Scientists have found that a large proportion of acidic emissions over Scandinavia at certain times of the year come from natural sources in the oceans rather than man-made pollution in the UK.
Up to three-quarters of the sulphur-containing substances emitted over Scandinavia in June come from plankton in the North Sea, scientists said. These substances eventually break down into the chemicals that cause acid rain.
Scientists from the Natural Environment Research Council have calculated that natural sulphur emissions from plankton play a significant role in contributing to the total emissions over Ireland, the UK and Scandinavia. The new calculations were published yesterday in the annual report of the research council.
Some microscopic plankton produce a gas called dimethyl sulphide. This quickly passes from the sea into the atmosphere, where it is converted into sulphur dioxide and acidic sulphates, which are identical to the products of burning fossil fuels in power stations and car engines.
Scientists have known of the process for many years but have been unable to assess its contribution to the overall sulphur emissions that result in acid rain.
Peter Liss, professor of environmental sciences at the University of East Anglia, said: 'What we have done is to firm up the numbers. Whatever targets (on man-made sulphur emissions) are set, they should take into account the natural background levels.'
He said that 75 per cent of sulphur substances emitted over Scandinavia in June came from the North Sea. During the spring-summer period about 30 per cent of the sulphur-containing gases over Ireland originated from plankton living in the North Atlantic. 'Some people find this surprising,' he said.
In places next to industrialised areas, such as Wales, the natural background level of sulphur-containing emissions may be nearer 10 per cent of the total, Professor Liss said. 'But that is a guess.' Work was under way to estimate the precise figure.Reuse content