (oligomers). However, these oligomers are mostly unexplored due to the demanding challenge for analyzing the broad and complex intermediates/products, not to mention the formation mechanism of SOA.  In this study, the photosensitized reaction of carbonyl compounds was investigated at the molecular level by means of two state-of-the-art, ultrahigh-resolution mass spectrometric instruments: Orbitrap Fusion Trihybridmass Spectrometer (ORBITRAP) and Ultrahigh-Resolution Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (FT-ICR-MS). Glyoxal (GL), a typical carbonyl compound and pyruvic acid (PA), an extensively studied photosensitizer, were selected as probe compounds.  An unexpected large number of highly oxidized multifunctional organic compounds (oligomers) were unambiguously identified, which illustrates the progression from C3 to C20 molecules. The possible transformation mechanism investigated by quantum-mechanical calculations using density functional theory (DFT) by the Gaussian 03 suite of programs, suggests that the oligomers are thermodynamically more likely from the cross-reactions between PA and GL rather than from the self-reactions of PA. We are confident that the better understanding in the formation mechanism of GL into these oligomeric products, as shown here, will provide a new insight into the change in the physical-chemical properties of the atmospheric aqueous phase and also the sources of SOA.

Oligomers formation from cross-reactions of carbonyl compounds in the atmosphere: Insight at a molecular level

2019 Fall National Meeting

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