Single Organic Aerosol Particle HYdration and microsphysical properties – SOAPHY

Secondary Organic Aerosol (SOA) particles in the atmosphere are recognized to affect both climate change and human health. Biogenic SOA (BSOA), which constitutes 30 to 50 % of the global organic aerosol budget, may be present in various particle phase states. To date, the formation and evolution (i.e. the atmospheric aging during air mass transport) of BSOAs have been investigated by performing both field measurements and laboratory experiments, highlighting the complexity of related physico-chemical processes, due to the large diversity of their chemical makeup. Actually, interactions between water vapor and BSOA play key roles in air quality and climate change, requiring an accurate scrutinization. As aerosol particles may be considered as micro-reactors, a key bridge between individual process studies and the complexity of in situ atmospheric chemistry can be provided by lab-single particle investigations. Interactions between gases and particles may be confined to the surface region for particles in solid and semi-solid phases but may also occur in the bulk for particles in the liquid phase. In addition, the condensed water may serve as a reaction medium for multiphasic reactions. The condensed-phase water may be regarded as a plasticizer whose presence results in changes in the particle phase state which may directly impact the molecular diffusion both at the particle surface and in the bulk. Thus, the particle phase state, which is determined by the particle viscosity, has emerged as a research focus during the last decade in the atmospheric science community. So far, most studies on aerosol hygroscopicity, phase states and viscosity have been performed with a focus on laboratory experiments, which demonstrated that atmospheric particles can adopt not only a liquid phase state, but also semi-solid and even solid states, depending on their chemical composition (including inorganic/organic mixing) and on the atmospheric relative humidity.
The SOAPHY project aims at deepening our understanding of BSOA-water particle interactions by determining the main physical and/or chemical factors that drive and/or influence the water-particle interactions during atmospheric aging processes at the particle scale. In this project, the main scientific questions to be answered are: what is the influence of chemical transformations, including photochemical reactions, upon induced changes in the composition, chemical heterogeneities, molecular organisation, hygroscopicity behaviour, water diffusion and, ultimately, the particle viscosity at the particle (and/or the surface) level? Special attention will be paid to the role of the particle surface and the inter- and intra-molecular organisation on the viscosity properties of BSOAs. SOAPHY will focus on BSOAs formed from (photo)oxidation pathways of alpha-pinene, as its strong atmospheric representativeness makes it a relevant model system to study biogenic VOC atmospheric fate. Laboratory studies will use a panel of original and complementary on-line and in situ experimental set ups. Investigations on model BSOA particles in ambient conditions will be performed through experiments conducted on single particles either when deposited on substrates or in total levitation. Molecular scale BSOA-water processes will be investigated using low temperature matrix isolation experiments.
The outcomes of the SOAPHY project will lead, first, to the proposal of an original experimental device dedicated to individual gas-particle interaction investigations and, second, to novel multiscale concepts including physico-chemical markers related to BSOA-water interaction processes, which will provide key understanding of direct effect of aerosols on climate change and air quality.