The overall objective of the BG experiment is to better understand the processes involved in maintaining the stratospheric background aerosol layer, i.e. stratospheric aerosol not resulting from direct volcanic injections into the stratosphere. The simulations prescribed for this experiment are time-slice simulations for the year 2000 with prescribed SST including all sources of aerosols and aerosol-precursors except for explosive volcanic eruptions. The result of BG will be a multi-model climatology of aerosol distribution, composition, and microphysical properties in absence of volcanic eruptions. By comparing models with different aerosol microphysics parameterization and simulations of background circulation with a variety of observational data, we aim to assess how these processes impact the simulated aerosol characteristics.