Rings of evidence

chemists at Sheffield University in the uk have discovered a new method to determine the extent of wartime air pollution by studying rings formed around tree trunks.

Cameron McLeod, Alan Cox and their colleagues have looked into Sheffield's war-ravaged past by examining felled trees. Sheffield's air choked with metals during the Second World War because this city was home for armament factories.

Measuring levels of pollutants in tree rings is an established method but sometimes it is an unreliable technique. With time, says Cox, pollutants disperse throughout the tree and gets diluted. So, the chemists turned to a novel method developed by their collaborator Kenichi Satake of Japan's National Institute of Environmental Studies in Tsukuba. Japanese scientists had modified the technique after he discovered that uranium did not appear in the rings of trees around Nagasaki until 15 years after the atomic bomb was dropped.

Satake turned to tree-bark pockets, pea-sized scars that form whenever a tree is injured, for example by loosing a branch. The scars eventually become part of the trunk. "The tree protects itself to cover the wound. Any pollution is trapped in the scar and held, and there is no absorption or leaching from the wood into other parts of the tree. They are like tiny time capsules' says Cox.

The Sheffield team measured concentrations of trapped metals by vaporising material from bark pockets. In the 100-year-old beech tree on the outskirts of Sheffield, they have found vastly increased levels of lead, uranium, cobalt, vanadium, chromium and mercury dating back to the wartime.

"We did more than one sample because we could not believe the levels of lead were so high. All the large bombs such as Talboys and Grand Slams were made in Sheffield and it must have taken a phenomenal amount of steel,' says Cox.