Photosensitization: A novel pathway to SOA generation and property change in tropospheric particles

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Tropospheric aerosol particles have often been described and represented in models in a simplistic way considering them as non-volatile and chemically inert. Such assumptions were recently been challenged by frontline research, according to which volatile organic compounds (VOCs) and secondary organic aerosols (SOA) form a system that evolves in the atmosphere by chemical and dynamical processing. A current key issue concern in the physico-chemistry of atmospheric organic particulate matter is that the models based on available parameterizations from laboratory studies strongly underestimate SOA and do not adequately account for particle growth as it is observed in the atmosphere. The difference between ambient and modeled SOA concentrations clearly suggests that other significant SOA sources have not yet been identified and characterized. Important efforts were consequently made to explain and close this gap. For instance, it was shown that gaseous glyoxal, which was previously considered as too volatile to noticeably partition into the particulate phase, could significantly contribute to SOA mass through multiphase chemistry. Glyoxal, and other small dicarbonyls are formed in large amounts during VOC oxidation. Condensed phase sinks for these gases are indeed able to explain an important part of the missing SOA mass in models, often addressed as aqSOA. However, observations imply that there are still large uncertainties about the tropospheric SOA formation - conventional aqSOA apparently cannot explain all missing SOA. Furthermore, multiphase processes have also been shown to produce light absorbing compounds in the particle phase. The formation of such light absorbing species could induce new photochemical processes within the aerosol particles and/or at the gas/particle interface. A significant body of literature on photo-induced charge or energy transfer in organic molecules from other fields of science exists. Such organic molecules are aromatics, substituted carbonyls and/or nitrogen containing compounds - all ubiquitous in tropospheric aerosols. Therefore, while aquatic photochemistry has recognized several of these processes that accelerate degradation of dissolved organic matter, only little is known about such processes in/on atmospheric particles. Therefore, within PHOTOSOA it is suggested to study photosensitization in the troposphere as it may play a significant role in SOA formation and ageing. Such photosensitization may introduce new chemical pathways so far unconsidered impacting both the atmospheric chemical composition and can thus contribute to close the current SOA underestimation. This project aims at tackling such issues by combining different laboratory based activities focusing on the chemistry of triplet state compounds of relevant photosensitizers, in various phases and their role in SOA processing. Clearly, frontline basic research studies on such processes are needed in order to be able to assess their importance.