Acid Rain


Earth Journalism Network, Global

Acid rain when precipitation includes acidic compounds. This precipitation could include snow, fog or bits of dry material in the atmosphere which drift down to Earth.

Acid rain is formed when chemicals such as sulfur dioxide (SO2) or nitrogen oxides (NOx) react with water in the atmosphere to form mild solutions of sulfuric or nitric acid.

The acidity of solutions is measured on a logarithmic pH scale from 0 to 14 with 0 being most acidic and 14 being most alkaline and 7 as neutral. Distilled water is at 7 with normal rainwater naturally being from 5.3 to 6.0. Any measurement below 5.3 is commonly considered to be acid rain.

Sulfur dioxide and nitric oxides can be produced either by human activity or by nature. Burning coal and oil and volcanic eruptions produce sulfur dioxide while the cars and lightning strikes produce nitric oxides.

Acid rain has been linked to a number of harmful impacts on the natural environment and human activities.

  • Soil – Acid rain draws toxic metals such as aluminum out of the soil which then gets carried by rainwater to water sources. Some soils may be naturally better at neutralizing the acidity than others. This ability is known as the “buffering capacity”
  • Forests and vegetation – Acid rain damages trees and the increased aluminum in the soil inhibits the uptake of water. The decline of tree growth in Europe and North America has been linked to acid rain.
  • Wildlife – Acid rain is particularly harmful against aquatic life. Some life forms are more tolerant of acidity than others which has resulted in the “jellifying” of certain lakes of North America where small microorganisms thrive on the lack of competition and the acidity. Because ecosystems are interconnected, the effects of acid rain can be felt by many animals and the overall biodiversity of the area.
  • Human Activities – Acid rain can corrode and weather buildings, especially those constructed out of limestone. While acid rain itself is too dilute to cause serious health concerns, the dry depositions of the acid can cause serious respiratory issues such as lung cancer.

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 centers 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.

The most 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 mixture of water and powdered limestone) to remove sulfur before it reaches the outside air.

Other approaches in the energy sector include using low-sulfur coal, converting coal to cleaner gas via gasification, or burning coal in the presence of calcium in limestone, which reacts with the sulfur 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 a complex story to cover due to its many causes and effects. It is an example of a trans-boundary problem – where the impacts can occur very far from the source of the pollution making it hard to pinpoint. Additionally, impacts take on many different forms, some of which are not able to be seen quickly. Journalists must be sure to investigate closely to correctly attribute the effects of acid rain to its sources whether they are natural or man-made.

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.

Acid News is a quarterly magazine that focuses on acid rain stories from a largely European perspective.

A good source for journalists covering acid rain in Asia is the Acid Deposition Monitoring Network in East Asia, which operates in 13 countries.


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 centers.

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 sulfur dioxide are expected to rise rapidly, nearly doubling between 2000 and 2020 according to some estimates.