Measurements of Ash Particles in the Tropical Lower Stratosphere

We report here on measurements of relatively large (up to 5 microns maximum dimension) particles detected in the tropical western Pacific lower stratosphere during the NASA Airborne Tropical TRopopause EXperiment (ATTREX) Global Hawk flights in early March, 2014. The particles were observed at low relative humidities and contained no detectable ice mass. We conclude that these particles were silicate ash injected by the Mount Kelud eruption in Indonesia a few weeks prior to the ATTREX flights. The Kelud ash particles were detected by cloud probes on the Global Hawk between the lapse-rate tropopause (about 16.5 km) and the aircraft ceiling (18.5 km). The ash particles sampled in the lowermost stratosphere by the aircraft extended further north than the Kelud plume measured by the CALIPSO lidar at higher altitudes. The concentration of ash particles dropped off rapidly below the lapse rate tropopause, presumably because of rapid removal by ice nucleation on the ash particles followed by sedimentation. Although ash particles are very effective ice nuclei, the low particle concentrations detected in the upper troposphere precludes a significant impact on cirrus microphysical properties at the time of the ATTREX observations.