Acid rain is a classic example of an environmental threat that originates in one place and causes problems in another, often very far away.
Acid rain can occur when certain chemicals react with air and water in the atmosphere to form mild acidic compounds, which can fall in rain and snow.
The main problem chemicals are sulphur dioxide (produced when coal or oil are burnt to generate power, and when volcanoes erupt) and nitrogen oxides (which arise from cars and other burning processes, including natural lightning strikes).
Acid rain has been linked to a number of harmful impacts on forests, soils, lakes and human health, but there are some scientific disagreements about how serious the problem is.
It has been blamed for a decline in tree growth in European and North American forests, possibly by reducing the nutrient content of soil or damaging the protective surface of leaves.
A side effect of acid rain is that it can enhance the ability of rainwater to draw toxic metals such as aluminium out of the soil and into rivers, lakes and water supplies.
An increase the acidity of lake water can also threaten plant and animal life directly — especially invertebrates such a crustaceans and molluscs and the eggs of bigger creatures such as fish.
The airborne acids that combine with water to form acid rain can also cause problems in their ‘dry’ state, forming a dust that some research suggests can increase lung disease if it is inhaled.
Acid rain became a well-known problem in Western Europe and North America in the 1970s and 1980s, and this led to legislation to curtail the threat.
Today, there are major new sources of acid rain such as Asia’s industrial centres and major cities with growing numbers of cars.
An early response to acid rain was to use taller chimneys at power stations, but this just meant the acid rain fell further away.
A more effective solution is to reduce air pollution at its source. This can be done by reducing reliance on fossil fuels and by taking steps to stop pollutants from entering the atmosphere.
One way to do this is by installing ‘scrubbers’ in the chimneys of power stations and industrial facilities. Scrubbers use wet lime/limestone (or a a mixture of water and powdered limestone) to remove sulphur before it reaches the outside air.
Other approaches in the energy sector include using low-sulphur coal, converting coal to cleaner gas via gasification, or burning coal in the presence of calcium in limestone, which reacts with the sulphur in coal to produce a harmless waste product.
Cars can be fitted with catalytic converters, which remove chemicals that cause acid rain from exhaust fumes.
In areas where acid rain falls, limestone can be added to soils, lakes and rivers to reduce their acidity but this ‘liming’ is an expensive, and only temporary fix.
Some countries have used legislation to promote such solutions to acid rain. The Acid Rain Program under the US Clean Air Act led to a rapid and cost-effective decline in pollutants through a ‘cap and trade’ approach, as described in this SolveClimate blog post.
Acid rain is an example of a trans-boundary problem — the impacts can occur very far from the source of the pollution. This makes it hard to prove the cause of the problem.
Acid rain is also a complex story to cover as it is hard to assess the extent of impacts (on forests for instance) and link them to pollution as opposed to other causes such as droughts, fires or pests.
Journalists must also remember that there are also natural sources of the pollutants that cause acid rain, such as wildfires and volcanic eruptions. However, it is though that the majority of these chemical come from fossil-fuel use and industry.
A good source for journalists covering acid rain in Asia is the Acid Deposition Monitoring Network in East Asia, which operates in 13 countries.
The UNEP Acid Rain and Emissions Reduction in Asia project created a database of experts that journalists might also find useful.
For news and story ideas, the India Environmental Portal produces summaries of research papers and online news about acid rain in India, China and other Asian nations.
CASE STUDY – Impacts of Asia’s acid rain
According to the World Resources Institute, acid rain is now particularly problematic in areas of southeast China, northeast India, Thailand, and the Republic of Korea that are in or downwind of urban and industrial centres.
Research in the late 1990s has shown high levels of acid rain in these areas and has linked this to declines in crop yields and tree growth.
As Asia industrializes its emissions of sulphur dioxide are expected to rise rapidly, nearly doubling between 2000 and 2020 according to some estimates.
The Air Pollution & Climate Secretariat (formerly the Swedish NGO Secretariat on Acid Rain)