Abstract. The presence of organic sulfur compounds (OS) at the water surface acting as organic surfactants, may influence the air-water interaction and contribute to new particle formation in the atmosphere. However, the impact of ubiquitous anthropogenic pollutant emissions, such as SO2 and polycyclic aromatic hydrocarbons (PAHs) on the formation of OS at the air-water interface still remains unknown. Here, we observe large amounts of OS formation in the presence of SO2, upon irradiation of aqueous solutions containing typical PAHs, such as pyrene (PYR), fluoranthene (FLA), and phenanthrene (PHE) as well as dimethylsulfoxide (DMSO). We observe rapid formation of several gaseous OSs from light-induced heterogeneous reactions of SO2 with either DMSO or a mixture of PAHs and DMSO (PAHs/DMSO), and some of these OSs (e.g. methanesulfonic acid) are well established secondary organic aerosol (SOA) precursors. A myriad of OSs and unsaturated compounds are produced and detected in the aqueous phase. The tentative reaction pathways are supported by theoretical calculations of the Gibbs energy of reactions. Our findings provide new insights into potential sources and formation pathways of OSs occurring at the water (sea, lake, river) surface, that should be considered in future model studies for a better representation of the air-water interaction and SOA formation processes.

The photochemical reactions of organic compounds at the water surface lead to the formation of gas‐phase molecules that may contribute to new particle formation in the atmosphere. Here, we observe the formation of organic sulfur (OS) compounds that are known secondary organic aerosol (SOA) precursors, upon irradiation of aqueous solutions containing typical polycyclic aromatic hydrocarbons (PAHs) such as pyrene, fluoranthene, and phenanthrene as well as dimethylsulfoxide (DMSO). The reactivity between the excited triplet states of PAHs (3PAHs*) and DMSO was determined by transient absorption spectroscopy and a tentative reaction mechanism for DMSO degradation was proposed, supported by theoretical calculations of the reaction Gibbs energies. In all cases, we observe rapid formation of methanesulfonic acid, methanesulfinic acid, methylsulfonylmethane, ethyl methanesulfonate, hydroxymethanesulfonic acid, and 2‐hydroxyethanesulfonic acid by use of novel membrane inlet‐single photon ionization‐time of flight mass spectrometry. These results suggest that ubiquitous PAHs and DMSO at the sea surface may represent an alternative source of OS compounds during daytime, through photochemical processes that should be considered in future models to better represent the SOA formation processes in the atmosphere.

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Section S1. Description of MI-SPI-TOF-MS Section S2. Description of FT-ICR-MS Section S3. Analysis of FT-ICR-MS aqueous phase products based on DBE vs carbon number iso-abundance plot Section S4. Reaction mechanism of the aqueous phase product compounds Section S5. Tables, Figures and Scheme Number of Pages: 47 Number of Tables: 6 Number of Figures: 16 Number of Schemes: 1 S3 C16H8-(8H,9H)-8,9-(cyclo-OS(=O)2O) -1314.61 C16H9-8-(OS(=O)2OH) -1314.66 C16H8-(1H,2H)-1,2-(cyclo-OS(=O)2O) -1314.62 S11Table S3: Calculated Gibbs free energies for all molecules employed in the derivation of aqueous-phase reaction Gibbs energies in transformation process initiated by 3 PHE* and 3 PYR*, arranged in the order of increasing molecular mass.Chemical Formula or Name Gm Θ /Hartree Phenathrene -539.34 ( 3 Σ)Phenathrene -539.26 Pyrene -615.56 ( 3 Σ)Pyrene -615.48 C14H8-(2H,3H)-2,3-(cyclo-OS(=O)O) -1163.17 C14H8-(9H,10H)-9,10-(cyclo-OS(=O)O) -1163.20 E-C10H6-1-(CH=CHCH=CH2)-2-(OS(=O)OH) -1164.35 C12H8-2-(CH=CH2)-2'-OS(=O)OH -1164.36 C14H8-(2H,3H)-2,3-(cyclo-OS(=O)2O) -1238.40 C14H9-3-(OS(=O)2OH) -1238.46 C14H8-(9H,10H)-9,10-(cyclo-OS(=O)2O) -1238.44 C14H9-9-(OS(=O)2OH) -1238.46 E-C10H6-1-(CH=CHCH=CH2)-2-(OS(=O)2OH) -1239.59 C12H8-2-(CH=CH2)-2'-OS(=O)2OH -1239.60 C16H8-(2H,3H)-2,3-(cyclo-OS(=O)O) -1239.38 C16H8-(4H,5H)-4,5-(cyclo-OS(=O)O) -1239.43 E-C13H7-1H-1-(=CHCH=CH2)-9-(OS(=O)OH) -1240.56 C14H8-4-(OS(=O)OH)-5-(CH=CH2) -1240.56 C14H8-2-OH-3-(OS(=O)2OH) -1313.69 C14H8-9-OH-10-(OS(=O)2OH) -1313.68 E-C10H6-1-(CH=CHCH2CHO)-2-(OS(=O)2OH) -1314.83 C12H8-2-(CH2CHO)-2'-OS(=O)2OH -1314.84 C16H8-(2H,3H)-2,3-(cyclo-OS(=O)2O) -1314.61 C16H9-2-(OS(=O)2OH) -1314.66 C16H8-(4H,5H)-4,5-(cyclo-OS(=O)2O) -1314.66 C16H9-4-(OS(=O)2OH) -1314.68 E-C13H7-1H-1-(=CHCH=CH2)-9-(OS(=O)2OH) -1315.79 C14H8-4-(OS(=O)2OH)-5-(CH=CH2) -1315.80 C16H8-1-OH-2-(OS(=O)2OH) -1389.91 C16H8-2-(OS(=O)2OH)-5-OH -1389.90