The Stratospheric-Troposphere Exchange Project (STEP) has two objectives: A. Investigate the mechanism and rates of irreversible transfer of mass, trace gases, and aerosols from troposphere to stratosphere and within the lower stratosphere. B. Explain the observed extreme dryness of the stratosphere The first objective derives from the need for a better description of how natural and manmade chemicals move from their tropospheric sources to the stratospheric ozone shield.
