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

Abstract

Abstract. The impact of volcanic emissions, especially from passive degassing and minor explosions, is a source of uncertainty in estimations of aerosol indirect effects. Observations of the impact of volcanic aerosol on clouds contribute to our understanding of both present-day atmospheric properties and of the pre-industrial baseline necessary to assess aerosol radiative forcing. We present systematic measurements over several years at multiple active and inactive volcanic islands in regions of low present-day aerosol burden. The time-averaged indirect aerosol effects within 200 km downwind of island volcanoes are observed using Moderate Resolution Imaging Spectroradiometer (MODIS, 2002–2013) and Advanced Along-Track Scanning Radiometer (AATSR, 2002–2008) data. Retrievals of aerosol and cloud properties at Kīlauea (Hawai'i), Yasur (Vanuatu) and Piton de la Fournaise (la Réunion) are rotated about the volcanic vent to be parallel to wind direction, so that upwind and downwind retrievals can be compared. The emissions from all three volcanoes – including those from passive degassing, Strombolian activity and minor explosions – lead to measurably increased aerosol optical depth downwind of the active vent. Average cloud droplet effective radius is lower downwind of the volcano in all cases, with the peak difference ranging from 2–8 μm at the different volcanoes in different seasons. Estimations of the difference in Top of Atmosphere upward Short Wave flux upwind and downwind of the active volcanoes from NASA's Clouds and the Earth's Radiant Energy System (CERES) suggest a downwind elevation of between 10 and 45 Wm−2 at distances of 150–400 km from the volcano, with much greater local (< 80 km) effects. Comparison of these observations with cloud properties at isolated islands without degassing or erupting volcanoes suggests 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.

The vertical distribution of volcanic SO<sub>2</sub> plumes measured by IASI

“…Volcanic eruptions are a significant source of atmospheric SO 2 , with its effects and lifetime depending on the SO 2 injection altitude. In the troposphere these include acidification of rainfall, modification of cloud formation and impacts on air quality and vegetation (Ebmeier et al, 2014;Delmelle et al, 2002;Delmelle, 2003;Calabrese et al, 2011). In the stratosphere, SO 2 oxidizes to form a stratospheric H 2 SO 4 aerosol that can affect climate for several years (Robock, 2000).…”

The aerosol-cyclone indirect effect in observations and high-resolution simulations

“…The effects of enhanced aerosol on clouds was investigated by increasing aerosol at the surface to 2000/cc in a channel between 30°N and 60°N. Ice number was controlled using a simple temperature-dependent relationship (Cooper, 1986). Simulations were then run for 15 days.…”

“…Further observational evidence is supplied by the natural laboratory provided by a degassing volcano in the North Atlantic. Some recent studies have used volcanoes to show that 5 transient changes in aerosol load lead to changes in cloud properties (Mace and Abernathy, 2016;Gassó, 2008;Ebmeier et al, 2014;Yuan et al, 2011;Schmidt et al, 2012;Malavelle et al, 2017), but due to the difficulty in separating meteorological and aerosol effects this is the first time that a liquid water response has been demonstrated for extratropical cyclones.…”

Temporal variations of flux and altitude of sulfur dioxide emissions during volcanic eruptions: implications for long-range dispersal of volcanic clouds

“…However, sulfate aerosols may reduce ice crystal nucleation rate and impact the properties of high altitude cirrus clouds which play a crucial role in the climate system (Kuebbeler et al, 2012). Furthermore, even the degassing processes of lowest intensity, such as persistent passive degassing outside of eruptive episodes, may provide a large natural background of aerosols which may substantially affect the properties of low altitude meteorological clouds and the radiative state of the atmosphere (Yuan et al, 2011;Schmidt et al, 2012;Ebmeier et al, 2014).…”