Systematic satellite observations of the impact of aerosols from passive volcanic degassing on local cloud properties

Aerosol indirect radiative effects - the alteration of cloud properties by atmospheric aerosol – have a large, but relatively uncertain impact on the Earth’s radiative balance. Quantification of volcanic aerosol indirect effects contributes to our understanding of both present-day atmospheric properties and of the pre-industrial baseline necessary to assess aerosol radiative forcing. The impact of emissions from passively degassing volcanoes and minor volcanic explosions are particularly poorly constrained.

We present systematic satellite measurements of the time-averaged indirect aerosol effect over several years at multiple active and inactive volcanic islands (Moderate Resolution Imaging Spectroradiometer, 2000-2013 and Advanced Along-Track Scanning Radiometer 2002-2008). Retrievals of aerosol and cloud properties at Kilauea, Yasur and Piton de la Fournaise are rotated about the volcanic vent to be parallel to wind direction, so that average upwind and downwind values can be estimated. The emissions from all three volcanoes, including those from passive degassing, strombolian activity and minor explosions lead to measurably increased aerosol optical depth (<0.1) and decreased cloud droplet effective radius (<8 μm) downwind of the volcanoes. Furthermore, Top of Atmosphere Short Wave flux from NASA’s Clouds and the Earth’s Radiant Energy System (CERES) show downwind perturbations ranging from 10 to 45 Wm^-2 within 400 km of degassing volcanoes. Comparison of these observations to cloud properties at isolated islands without degassing volcanoes demonstrates that these patterns are not purely orographic in origin. Our observations of unpolluted, isolated marine settings may capture processes similar to those in the pre-industrial marine atmosphere.