Aerosols represent one of the largest uncertainties in current understanding of the Earth’s radiative balance.  Direct perturbations of the radiation balance result from aerosol scattering and absorption of solar and longwave radiation.  Aerosols also indirectly modify the radiation balance by altering cloud microphysical properties.  Particles act as cloud condensation nuclei reducing cloud droplet size and increasing cloud albedo.  Reducing cloud droplet size can also suppress drizzle formation and extend cloud lifetime.

Aerosols have a wide range of sources, with atmospheric lifetimes that typically range between hours to days, reflecting aerosol size and composition. This consequently leads to rapid spatial and temporal variations in the loading and composition of atmospheric aerosols.  These rapid variations combined with the difficulty in scaling up the microphysical processes governing aerosol forcing into global models limits our ability to quantify aerosol radiative forcing.