Development of a Dual-Beam Optical Trap for Monitoring Water Uptake and Activation of Single Aerosol Particles

The microphysical properties of natural and anthropogenic aerosols play a crucial role in cloud formation, particularly in water uptake and droplet activation. Köhler's theory provides a framework for predicting the critical supersaturation at which a droplet activates, combining the effects of solute-induced water vapour reduction and surface curvature. While effective for inorganic compounds, the theory inaccurately predicts water uptake in droplets containing organic species. Models incorporating surfactant effects offer potential improvements but require robust experimental data for validation. At the same time, conventional ensemble measurements average over droplet size and compositional heterogeneities, obscuring critical single-particle behaviours.

To address these limitations, we present a dual-beam optical trap for studying droplet activation in single aerosol particles. The setup uses counter-propagating laser beams to stably trap individual particles, enabling precise size and refractive index measurements via Cavity-Enhanced Raman Spectroscopy. A specially designed cell, featuring cooling and heating sections, establishes controlled temperature and relative humidity/supersaturation gradients, enabling the investigation of droplet growth under defined conditions. Additionally, the setup is equipped with a high-speed camera to monitor the activation and subsequent growth of droplets, allowing real-time visualization of growth dynamics. By systematically isolating individual particles and monitoring their behaviour, this technique avoids the averaging effects inherent to ensemble methods, providing high-resolution data critical for validating and refining models of organic aerosol activation.