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Acid fog found to pollute forests

Innocuous looking fog can actually be more harmful to forests than acid rain

(Keystone)

Fog is contributing to the pollution of forests - and the problem is far worse than previously thought.

That is the conclusion of a new survey funded by the Swiss National Science Foundation and conducted by researchers at Bern University.

For the past four years, scientists have been studying the effects of sulphur and nitrogen carried in fog on forest ecosystems in Germany and Switzerland.

In Switzerland, they collected data from a hill above Wettingen, near the city of Zurich.

One of the researchers on the project, Werner Eugster, says the results show that fog can - in a similar way to rain - be acidic and therefore harmful to forests.

Acid fog

Eugster explains that sulphur in small amounts is a nutrient - but in high levels can make fog and rain more acidic.

"Fog is normally even more acidic than rain," Eugster told swissinfo.

"Fog droplets are much smaller than rain droplets but they can collect almost as many pollutants."

Until now, scientists had assumed that fog carried one sixth of the nitrogen needed by a forest to grow.

But researchers found that the particles actually contained one third of the necessary nitrogen - twice as much as had been previously thought.

Surprising findings

The high quantity of pollutants found in fog surprised the team of scientists conducting the research, who had been expecting a decrease in nitrogen content - due in part to more stringent laws governing pollution.

"You don't see a big difference between the concentrations in fog from the measurements performed by other people 15 or 20 years ago," comments Eugster.

Eugster says the increased level of nitrogen has the effect of speeding up the growth of forests because it is a nutrient.

But he adds that too much nitrogen could be harmful to forests and lead to more damage during stormy weather because the trees become malnourished.

"I compare it to someone who's going to fast food restaurants all day," he told swissinfo.

"If you go once there is not a problem, but if you only eat fast food you could get malnutrition - and the same probably happens in forests."

Homemade pollution

According to the Swiss Science Foundation, the high levels of nitrogen are thought to be a result of an increase in traffic across Switzerland.

The researchers found it more difficult to find an explanation for the increase in sulphur. The fact that Zurich and its busy airport were not far from the test site is thought to be one possible explanation.

Eugster believes that a solution to the problem can only be found by generating a wider discussion on Switzerland's pollution strategy.

"There is still need for more action and to consolidate action that has already been started in Switzerland, to really get Swiss people to take part in sustainable activity and to reduce the pollutant levels overall."

swissinfo, Sally Mules and Isobel Johnson

In Brief:

Scientists have discovered that acid fog is contributing to the pollution of forests.
Sulphur in small amounts is a nutrient - but in high levels can make fog and rain more acidic.
Experts say more needs to be done to persuade people to curb pollution.

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