In the morning of April 11, Mt. Eyjafjallajökull, a volcano which is covered by a glacier, erupted in the southwest of Iceland. Its cloud of ashes rises up to altitudes of 10 – 12 kms and has been shifted towards Central Europe by a northerly airstream since Thursday (animation).The ash particles are slowly sinking downwards in the air, obstructing aviation in many places. In the atmosphere they dim the light (photos C. Hinz 1-2-3) and make Bishop´s Ring visible (photo P. Krämer), which is caused by light refraction on the aerosoles.
In high levels of the atmosphere, the particles act as additional nuclei for condensation, on which humidity (which under normal circumstances is not sufficient for cloud formation) freezes and forms ice crystals generating so-called “Invisible Cirrus Clouds”. Size and/or density of the ice crystals is in most cases not high enough to make the clouds visible, but their existence can be proved by the formation of faint halos such as sun pillars (photo Ina Rendtel), sundogs (photo Reinhard Nitze), or the 22°-halo (photo Brigitte Rauch).
There are still doubts regarding the appearance of the colourful twilight effects known from the eruption of Mt. Sarychev. Measurements with a Lidar effected by the Hohenpeissenberg Meteorological Observatory have shown that most of the aerosoles are at altitudes between 3.000 and 7.000 meters. A heavy rainshower should be enough to wash them out of the atmosphere and make the air clean again. An elevated concentration of sulphuric acid, which after the eruption of Mt. Sarychev formed several layers at different altitudes and caused beautiful purple light and afterglow effects, has not been measured at all. Probably the SO2 ejected by Mt. Eyjafjallajökull is chemically combined to water at the moment when the ash cloud is formed. The explosions, however, are generated by the contact of lava with ice, and every time a part of the glacier falls into the lava, there is plenty of water provided for such a reaction.
Authors: Claudia Hinz, Peter Krämer and Wolfgang Hamburg
2 comments:
Does volcanic ash ionize the gases in the atmosphere at all, producing an aurora-like effect? It's always been my impression that the only atmospheric phenomena associated with volcanic ash is "volcanic twilight", which is merely an extended twilight as a result of high-altitude ash particles reflecting sunlight. I'd love to be set straight on all of this!
Eyjafjallajökull had a very small amount of SO2 in its eruptive plumes, it can't even be compared to any of the last two years' explosive eruptions in the North Pacific region. "Ozone Monitoring Instrument (OMI) images of the SO2 and ash cloud on April 15 at ~12:00 UTC. The total SO2 mass measured by OMI in the volcanic cloud was ~3000-4000 tons." This is really far from the 1,5 million tons of SO2 produced by Kasatochi in 2008 in two days of its eruption.
Doug: I think it is not possible to volcanic ash particles to have any kind of chemical connection with atmospheric gases. Ash is not something that holds energy (like the high energy particles coming from Sun) so it has no similar effect. And ionization is not enough, the atmospheric gases must be excited to produce light (aurora). Ionization means getting any extra or losing some of its own electrons. When an aurora is forming, the gas's electrons are forced to another energy level (different from their natural level), this causes the emission of photons. These energy levels of electrons are fixed, they refer to the atom or ion or molecule itself. It's easy to imageine it like a rubber onion: you have to push its outer layer with the power of your finger, but as below the layer you push there is another layer, it will spring back the outer one that you have pushed. The more energy you give into the push, the more energy is given back with the spring-back.
The twilights of volcanic sulphur dioxide (not ash) is simply a Rayleigh scattering that occures much higher than the tropospheric clouds dwell, so the sunlight reaches that higher altitude at a later moment that it is needed for normal twilight. The SO2 particles are really tiny, about 0,3-0,5 microns, the ash particles are over the size of a micron, mostly tens of microns, so they can't produce Rayleigh scattering. So ash can't make any extended twilight with vivid colours. Ash makes the colours being more dull, as Mie scattering fades them.
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