Ash Particles Detected in the Tropical Lower Stratosphere

We report here on measurements of relatively large (up to ≃5-μm maximum dimension) particles detected in the tropical western Pacific lower stratosphere. The particles were observed at low relative humidities and contain no detectable ice mass. We conclude that these particles are silicate ash injected by the Mount Kelud eruption in Indonesia a few weeks prior to the aircraft observations. The Kelud ash particles were detected by cloud probes on the National Aeronautics and Space Administration Global Hawk between the lapse rate tropopause (≃16.5 km) and the aircraft ceiling (18.5 km). The ash particles detected in the lowermost stratosphere by the aircraft extended well into the northern subtropics. The concentration of ash particles dropped off rapidly below the lapse rate tropopause, presumably because of rapid removal by ice nucleation scavenging. Although ash particles are very effective ice nuclei, the low particle concentrations detected in the upper troposphere precludes a significant impact on cirrus clouds.