Abstract: The physicochemical properties of aerosols and droplets, including phase state, pH and viscosity, impact processes from respiratory virus transmission to critical atmospheric processes to recent reports of incredible accelerations in chemical reactivity in microdroplets. Yet, these properties are challenging to measure directly in situ due to the small sizes and sensitive nature of the particles. Here, the use of fluorescence probe spectroscopy as a versatile tool for measuring these critical properties in situ in submicron aerosols is presented, along with a discussion of how the measured properties differ or not from those of larger particles or bulk phases, with implications for each of the aforementioned application areas.