Aerosols play a key role in atmospheric science but remain the most poorly quantified factor in predicting anthropogenic radiative forcing. The interaction of aerosol with water vapour regulates the lifetime and reflectivity of clouds, while coagulation governs aerosol removal and a better ' understanding of these processes is crucial. In particular, the ubiquity of organic aerosols, their characteristic phase-behaviour and surface chemistry, provides a fertile area for study and cannot be reproduced in bulk-phases. This thesis reports single-particle studies of aerosol using holographic optical tweezers for the trapping and manipulation of micron-sized aerosol particles. Novel microscopic and spectroscopic techniques were developed and used in tandem for their characterisation. A study of optical trapping forces and their interaction with micro particles was carried out. The influence of a range of experimental parameters and their influence on trapping geometry were explored through particle position tracking, allowing the complex interplay between thermal, optical and inter-particle forces to be studied. Notably, particle size-resolved measurements of Brownian dynamics have experimentally demonstrated resonant behaviour in trapping forces. Studies of aerosol coalescence in optical tweezers have allowed the hydrodynamics of single aerosol particles to be explored. In this regard, the underdamped relaxation of 'inviscid particles was observed from the elastically-scattered light signature. Correspondingly, the frequency of damped surface oscillations was used to probe aerosol surface tension, of key importance in regulating cloud droplet number concentration. This was extended to the study of viscous particles, critical in understanding the kinetics of cloud droplet activation. Overdamped relaxation in morphology was observed, spanning 12 orders of magnitude in time scale and viscosity in amorphous aerosol. Furthermore, the relationship between bulk-diffusivity and viscosity was shown to decouple from the classical theories of Stokes and Einstein, highlighting the microscopic and macroscopic factors governing the phase-behaviour of metastable states approaching a glass-transition.