What are Sulphur Oxides?

SOx refers to all sulphur oxides, the two major ones being sulphur dioxide (SO2) and sulphur trioxide (SO3). Sulphur dioxide is a colourless gas with a pungent, irritating odour and taste. It is highly soluble in water forming weakly acidic sulphurous acid. When sulphur dioxide combines with the oxygen (O2) in the air some sulphur trioxide is slowly formed. Sulphur trioxide rapidly combines with water to produce sulphuric acid. The lifespan of sulphur oxides in the atmosphere is from 4 to 10 days.

Sulphur dioxide is used in many industrial processes such as chemical preparation, refining, pulp-making and solvent extraction. Sulphur dioxide is also used in the preparation and preservation of food because it prevents bacterial growth and the browning of fruit.


Natural sources of sulphur dioxide include volcanoes and hot springs. Sulphur dioxide is also formed by the oxidation of hydrogen sulphide (H2S), a toxic gas that smells like rotten eggs. Oxidation occurs when hydrogen sulphide combines with the oxygen in air. Hydrogen sulphide is released by marshes and other places on land and in oceans where biological decay is taking place. Hydrogen sulphide is frequently found with natural gas. These deposits are referred to as sour gas.

Man-made sources of sulphur dioxide include sour gas processing, oil sands production, coal combustion, ore refining, chemical manufacturing and other fossil fuel processing and burning.

Canada's sulphur dioxide emissions are about 15 percent of those of the United States, and Alberta's emissions are about 15 percent of Canada's.

Of the 626 kilotonnes of sulphur dioxide emitted in Alberta in 1988, sour gas plants accounted for 38 percent, oil sands 29 percent and coal-fired power plants 16 percent. Sulphur trioxide is generally emitted with sulphur dioxide at about one to five percent of the sulphur dioxide emission rate.

The Effects

Sulphur dioxide can harm crops and trees, textiles, building materials, animals, and people either as a result of exposure to long-term low concentreations or short-term high concentrations. It turns leaves yellow and decreases the growth rate of crops. Sulphur dioxide corrodes metal, and causes building materials and textiles to deteriorate and weaken.

Sulphur dioxide irritates the throat and lungs and, if there are fine dust particles in the air, can damage a person's respiratory system. Sulphur oxides combine with other substances in the air to produce a haze that reduces visibility.

Sulphur dioxide is a mojor contributor to acid deposition, which is described in the fact sheet Acid Deposition (Acid Rain).

What Has Been Done So Far?

Ambient objectives were set years ago by federal and provincial governments. The concentration of sulphur oxides in the air is carefully monitored regularly around facilities that emit sulphur oxides, and in some urban centres, to ensure thos objectives are met. The Alberta objective for sulphur dioxide concentrations, averaged over one hour, is 0.17 parts per million (by volume) in air.

Alberta has had sulphur dioxide emission stnadards for gas processing plants since 1971. In August 1988, the Alberta government moved to further reduce the sulphur dioxide emissions produced by gas plants.

The new sulphur recovery guidelines require that for large new plants, 99.8 percent of the sulphur be removed, up from the previous 96.2 percent. Smaller gas plants must also provide sulphur recovery, ranging from 70 to 90 percent. The new requirements are not applied to existing plants unless they expand by more than 25 percent.

Alberta has adopted the natiional emission standards for new coal- or gas-fired power plants.

In 1985 Canada signed a United Nations Economic Commission for Europe (UN-ECE) protocol agreeing to a 30 percent reduction in national sulphur dioxide emissions. In view of the substantial acid deposition problem in Eastern Canada, the reduction was implemented as a 50 percent reduction in all provinces east of Saskatchewan.

The Future

The 1985 protocol, referred to above, is due for renegotiation in 1994.

Further Information

Air quality issues - greenhouse gases, acid deposition (acid rain) and smog - cannot be addressed in isolation. Their complex inter-relationships make achieving the goal of clean air for the future a challenge for individuals, industry and governments alike. The Clean Air Strategy for Alberta is providing an opportunity for Albertans to participate in meeting that challenge.

To assist Albertans in participating in the Clean Air Strategy for Alberta, the Alberta government has prepared a series of fact sheets and a glossary. Their purpose is to help Albertans understand the magnitude of the environmental and economic considerations, the complexity of the science, the potential requirements for changes in lifestyle, and the challenges facing individuals, industry and government.

List of Fact Sheets

An Overview, Greenhouse Effect, Acid Deposition (Acid Rain), Ozone - Stratospheric and Ground-Level, Carbon Dioxide, Sulphur Oxides, Methane, Nitrogen Oxides, Volatile Organic Compounds, Chlorofluorocarbons and Halons, Energy Efficiency, Policy Instruments, Glossary, Renewable Energy

Other information on sulphur oxides (SOx):

Industrial Sulphur Dioxide Emissions Inventory for Alberta. 1981-1985. Alberta environment. February 1988.

Sulphur Emissions Forecast for Alberta. Alberta Energy/Energy Resources Conservation Board, August 1990.

Clean Air Strategic Alliance, 9th Floor, Sterling Place, 9940 - 106 Street, Edmonton, Alberta, Canada, T5K 2N2, ph. 403/427-9793, fx. 403/422-3127, em:

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