Stratospheric Aerosol Plumes

Aerosol eruption plumes in the tropical lower stratosphere appear to rise whereas extra-tropical plumes do not. We show that this occurs because the Brewer-Dobson transport circulation is upward in the tropics, and the circulation lifts the tropical plumes. We compute the tropical upward transport circulation from periodic rising tropical water vapor anomalies. Using the Stratospheric Water and OzOne Satellite Homogenized water-vapor database, we estimate that the 2005-2020 average tropical transport velocity is ~0.3 mm/s at 21 km increasing to ~0.5 mm/s at 26 km. The aerosol plumes rise slower than this rate. The difference between the transport velocity and slower observed rate which aerosol plumes move upward is likely the result of aerosol gravitational settling. To test this hypothesis, we focus on two tropical volcanic eruption plumes, Ambae (mid-2018) and Ulawun (mid-2019). Using multi-wavelength extinction ratio methods, we compute the plume particle sizes to be ~0.15 µm. Using the observed volcanic plume rise and the transport circulation computed from water vapor, we compute the aerosol settling velocity. From the settling velocity we can use Stokes-Cunningham equations to estimate the average aerosol radius. For Ambae the radius is 0.26µm, and for Ulawun it is 0.17µm. Within the uncertainty of the calculation, these sizes are consistent with size estimates using extinction ratio-based retrievals. Key Points • Tropical stratospheric aerosol plumes are carried upward by the transport circulation • We compute the upward transport velocity using water vapor observations • From the observed volcanic plume rise rate we compute aerosol particle radii for two eruptions.