Suspensions of particles in a gas, called aerosols, play a central role in the atmospheric processes involved in climate change. Some aerosols are emitted directly into the atmosphere; others form there through oxidation of gaseous precursors. Both natural processes and anthropogenic sources contribute to the aerosol burden. Climate change is driven by greenhouse gases that warm the air, and by aerosols and the cloud droplets that form on them. Depending on their compositions, the aerosols cool the planet by scattering incident sunlight back to space, or warm it when soot and other particles absorb solar radiation. While the impacts of greenhouse gases are readily quantified, aerosols remain the greatest source of uncertainty in the global radiation budget. Only by understanding the global distribution of aerosols can their impact be properly assessed. Unfortunately, data of the atmospheric aerosol remain sparse. The Flagan group works to increase the level of understanding through a combination of laboratory and field measurements, and by advancing the state of the art in aerosol measurements.