Wednesday, December 22, 2010

How 'Large' is CO2?

CO2's Volume: It's very difficult to imagine GASES by weight, as they do not appear to weigh anything when considered in the atmosphere at sea level.
  You could imagine a large balloon of gas orbiting in the vacuum of space - that would definitely have a mass and momentum. How large would a tonne of that be?
  A plastic bag full of water floating in the sea is suspended weightlessly - lift it out into the air, and that bag becomes very heavy! A bag of pure CO2 in the atmosphere would fall gently, as it is 1.5 times the density of air, plus the bag would have some weight. Hot air balloons can rise or fall gently, just by varying the temperature of the gas inside compared with the cooler temperature of the same gas around the balloon.
  Another way of thinking about it is through wind forces - gas has momentum when it moves, and acts on things it meets - trees, building, people! A 5 metres/second wind on your building is like having 6 kilograms being thrown at every square metre of your building, every second! (that is only about 10 knots, by the way).

  The density of CO2 gas is 1.98 kg/m3 at atmospheric temperature and pressure. So one kilo is a sphere of nearly one metre diameter or a cube of 800mm dimension. A tonne of CO2 would be 1000 of those (would be 10x10x10 bigger), therefore a cube of 7.96m dimension, or a sphere of 9.88m diameter. Doesn't seem so large until you consider the next paragraph.

  The next problem is that pure CO2 cannot exist unless contained - it may be heavier than air, but it doesn't settle out into a simple layer, it wants to dissolve into the air it is released to. For many centuries, the CO2 concentration has hovered at 260-280 ppm, but the high present day CO2 concentration is 390 parts per million, and if it goes above 450 ppm, then the planet will have runaway Climate change. So at 390 ppm, a tonne of CO2 dissolving into air would require an air-cube of 110m dimension, or an air-sphere of 135m diameter. Considering that the air it is being released to already has this much CO2 in it, the volume required is even bigger than that, unless a conveniently close Rainforest can remove all that CO2... but there are declining numbers of them!

PS, I am grateful to Ted and Klaus of the Navi tron forum for elucidating some of these points, especially, as they took it a lot further, with Avogadro's numbers and Mols, bringing back memories of GCSE Chemistry lessons in the 1960s!

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