The persistence of ash in the stratosphere

On 13 February 2014, the Kelud volcano, considered one of the most dangerous in Java because of its frequent eruptions and deadly mudslides, spewed billions of tons of sulfur dioxide and ash into Earth’s stratosphere. Such large eruptions are known to cool the planet’s surface because sulfate aerosols, created by oxidation of the SO₂, can linger for months to years in the atmosphere, where they efficiently reflect solar radiation. The heavier ash particles, by contrast, are assumed to fall from the sky within days because of rain and gravity, and climate models almost universally neglect them. NASA atmospheric scientist Jean-Paul Vernier and his colleagues have now refuted that assumption. Following the Kelud eruption, they monitored the diffusive evolution of its plume using the CALIPSO (Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation) satellite. Polarization measurements can distinguish between the backscatter signals of volcanic ash, sulfate aerosols, and ice clouds. The researchers found that within 11 days of the eruption, sedimentation had caused much of the ash to settle to the bottom of the plume in the lower stratosphere. Surprisingly, though, the contribution of the fine ash to the plume’s optical depth—the logarithm of the ratio of incident to transmitted radiant power—three weeks later amounted to almost 40%. A subsequent balloon campaign conducted in late May confirmed the presence of ash even then and found that it accounted for up to 25% of the optical depth. Vernier and colleagues argue that upward-moving air currents in the tropics, where Kelud’s plume was largely confined, reduce the ash particles’ settling speed and account for the longevity. (J.-P. Vernier et al., J. Geophys. Res: Atmos. 121, 11104, 2016.)