Organic aerosols play a major role in the chemistry and physics of the atmosphere, serving as cloud condensation or ice nuclei, sites of interfacial chemistry, and scatters and absorbers of incident radiation. Recent field campaigns have affirmed their prevalence in the troposphere, and aircraft measurements have discovered large quantities of organic material within the nuclei of ice crystals in mid-latitude mixed-phase clouds. Despite their ubiquity and importance in the environment, little is known about organic aerosols, in particular, their phase state and influence over the partitioning of gas-phase compounds, rate of chemical reactions, water uptake and particle ability to act as CCN or IN. To elucidate the role of organic aerosols, I propose experimental studies on laboratory-generated organic aerosols by investigating their phase behavior under well-controlled, atmospherically-relevant temperature and relative humidity. For single-particle resolution, I will develop and employ a counter-propagating Bessel beams optical trap (BBOT), equipped with a positional sensor, a cold head (capable of lowering temperatures quickly down to 70 K) and RH control, the first of its kind. The laboratory-based instrument will be developed in the group of Prof. Ruth Signorell at ETH Zürich, Department of Chemistry, where the research will be performed. By optically trapping single aerosols and controlling the ambient relative humidity and temperature, the phase behavior of organic aerosols can be characterized, giving insight into their influence on cloud formation, precipitation, and overall climate.