Aerosol particles in the atmosphere affect the Earth’s climate, atmospheric chemistry, and human health. Characterizing the chemical and physical properties of tropospheric aerosols is important for understanding their role in the atmosphere. This research involves applying new, rapid, in-situ, on-line instrumentation to measure the chemical composition and size of atmospheric particles. Size-resolved chemical composition measurements are obtained with an Aerodyne Aerosol Mass Spectrometer. This research also involves developing and implementing other innovative methods for in-situ on-line aerosol composition and size distribution measurements on rapid time scales. Airborne and ground-based field studies with these aerosol and other gas phase measurements will be used to assess the processes involved in the production and growth of aerosols in various regions of the atmosphere.