Aerosol particles in the stratosphere affect Earth's temperature by scattering solar radiation back into space and facilitating heterogeneous chemistry that alters ozone abundance. These particles, composed mostly of sulfate, have long been measured by satellite and balloon-borne sensors, which have limited sensitivity for particles smaller than 0.15 um, resulting in a bias towards larger particles. While some evidence for particles smaller than 0.1 um in the stratosphere has been published, these observations are mostly limited to altitudes below 12 km and ozone levels lower than 500 ppbv-at the very bottom of the stratosphere. Efforts to estimate the abundance and characteristics of these particles deeper into the stratosphere have been limited to poorly constrained extrapolations (fits) from measurements of larger particles. As a result, the size distribution of smaller particles in the stratosphere, although crucial for understanding how both current emissions and proposed future geoengineering efforts would affect Earth's radiation budget, remains poorly quantified. Here, we present our findings on the particle size distribution and composition of aerosols in the stratosphere, ranging from 3 nm to 2.4 um at altitudes of up to 19 km, and ozone levels of 3000 ppbv. Our measurements reveal the prevalence of particles with diameters < 0.1 um in the stratosphere. Specifically, we have observed a mode of small particles, rich in organics and originating from the troposphere, in the lowermost stratosphere extending toward the middle stratosphere. These particles, along with larger background stratospheric sulfate particles, form a bimodal aerosol size distribution that is not successfully replicated by a state-of-the-art global model. Accurately representing the size and origin of these particles in global models is crucial to both understand the current stratosphere and provide robust predictions of future stratospheric conditions, both with and without geoengineering efforts.