Stratospheric Aerosol Plumes

Abstract 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 aerosol 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 2005–2022 average tropical transport velocity is ∼0.2–0.3 mm/s at 20 km increasing to ∼0.5 mm/s at 26 km. The aerosol plumes rise at a slower rate. The difference between the water vapor‐based transport velocity and the aerosol plume rise rate 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 a simple estimate of the volcanic plume rise, we estimate the aerosol settling velocity. From the settling velocity, we compute median aerosol radius for both Ambae and Ulawun to be ∼0.2 ± 0.08 μm. Using multi‐wavelength extinction ratio methods, we estimate the plume median radius to be ∼0.15 ± 0.01 μm which is consistent with other estimates of the Ambae and Ulawun aerosol median radii. Within their uncertainty, the settling and extinction ratio size estimates are in agreement.