Chemical processes in clouds have been suggested to contribute substantially to organic aerosol particle mass since a long time. Recent evidence from the HCCT-2010 field study and the CUMULUS chamber study suggest that this organic mass production can be substantial and depends on the concentration of available organic precursor compounds in the gas phase. However, considerable uncertainties exist, e.g. with regards to the nature of the resulting aerosol particles which might be metastable and loose at least part of their OM content during the cloud droplet evaporation. Hence, PARAMOUNT is aimed at the investigation of cloud processes which are able to process organic constituents and produce organic aerosol particle mass. The project will focus on the multiphase chemistry of very relevant polyfunctional precursors such as polyfunctional carbonyls and acids. With these precursors, a combination of aqueous-phase laboratory and CESAM chamber studies will be undertaken to examine the multiphase cloud processing. The planned aqueous-phase laboratory studies will lay the groundwork with regard to kinetics and mechanism of the multiphase processing of the mentioned compounds. The suggested CESAM chamber experiments, which are central in PARAMOUNT will mainly focus on studying the organic mass production by the chemical in-cloud processing of these compounds one by one, grouped or with mixtures with all of them. The planned chamber studies will use different seeds and oxidant precursors to examine the organic mass production under different environmental and diurnal conditions. The organic mass increases during the cloud episodes will be investigated. Besides the organic mass formation, the partitioning of organic compounds under cloud conditions should be studied to evaluate possible enrichments of organic carbonyl compounds observed during the cloud field campaign HCCT-2010. The organic aerosol fraction will be analysed on-line by two Aerosol Mass Spectrometer instruments and the processing of the interstitial gas phase and the phase partitioning will be investigated by PTR-MS and the use of a mini CVI (counter virtual impactor) followed by offline analysis. Finally, the performed CESAM experiments are being modelled with the complex multiphase chemistry mechanism MCM/CAPRAM. The multiphase modelling will be performed to both validate the mechanism and support the interpretation of the chamber experiments. Overall, the proposed project PARAMOUNT will be a scientific breakthrough for understanding of in-cloud processes clarifying the role of clouds for atmospheric organic aerosol mass production.

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.