Ask an Expert

Can acid rain go through anything? Why? How?

Acid rain not only kills off plants and animals by poisoning forests and lakes, but it also slowly eats away many man-made structures such as buildings and bridges.

Highly populated areas in the Northern Hemisphere were so badly polluted by acid rain in the '70s and '80s that it caused a reduction of plant and animal diversity, says Dr Greg Ayers, director of the Australian Bureau of Meteorology.

"There were some regions where thousands of small lakes became devoid of life altogether, says Ayres, who studied the effects of acid rain while the head of CSIRO Marine and Atmospheric Research during the '80s and '90s.

"Acid rain has also been blamed for eating away parts of sandstone buildings and statues by dissolving the calcium carbonate matrix that binds the silicates of the sandstone together. This can happen in just 10 or 20 years.

"And it's caused modern-day metal structures to degrade more quickly, forcing engineers to use stainless steel instead of normal steel to keep bridges safe. In some parts of China, acid rain causes galvanized rubbish bins to rust through so quickly, they only last a few years."

And in Sweden and Norway, corrosion of copper pipes by acid rain even turned some people's hair a blueish-green tinge.

^ to top

Chemical make up

Acid rain is composed mostly of sulphuric and nitric acid.

Both are common substances in the natural environment, occurring in trace amounts in the atmosphere because of trace gases containing sulphur and nitrogen being oxidised by sunlight, especially ultraviolet light.

"In fact they can even provide the seeding for rain droplets.

"But when human activities like industry and transport emit large quantities of sulphur and nitrogen through things like the burning of coal and oil, the natural cycle becomes swamped by the these chemicals and acid rain becomes a real problem.

"Work by governments in more recent times has seen a drop in sulphur emissions and a stabilisation of nitric acid levels, even though populations have continued to grow," says Ayres.

Acid rain levels are low in most of Australia compared to other parts of the world.

"That's mainly because of our sparse population and widely separated emission sources. And because Australian coal is very low in sulphur, up to a factor of five lower than overseas coal," says Ayres.

"Interestingly however, readings taken in the Northern Territory at places like Kakadu National Park produce very high levels of acid rain at the start of each wet season.

"These levels are as acidic as parts of the Northern Hemisphere, but are caused naturally by emissions from native tropical plants, rather than through any sort of human activities."

^ to top

Acid rain and people

According to the CSIRO, normal unpolluted rain water should have a pH value of about 5 and tap water should be around 7. Any measurement under 5 is considered acid rain.

Ayres says the levels of acid rain deposited into lakes and rivers, is still relatively low by the standards of acidity in the human gut, or in some of the foods we eat.

"It's about as strong as the acidic acid in vinegar. You wouldn't water your plants with it, but it's not bad enough to hurt humans."

Dr Greg Ayers is the director of the Australian Bureau of Meteorology, and former head of CSIRO Marine and Atmospheric Research. He was interviewed by Stuart Gary.