The Aerosol Physics Lab (APL) is primarily concerned with measurement and characterization of the particles upon which cloud droplets condense. These cloud condensation nuclei (CCN) are ubiquitous in the atmosphere, but they display considerable spatial and temporal variability with respect to concentration, composition, and origin. Anthropogenic CCN are responsible for the largest climate uncertainty, the indirect aerosol effect (IAE). This uncertainty stems from inadequate knowledge of natural CCN sources and concentrations, which would allow quantification of the alterations of clouds by anthropogenic CCN, which thus produce IAE. The major focus of APL has been deployment of the DRI CCN spectrometers in aircraft field projects that investigate IAE. The supersaturation range of the DRI instruments is much greater than that of other CCN instruments and unlike other CCN instruments the entire supersaturation spectrum is obtained simultaneously within time periods as short as 1 second. This is especially vital foraircraft deployments. APL is also equipped with condensation nuclei (CN) counters and differential mobility analysers (DMA) to make direct particle size distribution measurements and calibrate the CCN spectrometers. These instruments have also been deployed in the aircraft projects.