Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous at the sea surface. Dimethyl sulfoxide (DMSO) represents the most abundant organic sulfur compound in the oceans. Decades of research on the photodegradation of polycyclic aromatic hydrocarbons (PAHs) have provided fundamental and practical insights into these processes; however, the structure of many organic compounds formed during the photodegradation and the reaction pathways leading to their formation still remain unclear. This study provides an experimental evidence that PAHS, such as fluorene (FL) dissolved in DMSO, when exposed to sunlight, can initiate photosensitized chemistry at the air-water interface into highly oxygenated and toxic unsaturated multifunctional compounds which are known precursors of secondary organic aerosols (SOA). The photosensitized properties of excited triplet state (3FL*) and its reactivity have been investigated using transient spectroscopy with a laser flash photolysis. From the reaction between DMSO and 3FL*, the formation of organosulfur compounds (OS) was observed in both the aqueous phase and gas phase. Some of the organosulfur compounds identified here exhibit the same molecular composition with the organosulfur compounds detected in ambient aerosols, highlighting the importance of the photosensitized reactions of PAHs at the water surface.
We determine the generated liquid-phase and gas phase products by means of two state-of-the-art, ultrahigh-resolution mass spectrometric instruments: Ultrahigh-Resolution Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (FT-ICR-MS) and Ultrahigh-Resolution Hybrid Quadrupole Orbitrap Mass Spectrometer which was interfaced with a secondary electrospray ionization source (SESI-HRMS).
From the detected products in the gas phase and in the aqueous phase we propose a detailed reaction pathways for the photosensitized degradation of FL. The experimental results are supported by quantum-mechanical calculations using density functional theory (DFT) by the Gaussian 03 suite of programs.
The photosensitized chemistry suggested in this study may exert a very large impact, especially if in general the mere presence of a surface layer of PAHs can trigger this interfacial photosensitized chemistry at sea surface, cloud droplet, and the surface of aerosol deliquescent particles. Because PAHs-covered water surfaces are ubiquitous, the formation of unsaturated multifunctional compounds and OS compounds might also influence the formation of SOA in the atmosphere, which in turn can better explain field observations of new particle formation events which are occurring independently of the primary emitted particles from marine biological activity.
The van Krevelen graph for CcHhOo group of compounds, formed upon 4 hr of light irradiation of FL/DMSO and detected in ESI− mode. The recognized repeating mass building increments are labeled in the legend. All other molecules which do not participate in any building block increments are depicted in black color. A and B areas depict oxygenated aliphatic compounds and low‐oxygenated aromatic hydrocarbons, respectively. The aromaticity equivalent (black with Xc < 2.5, red with 2.5 ≤ Xc < 2.7, and green with Xc ≥ 2.7) is illustrated by the color bar, while the pie chart demonstrates the percentage value of each color‐coded compound in the sample.
Detailed reaction mechanism describing the formation of gas phase products initiated by 3FL*. Numbers in brackets, written below each molecule, present compound designations to follow the discussion better with the Scheme 2. All identified products are framed into red rectangles
The result was published in JGR-Atmosphere, recently.
Majda Mekic Jiafa Zeng Bin Jiang Xue Li Yannis G. Lazarou Marcello Brigante Hartmut Herrmann Sasho Gligorovski. Formation of Toxic Unsaturated Multifunctional and Organosulfur Compounds From the Photosensitized Processing of Fluorene and DMSO at the Air‐Water Interface. Journal of Geophysical Research: Atmospheres, 125, e2019JD031839.