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Regional Air Pollution

Air pollution can be carried far from the source. Pollutants can cause environmental problems of many types. One common issue is the formation of acid in clouds from nitrous and sulphuric oxides. Another famous regional air pollution problem is the formation of a cloud of particulate matter from burning forests in South-East Asia. Countries affected by air pollution from their neighbours are protected by transboundary pollution control agreements.

Acid rain is unusually acidic precipitation. Emissions from motor vehicles, industry and coal power stations contain pollutants, such as sulphur dioxide and nitrous oxide, which do not respect national borders, travel far and result in destructive acid rain.

Sulphuric acid/Schwefelsäure
Nitric acid/Salpetersäure

Acid rain is a transboundary problem, with clouds carrying the acid to other countries, and therefore the subject of international and bilateral agreements.

The emissions of SO2 and NO2 lead to the formation of sulphuric and nitric acid in clouds. Acid rain can fall far from the source of the pollution, and cause major damage to ecosystems, water resources, agriculture and infrastructure.

Sulphuric acid/Schwefelsäure

Sulphuric acid

Sulphuric acid (H2SO4) forms from SO2 gaseous emissions, from coal-fired power stations primarily, which reacts with atmospheric oxygen to form sulphuric trioxide, SO3:

2SO2 + O2 → 2SO3

This gaseous sulphur trioxide is carried to clouds, where it reacts with water droplets to form aqueous sulphuric acid, H2SO4:

SO3 + H2O → H2SO4

In mist, the sulphur dioxide can react directly with water:

SO2 + H2O → H2SO3

2H2SO3 + O2 → 2H2SO4

Nitric acid

Nitric acid forms from NO2 gaseous emissions, from petrol and diesel vehicle exhausts primarily, which react with atmospheric water to form nitric acid, HNO3:

2NO2 + H2O → HNO2 + HNO3

N2O4 can also form nitric acid:

N2O4 + H2O → HNO2 + HNO3

Acid rain is caused by the formation of sulphuric and nitric acid in clouds. These are formed by the emissions of SOx and NOx from fossil fuels.

Sulphur oxides SOx

Sulphur oxides, S0x (more correctly Sx0y), are a group of sulphur and oxygen compounds. Where the environment is concerned, these are mainly: S0, S02, and S03.

SOx Sources

Fossil fuels (mainly coal): 72% power generation, 18% industry, 2% road vehicles, 3% domestic (heating)


Local and Regional


Atmospheric particulates lead to respiratory diseases, and sulphur oxides lead to regional acid rain.


To prevent the kind of smog events, such as London's Great Smog of 1952, which killed more than 12,000 people, and injured 100,000, coal has been banned for domestic heating in many countries. Flue gases are scrubbed to remove most sulphur from power station emissions, and prior-purification of fuels can eliminate much sulphur from combustion sources.

Threshold Levels

EU: sulphur dioxide 1-hour threshold = 350 µg/m3, may be exceeded no more than 24 times in a year. 1-day threshold value = 125 µg/m3, and may be exceeded up to three times in a calendar year. For the protection of vegetation, the critical value of 20 µg/m3 holds for annual and for winter (October to March) average values.


In MDCs (more developed countries), the dependence on sulphur-bearing fuel (mainly low-grade oil) is decreasing, and the technology to scrub flue gases is finding more widespread use. In emerging economies, S0x remains a primary pollutant, with increasingly serious local and regional consequences for human and ecosystem health.

In Germany, since the 1990s the SO2 threshold values for human health have been consistently maintained.

Nitrous oxides SOx

Nitrogen oxides, N0x, refers to the group of nitrogen and oxygen compounds, mainly: N0 N20 and N02.


Fossil fuels (mainly petrol and diesel): 51% road vehicles, 28% power generation, 10% industry, 2% domestic (heating)


Local and Regional


N0x is a cause of photosmog, responsible for the development of terrestrial ozone, and other pollutants which cause respiratory ailments and disease. N0x joins S0x in forming cloud-borne acid rain (nitric and sulphuric acid), which damages property, crops and natural systems far beyond the immediate region of the source.


Vehicle emissions can be reduced to some degree at source through catalytic converters, and improved fuel quality. However, the scale of the problem leads to some cities imposing traffic limiting regulations, especially in summer, when photosmog is most prevalent.


The transition from fossil fuels to renewable energy sources will lead to reductions in this principal pollutant. Low-emission vehicle solutions such as hydrogen fuel cells, promise to reduce emissions. However, some proposed solutions, such as electric vehicles, can result in only shifting the location of the pollution emission source, rather than solve the overall pollution problem.

The Atmospheric Brown Cloud, also referred to regionally as the Asian Brown Cloud, South-East Asian Brown Cloud, and Indian Ocean Brown Cloud, is a large, seasonal cloud of air pollution, consisting of primarily smoke from forest fires, which covers large areas of Asia mainly from January to March.

Just how bad systematic failure to curb air pollution can get can be seen in the perennial cloud of smog, which covers large areas of the Indian Ocean and countries from south-east Asia to Pakistan. The cloud intensifies between January and March, during the dry season related to the winter monsoon. The later summer monsoon rains clear the air of the pollutants.

As opposed to the specific forest fire cause of the south-east Asia haze, the Asian Brown cloud is caused by an accumulation of all types of air pollution which produce particulate matter, industry, vehicle emissions, and incomplete combustion of biomass.

South-east Asia Haze

This cloud is caused by the burning of forest, legal and illegal, in Indonesia and Malaysia.

Every year a giant brown cloud of pollution grows and moves across South-East Asia. In this cloud are smoke from deforestation, but also huge quantities of SO2 and NO2 from the burning of fossil fuels.

These chemicals lead to the formation of sulphuric and nitric acid in clouds, which do not respect national boundaries. The acid rain they produce can fall far from the source of the pollution, and cause major damage to ecosystems, water resources, agriculture and infrastructure. The economic costs are far from insignificant.

There is a long-established link between poor pollution control and economic and environmental impacts. But what are developed and developing nations doing in their struggle to reduce this perennial, transboundary problem?

An environmental catastrophe occurs every year during the dry season, affecting hundreds of millions in the Asian/Indian region. Along with serious human health impacts, the Atmospheric Brown Cloud exacerbates and complicates the effects of climate change, and is a cause of both direct and indirect forcing of the radiation budget.

Previously known as the Asian Brown Cloud, technically it is a haze, consisting of fine particles and dust, rather than liquid water drops like normal clouds. However, these particles can form the nuclei of droplets. Related phenomena exist in many other parts of the world: the North American haze (now extending as far as Europe), the North and Eastern European aerosol haze, the biomass burning aerosol hazes in the Africa and the Amazon regions, and a Saharan dust plume extending over the tropical Atlantic.

The Atmospheric Brown Cloud appears as a brown smear from space. It consists of a mixture of anthropogenic pollutants such as sulphate, nitrate, VOCs, black carbon, dust and fly ash, as well as naturally occurring aerosols, such as sea salt and mineral dust. In the South-East Asia - India region, three-quarters of the annual cloud is due to biomass and fossil fuel burning.

In India, during the dry winter monsoon period, which is at least January to March, but may extend by a month either side, the lack of rain results in a peak accumulation of the cloud. The effects of the cloud are many, and tend to manifest according to prevailing conditions in each region. It is causing a change in rainfall patterns. The Asian monsoon is known to have been delayed by as much as a month by the cloud. The Indian monsoon is seriously weakened, and China experiences drought in the north, and flooding in the south, as a result of the cloud.

Australia accuses the cloud of pushing the thermal equator further south, causing tropical storms and increased rainfall across northern Australia.

China smog
China's cities are now swathed in permanent man-made clouds

The cloud also has an impact on global warming, with aerosols and ozone both reflecting sunlight and trapping infrared radiation from the ground. The black carbon settles on glaciers and snow in the Hindu Kush Himalayas, causing them to absorb solar energy, and to lose their reflective qualities. The resulting melting causes both flooding and water shortages right across the region, including the Ganges catchment. The gases in the clouds form acid rain, and the increase in terrestrial ozone results in decreased crop yields and human health issues.