Stratospheric aerosol, a persistent yet highly variable component of the stratosphere, has impacts on radiation and stratospheric ozone, and thus climate. Since the 1960s the study of stratospheric aerosol has been substantially influenced by the large volcanic eruptions of: Agung, Fuego, El Chichón, and Pinatubo. This paradigm of decadal major volcanic eruptions changed after Pinatubo in 1991. Since then, no volcanic eruptions have had similar magnitudes and in most cases the stratospheric signature was lost within one year. The period following Pinatubo to the present overlaps with a golden age for stratospheric aerosol and gas phase sulfur measurements from satellites, aircraft, balloons, and the ground, representing an optimum period for testing climate models that use prescribed stratospheric aerosol and gas phase sulfur, and those that use surface emissions and calculate stratospheric aerosol and gas phase sulfur. Historically such a period of quiescent to mild volcanism is perhaps more common than the active volcanism of the 1960s through the 1990s, and is more suited to study troposphere-to-stratosphere transport, background stratospheric chemistry, trends in stratospheric sulfate aerosol, and the sources and potential climate impact of non-sulfate stratospheric aerosol. This Chapman Conference will be focused on addressing the following scientific questions that have grown out of this period of observations.

1)   What sources (volcanic, natural non-volcanic, and anthropogenic) and processes have controlled stratospheric aerosol levels since the decay of the Mt. Pinatubo aerosol?

2)   How do the observed variations in stratospheric aerosol, over the recent quiescent and mild volcanic period, impact climate and the composition of the stratosphere?

3)   How well do we understand the sulfur budget over the “golden period” of stratospheric observations, in the post-Pinatubo era?

4)   What is the role of non-sulfate aerosol in the stratosphere and how much does it change the current sulfate-dominated picture of stratospheric aerosol?

5)   How well do the global aerosol climatologies, derived primarily from satellite measurements, capture the climate relevant quantities during the quiescent to mild volcanic period?

6)  How well do models represent the stratospheric aerosol over quiescent to mild volcanic periods?