Aerosol-cloud interactions are one of the main sources of uncertainties in modeling and predicting the Earth’s climate. To overcome this uncertainty, we need to improve the understanding about the processes and parameters defining how aerosol particles turn into cloud condensation nuclei (CCN) or ice nuclei (IN) to produce cloud droplets or ice crystals. The focus of this dissertation is on liquid phase cloud droplets. The thesis investigates the effect of water solubility and surface tension on the CCN activity of atmospheric aerosol particles. These parameters are among the key properties defining how an aerosol particle can turn into a cloud droplet. The main goals of this thesis are to investigate 1) the CCN activity of aerosol particles containing both water soluble and insoluble substances and 2) the contribution of molecular-scale surface structure to the surface tension and CCN activity of atmospherically relevant aqueous mixtures.

In the first part of this thesis, the CCN activity of water-insoluble aerosol constituents coated by water-soluble or sparingly soluble species was investigated. The results showed that the CCN activity of the insoluble silica and black carbon particles, with sizes between 100 and 300 nm, increased with the amount of the coating on the insoluble cores and at thick enough coating approached the CCN activity of the soluble species. Moreover, controlled dry coating of the insoluble BC cores yielded a size-independent distribution of the coating material on the insoluble cores, which was not achieved by wet coating of the silica particles. The results also confirmed that by knowing the fraction of soluble material (coating thicknesses), the existing theories gave a reasonable estimate of the CCN activity for the mixed soluble-insoluble particles. Finally, the results highlight the need for including the impacts of co-emitted or later condensed compounds in estimates of the climate impacts of atmospheric insoluble aerosol species.

In the second part of the thesis, surface propensity of succinic acid, pure or mixed with soluble inorganic salts in the aqueous droplets, were quantified via molecular-level surface composition measurement by X-ray Photoelectron Spectroscopy (XPS). The XPS and molecular dynamic (MD) simulations of succinic acid aqueous solutions showed strong enrichment of the succinic acid at the surface of the liquid droplets compared to the bulk solution. This effect was more pronounced in the presence of the highly soluble inorganic salts like NaCl and (NH₄)₂SO₄ in the system. The modeled surface tension of the pure organic or mixture of organic and inorganic substances, using surface enrichment factors derived from the XPS experiments were in good agreement with the experimental surface tension data. This demonstrates the high potential of XPS for direct measurements of the surface composition of atmospherically relevant aqueous mixtures. The results suggest that for modeling the phase-state and water content of the atmospheric particles, the contribution by the surface layer needs to be considered, because aqueous droplet can contain larger amounts of organic compounds than the bulk solubility limit of the solutions. However, the effect of the aqueous surface composition on the CCN activation of particles consisting of the studied mixtures was estimated to be very small.

The results presented in this thesis provide new insights into the relationship between aerosol particle composition and cloud condensation nuclei activity. However, the effect of more realistic complex mixtures will require more research. The results showed that for modeling semi-volatile species, the partitioning between the gas and condensed phase needs to be considered. In addition, along with the liquid-phase cloud activation, the ice nucleation ability of the particles made of soluble and insoluble species requires to be further investigated.

Atmospheric aerosols particles may act as cloud condensation nuclei (CCN) that provide sites for condensation of water vapour for the formation of cloud droplets, called cloud droplet activation. Whether aerosol particles are CCN is determined by their size, composition and the ambient humidity. Cloud macrophysical properties together with the size and number concentration of droplets determine the optical properties of liquid phase clouds. Clouds are an important component in the Earth's radiation balance and aerosol-cloud interactions (ACI) are associated with the largest uncertainty in estimates made of anthropogenic radiative forcing in earth system models.

To constrain ACI and reduce uncertainties, an improvement in our understanding of CCN activation is required. Owing to its complex phase structure and chemical heterogeneity, the organic fraction of atmospheric aerosol introduces significant challenges in developing an exact description of cloud formation. In this thesis, a cloud parcel model is employed to systematically address parametric and process uncertainties in estimates of cloud droplet sizes and number concentrations (CDNC). To do so, the unified framework for organic aerosol (UFO) scheme was developed and embedded into the cloud parcel model, ICPM-UFO. The ICPM-UFO simulates partitioning of organic mass between the gas and aqueous bulk and surface phases, thereby providing means to theoretically diagnose changes in droplet nucleating potential of aerosol particles due to organic aerosol mass transfer processes.

Partitioning of surface active organic aerosol mass from the bulk particle phase to the surface phase results in a lowered, size-dependent surface tension that enhances activation potential of CCN and therefore simulated CDNC. A large fraction of organic aerosol constituents exist partitioned across particle and gas phases and simulation of cloud formation events show this semi-volatile organic mass to condense to the particle phase as humidity increases through the cloud base. This additional particle phase mass may be partially soluble. The more soluble component increases the activation potential by lowering the water activity, while the less soluble but more surface active component also increases the activation potential by further lowering of the surface tension. The compounding effects of the gas-particle and bulk-surface partitioning processes result in significant changes in CCN concentrations and CDNC for simulation on boreal aerosol. These results exhibit a significant over prediction of typical boreal CCN concentrations relative to in-situ measurements, though further sensitivity analysis with respect to the soluble fraction and surface phase description may be advantageous. Based on multivariate statistical approaches applied, resolution of the surface phase in cloud formation parameterisations within climate models is however not currently recommended.

Theoretical description of both partitioning processes require prescription of input parameters that are challenging to measure in-situ. These parameters include: SVOC volatility and enthalpy of vaporisation and organic component surface tension and film thickness. Further work using the inverse modelling framework established herein is recommended to provide estimation of these parameters while simultaneously matching simulated CDNC and/or CCN concentrations with observational data. It is envisaged that such an investigation will also yield insights into structural uncertainties associated with the choice of surface phase model - a point of contention both within this thesis and the wider literature.