Chemical and physical transformation of aerosol droplets in Raman tweezers

Abstract

Calcium carbonate (CaCO3) particles are one of the most abundant mineral aerosol totalling to approximately 8% of the total mass of mineral aerosols in the atmosphere, amounting to a loading of ~1.3 Tg. It is possible to produce CaCO3 via a “simple” ion exchange reaction between calcium chloride (CaCl2) and sodium carbonate (Na2CO3). This reaction seems simple, however as a mineral CaCO3 can also have multiple polymorphs, such as calcite, vaterite and a hydrated crystal ikaite (calcium carbonate hexahydrate, CaCO3∙6H2O). This reaction and its mechanism is important in both inorganic and biological chemistry, as CaCO3 is major component of bones and shells of living organisms, and the exact mechanism of formation and transformation of the different polymorphs is not known. In the atmosphere it can react with various reactive gases, such as NOx and SOx gases. This thesis can be split in to two major parts: a comparison of the hygroscopic responses of CaCl2, sodium chloride and a hydrocarbon (2-Hydroxypropyl)-βcyclodextrin using aerosol optical tweezing and dynamic vapour sorption, and the other focusing on the investigation of the formation and transformation of CaCO3 polymorphs in an optically tweezed aerosol droplet using Raman spectroscopy. In this thesis the formation and transformations CaCO3 is looked at from different perspectives: a theoretical model of the changes in the equilibrium size of the particle at a set relative humidity as a function of depletion of CaCl2 available for the reaction, and a spectroscopic point of view focusing on the evolution of the Raman spectrum of the droplet as the CaCO3 forms and transitions between polymorphs. The thesis also looks at potential ways of overcoming the issues encountered in the previously mentioned parts of the thesis, such as the use of Brownian motion of the droplet in a video recording for sizing.