Sulfur Dioxide (SO2)

Sulfur dioxide (SO2) is an air pollutant.

It is a colorless gas with a pungent, suffocating odor. It is a dangerous air pollutant because it is corrosive to organic materials and it irritates the eyes, nose and lungs. (1)

Anthropogenic Sources of Sulfur Emissions

Sulfur is contained within all fossil fuels, and is released in the form of sulfur dioxide (SO2) during fossil fuel combustion. Fossil fuel combustion accounts for almost all anthropogenic (human-caused) sulfur emissions. (2)

Sulfur contents in fossil fuels range between 0.1% and 4% in oil, oil by-products and coal, and up to 40% in natural gas (3) (when immediately extracted from the well; however, the sulfur is efficiently removed during the processing of gas before distribution (4); therefore, combustion of natural gas is not a major source of sulfur emissions (5)).

Historically, the use of coal in domestic heating was a major source of sulfur dioxide emissions (at least in the UK), but it has declined substantially over time. (6)

Over the last several decades the industrial use of coal in the UK has also declined, whereas the use of oil and natural gas has gradually increased. (7)

In the UK, the sulfur dioxide emissions declined significantly since 1970, thanks to the introduction of low sulfur fuels, the switch from coal to gas and increased energy efficiency. (8)

This trend is possibly true for other industrialized countries as well, though the US as the most important economy in the world is still a large consumer of energy derived from coal.

Below is a breakdown of all the significant sources of SO2 emissions (based on sources of the UK emissions) (9):

• Energy Production
  
  
  • Electric power generation
  • Petroleum refining
  • Other combustion
• Commercial and residential use
• Combustion for industry use
• Production processes
• Extraction and distribution of fossil fuels
• Transport
  
  
  • Road transport
  • Other Transport (such as aviation, ships, trains).

Currently, the most important sources of SO2 emissions (as a result of fossil fuel combustion) are electric power generating plants.

For example, as of 1998, 66% of all sulfur dioxide emissions in the UK came from power plants. In contrast, transport contributions of sulfur dioxide emissions are among the smallest ones. (10)

The biggest sulfur dioxide emitters: US, China and Russia. (11)

In fact, you may be surprised to learn that just one Siberian city in Russia – Norilsk – produces 1% of the total global emissions of SO2. In 2007, Norilsk was considered to be one of the most polluted places on Earth. (12)

Natural Sources of Sulfur Emissions

There are also significant sulfur emissions generated by natural sources.

The main natural sulfur emissions come in the reduced forms of sulfur compounds such as (13):

• hydrogen sulfide (H2S)
• carbon disulfide (CS2)
• carbonyl sulfide (COS)

and in the organic forms of:

• methyl mercaptan (CH3SH)
• dimethyl sulfide (DMS) (CH3SCH3)
• dimethyl disulfide (DMDS) (CH3SSCH3)

Most of these compounds get oxidized to SO2 or to sulfate aerosols in the atmosphere. (14)

Marine phytoplankton produce dimethyl sulfide (DMS) which is then oxidized to SO 2 in the atmosphere; decay processes in soil and vegetation produce H 2S (as one of sulfur compounds); and SO2 is emitted into the atmosphere by volcanoes. (15)

Around 90% of all natural sulfur emissions come in the form of DMS. (16)

Most recently the natural sources have been by far surpassed by anthropogenic sources. Natural sources have been estimated to produce around 24% of all sulfur dioxide emissions, whereas human-caused emissions made up around 76%. (17)

Effects of Sulfur Emissions

Sulfur dioxide found in the air produces following effects (18, 19):

• Irritates eyes, nose, throat
• Damages lungs when inhaled
• As part of acid rain:
  
  
  • acidifies lakes and streams
  • destroys plant and fish life in lakes and streams
  • may deplete mineral nutrients in the soil
  • may cause reduction of forest and agricultural yields
  • corrodes metals
  • damages surfaces of buildings.

Last Update: October 2008