Professor Flagan focuses on aerosols, and includes studies of secondary organic aerosols in the atmosphere, of biological particles such as pollen and their health impacts, and of the formation of particles and clouds in the atmosphere of Titan. At the center of his work is the development of methods for the physical, chemical, and biochemical characterization of aerosol particles ranging from particles as small a 1 nm diameter to pollen grains that can exceed 100 µm in size. He also applies methods derived from aerosol science to the study of phase transitions in materials, and the development of separations technologies.