Technical note: Characterization of a single-beam gradient force aerosol optical tweezer for droplet trapping, phase transitions monitoring, and morphology studies

Abstract. Single particle analysis is essential for a better understanding of the particle transformation process and predicting its environmental impact. In this study, we developed an aerosol optical tweezer (AOT)-Raman spectroscopy system to investigate the phase state and morphology of suspended aerosol droplets in real time. The system comprises four modules: optical trapping, reaction, illumination and imaging, as well as detection. The optical trapping module utilizes a 532 nm laser and a 100x oil immersion objective to stably trap aerosol droplets within 30 seconds. The reaction module allows us to adjust relative humidity (RH) and introduce reaction gases into the droplet levitation chamber, facilitating experiments to study liquid-liquid phase transitions. The illumination and imaging module employs a high-speed camera to monitor the trapped droplets, while the detector module records Raman scattering light. We trapped sodium chloride (NaCl) and 3-methyl glutaric acid (3-MGA) mixed droplets to examine RH-dependent morphology changes. Liquid-liquid phase separation (LLPS) occurred when RH was decreased. Additionally, we introduced ozone and limonene/α-pinene to generate secondary organic aerosol (SOA) particles in situ, which collided with the trapped droplet and dissolve in it. To determine the trapped droplet’s characteristics, we utilized an open-source program which based on Mie theory to retrieve diameter and refractive index from the observed whispering gallery modes (WGMs) in Raman spectra. It is found that mixed droplets formed core-shell morphology when RH was decreased, and the RH dependence of the droplets phase transitions generated by different SOA precursors varied. Our AOT system serves as an essential experimental platform for in-situ assessment of morphology and phase state during dynamic atmospheric processes.