Aerosol-cloud interactions are among the largest sources of uncertainty in understanding climate change and predictions of the future climate. Besides the physical dimensions of aerosol particles, their water-solubility and surface tension are the key properties needed to describe their potential to act as cloud condensation nuclei (CCN). This thesis aims to improve the understanding of the role of atmospherically relevant aerosol mixtures in cloud formation by 1) cloud activation measurements of aerosol particles containing both water-soluble and insoluble components; 2) molecular-level measurements of the surface properties of aqueous solutions for atmospherically relevant organic compounds.
In the first part of our study the cloud activation of particles containing ammonium sulfate, sucrose or BSA (bovine serum albumin), and their mixtures with insoluble silica was investigated through laboratory experiments. The CCN activity of the particles was governed by the fraction of soluble material in them. If the soluble fraction could be reliably estimated, the CCN activation of these particles could be successfully predicted with existing theoretical frameworks. Our results showed an uneven distribution of soluble material on the insoluble silica cores in our wet-generated particles. This highlights the need for controlled particle generation and coating techniques.
In the second part of this work, the microscopic surface structure of aqueous succinic acid (an atmospherically relevant organic compound) solutions were determined and connected to the macroscopic surface tension of the solution. The molecular data derived from the combination of X-ray Photoelectron Spectroscopy (XPS) experiments and Molecular Dynamics (MD) simulations allowed for derivation of surface enrichment factors for succinic acid in aqueous solutions. These enrichment factors were used for direct estimations of the surface tensions of these mixtures, which showed a good agreement with experimental data. This study thus confirmed the connection between microscopic surface structure and macroscopic surface tension.
Despite our effort to investigate the aerosol properties controlling CCN activation and cloud formation, there is still much to learn about cloud activation of complex atmospheric aerosol particles. In this regard, further examining the CCN activation of other insoluble materials like soot or dust, developing controlled coating techniques, investigating the molecular properties of particles consisting organic and inorganic compounds, as well as studying the ice nuclei activity of mixed particles are of particular importance.
Stockholm University, 2014. , 66 p.