The question about volcanic ashes

  1. Good day :smile:
    It is well-known fact that the volcanic eruptions cause the global decrease of average temperature. I quickly reviewed the article in wikipedia and several questions arised regarding this issue. There is written in this article (I do not trust Wikipedia very much):
    1) So, the most dangerous/active factor that cools our planet is aerosols that are made of volcanic ash+ sulfuric acid, right?
    2) What about the sizes of these aerosols?
    3) What can you tell me about their exact chemical composition?
    4) How are they distributed in stratosphere? Do they occupy the thickness of 10–50 km (these are lower and upper boundaries of stratosphere) equally?
    5) After some amount of time (months, years) the effect of “Volcanic winter” weakens, apparently because of the weight of the volcanic aerosols. Do they (chemically) decay or do they sink down towards the Earth’s surface?
    6) Are they subjected to the solar radiation? Can this radiation (if strong) evaporate/decay them? :rolleyes:
     
  2. jcsd
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  3. Simon Bridge

    Simon Bridge 15,259
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    Define "dangerous".
    I understand that there is a difference of opinion about what is currently having the largest "cooling effect" on the climate. It seems a fair bet that volcanoes both heat the atmosphere directly and cool it indirectly ... volcanic activity is usually considered to have had an important role in the pre-industrial climate.

    Yes. What about the size of these aerosols? Is there something you want to know about this?
    Ejecta from a volcano varies from simple gas molecules to huge rocks.
    Expect aerosols in the entire range that can be called by the name "aerosol".

    Not much - the exact composition varies too much so the list would be rather large for this forum.
    You can find papers listing various compositions for different eruptions in journals of vulcanology.
    Most models would use average compositions of the major substances of concern.

    TLDR: It's complicated.
    Especially in light of the request for "exact compositions" prev.

    No. Different chemicals would be distributed differently at different elevations.

    You can find examples in atmospheric studies.

    They are generally heavy - they sink. They also get caught up in rainwater and washed down.

    Yes. Everything is subject to solar radiation.

    If strong yes - but define "strong". The term "solar radiation" is very general and covers a very wide range of energies. Most solar radiation is in the form of infra-red light - which won't do much to the relative concentrations of volcanic ash in the upper atmosphere.

    I think you need to have a closer look at what the processes of "evaporation" and "decay" actually are. It will help you ask more specific questions.

    In evaporation, a liquid droplet breaks up into much smaller droplets ... allowing a more even distribution. Solar radiation can help this along by heating the parent droplet.
    http://techalive.mtu.edu/meec/module01/EvaporationandTranspiration.htm
    ... usually droplets in the upper atmosphere are already smallest for vapor phase, but they can condense on suspended dust particles and these bigger drops evaporate.

    Of course, solar radiation can also vaporize a liquid droplet - turning it into a gas.

    In "decay" - depends on the kind. In nuclear decay, for eg, higher energy radiation would get captured by the atoms in the droplet, forming an unstable isotope. i.e. oxygen 19 beta-decays into Florine. This would require regular oxygen 16-18 gaining neutrons from the solar radiation flux or secondary interactions.


    I think you will benefit from asking more specific questions, but you need more grounding in regular science in order to do this carefully. You seem to be trying to ask very careful questions. I'd suggest leaning on your inexperience and just ask less-careful questions.
     
    Last edited: Feb 13, 2014
  4. Volcanic eruptions that put material into the stratosphere (causing temporary cooling) are very isolated events. Most eruptions are not strong or high enough to inject significant mass. Some famous eruptions that did have a cooling effect were Krakatoa in 1883, and Tunguska in the early 1900s. Krakatoa put 1 cubic mile of debris into the stratosphere, and there was said to be no summer in the northern hemisphere that year. Snow fell in June.
    micron size
    Like you said, sulfuric acid.
    The stratospheric aerosol layer is located in the very lower stratosphere, close to the tropopause.

    Sulfate aerosol can evaporate, and there are also gaseous sulphur compound present.
     
  5. SteamKing

    SteamKing 9,391
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  6. Yes. You're right. Sorry for any misinformation I created.

    Chet
     
  7. davenn

    davenn 3,799
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    Tunguska wasnt an eruption ... not sure where you heard that from ??
    it was a comet/meteorite type impact

    a more recent eruption, 1991, Mt Pinatubo in the Philippines put out enough material to slightly affected global temps

    from wiki.....

    Dave
     
  8. Simon Bridge

    Simon Bridge 15,259
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  9. davenn

    davenn 3,799
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  10. davenn

    davenn 3,799
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    just watching some pinatubo eruption videos on youtube at the moment
    it was truely massive
    My wife is from the Phil's, she was 21 when the eruption occurred, she remembers it well

    Dave
     
  11. SteamKing

    SteamKing 9,391
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    It's not even clear that Tunguska was an impact event. Like it recent event over Chelyabinsk, it appears that a meteorite exploded above the surface, and the shock wave produced did all the damage on the ground.

    http://en.wikipedia.org/wiki/Tunguska_event

    Due to the remote location, and the amount of time elapsed until scientists could survey the damage, it is not clear if some of the fragments of the explosion made it to the ground. In any event, it appears that none have been recovered so far, unlike in the Chelyabinsk event.
     
  12. davenn

    davenn 3,799
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    yes thats true, it is deemed an airburst

    but it wasnt a volcanic eruption :smile:

    D
     
  13. Simon Bridge
    The huge rock will fall down on Earth quickly, the gas molecules will not at all. I want to know the size of the aerosols that block solar radiation and stay in stratosphere relatively long-several months/years.
    But probably the most common chemical found in aerosols will be SiO2, right?
    Where exactly?

    Chestermiller

    What means “high” in this case? Do you mean the height of the column of the volcanic ashes?
    [​IMG]
    Thanks :smile:
    But with SiO2, right? :rolleyes:
    That is at the altitude 10 km, right?

    SteamKing
    No, I did not mix them up. I know that eruption of Tambora was much stronger than eruption of Krakatoa :wink:

    davenn

    Well, as I see the aerosols from this eruption was made up with ONLY sulfuric acid, apparently in solid/frozen state, right? But what about the SiO2 that must have been there? Our planet contains this substance in much larger amount than sulfuric acid.
     
  14. davenn

    davenn 3,799
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    yes, the height's of the volcano in comparison are small, a mountain of say ~ 2- 3,000 metres
    compared to the ash column that may go up to 15 - 20,000 metres or more

    oh and in general, the SO2 doesn't become sulphuric acid till it mixes with the water (vapour) in the atmosphere

    Most of the SiO2 will be in a big enough particulate size that it will either fall out under gravity or be washed out with rain .... that doesn't stop it from staying up long enough to encircle the world
    I remember when living in New Zealand, the incredible sunsets caused by the Pinatubo ash high in the atmosphere
    The aerosols on the other hand are primarily micron size to gasses

    Dave
     
    Last edited: Feb 16, 2014
  15. Simon Bridge

    Simon Bridge 15,259
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  16. davenn

    davenn 3,799
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    Simon

    thanks particularly for that first link
    had a quick scan ... think Im going to learn some new things from that
    when I get a chance to read it fully :)

    Cheeras
    Dave
     
  17. I see no reason why SiO2 should be dominant among the ash. Many volcanoes contain no free silica (quartz). Granted that explosive eruptions are more likely with an acidic lava, but in these the quartz percentage rarely exceeds 35%. Perhaps the OP means silicate minerals, including the feldspars and ferromagnesians as well.
     
  18. Try the prestigious Journal of Geophysical Research. Also check out the book by Susan Solomon and Guy Brasseur on atmospheric chemistry (I forgot the name). Also check out Prupacher and Kleck (again, l I forgot the name) which covers atmospheric physical chemistry, particularly with regard to ice and other aerosols.
    Both altitude and and quantity of injectate. 1 cubic mile of injectate into the stratosphere will do the trick for causing a big effect.
    The tropopause is located at roughly 8 - 15 km, depending on latitude - closer to 15 km at the equator, and 8 km at the poles.
     
  19. davenn
    And as I guess only such volcanoes with long/high ash column can cause global cooling, right?
    So, we can conclude that mainly SO2 (plus water, that is sulfuric acid eventually) causes global cooling, right? But SiO2 also can stay a long time in stratosphere, right?

    Simon Bridge
    Thanks :smile:

    Chestermiller
    Thanks, I think that the information gained in this topic will be enough for me :rolleyes:
     
  20. Simon Bridge

    Simon Bridge 15,259
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    Not really - "SO2 (etc) causes global cooling" is a bit of a strong statement - what we'd say is that these substances are contributing factors to global climate models - with a short-term cooling effect.

    The greatest volcanic impact upon the earth's short term weather patterns is caused by sulfur dioxide gas. In the cold lower atmosphere, it is converted to sulfuric acid by the sun's rays reacting with stratospheric water vapor to form sulfuric acid aerosol layers. The aerosol remains in suspension long after solid ash particles have fallen to earth and forms a layer of sulfuric acid droplets between 15 to 25 kilometers up. Fine ash particles from an eruption column fall out too quickly to significantly cool the atmosphere over an extended period of time, no matter how large the eruption.

    Sulfur aerosols last many years, and several historic eruptions show a good correlation of sulfur dioxide layers in the atmosphere with a decrease in average temperature decrease of subsequent years. The close correlation was first established after the 1963 eruption of Agung volcano in Indonesia when it was found that sulfur dioxide reached the stratosphere and stayed as a sulfuric acid aerosol.

    Without replenishment, the sulfuric acid aerosol layer around the earth is gradually depleted, but it is renewed by each eruption rich in sulfur dioxide. This was confirmed by data collected after the eruptions of El Chichon, Mexico (1982) and Pinatubo, Philippines (1991), both of which were high-sulfur compound carriers like Agung, Indonesia.

    -- http://volcanology.geol.ucsb.edu/gas.htm

    Note: if H2SO4 aerosols mean cooler planets, then Venus should be very chilly right?
    In fact Venus is very hot. The greenhouse agents sustaining it are water vapour, carbon dioxide and sulphuric acid aerosols. Sound familiar?
    http://www.esa.int/Our_Activities/Space_Science/Venus_Express/Greenhouse_effect_clouds_and_winds
    (... or pretty much any tract on Venus.)

    See the first link - solids from eruptions generally don't stay a long time in the atmosphere.
     
  21. Simon Bridge
    Well, thanks a lot for this information; this was what I wanted to know :smile:
     
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