I found this information at the Findhorn Community in Scotland. They are using a tree to illustrate the volume occupied by one tonne of carbon dioxide. A 10 metre diameter sphere has a volume of 523 cubic metres. There are 1000 litres in 1 cubic metre so 523000 litres in the sphere. At room temperature and pressure, one mole of a gas occupies 24 litres. This means there are about 22000 moles of carbon dioxide. One mole is 12 + (2x16) = 44 grams. So the mass of gas in the sphere = 968000 grams which is 968kg so almost 1000kg or 1 tonne. It works.
