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DESIGNATIONS
CAS No.: 7446-09-5
Registry name: Sulphur dioxide
Chemical name: Sulphur dioxide
Synonyms, Trade names: Sulphur(IV)oxide, sulphur oxide, sulphurous acid anhydride, sulphurous anhydride
Chemical name (German): Schwefeldioxid
Chemical name (French): Dioxyde de sulfure
Appearance: colourless, non-combustible, pungent gas with odour similar to burning sulphur; vinegar-like odour when diluted
BASIC CHEMICAL AND PHYSICAL DATA
| Empirical formula: | SO2 | |
| Rel. molecular mass: | 64.06 g | |
| Density: | 1.46 g/cm3 at -10°C (liquid); 2.93 g/l at 20°C (gas) | |
| Relative gas density: | 2.26 | |
| Boiling point: | -10°C | |
| Melting point: | -75.5°C | |
| Vapour pressure: | 331 kPa at 20°C, 462 kPa at 30°C, 842 kPa at 50°C | |
| Odour threshold: | 0.3 - 1 ppm (in air) | |
| Solvolysis/solubility: | in water: | 112.7 g/l at 20°C (1013 mbar); |
| 228.3 g/l at 0°C (1013 mbar); | ||
| readily soluble in alcohol, benzene, acetone, carbon tetrachloride; | ||
| fully miscible with ether, carbon disulphide, chloroform, glycol | ||
| Conversion factors: | 1 ppm = 0.376 mg/m3 | |
| 1 mg/m3 = 2.663 ppm | ||
ORIGIN AND USE
Usage:
There are many uses of sulphur dioxide. It is used e.g. as a
reducing agent in metallurgy, as a coolant in the refrigeration
industry, as a disinfectant and bleach, in the preservation of
foodstuffs, for dechlorination and as a fumigant. Sulphur dioxide
is one of the most important compounds in the chemical industry.
98% of technical SO2 is used in the production of
sulphur trioxide as a precursor of sulphuric acid.
Origin/derivation:
Sulphur dioxide is released naturally into the atmosphere from
volcanoes and combustion processes. The anthropogenic impact on
the environment primarily results from the combustion of
sulphurous fossil fuels (e.g. coal, oil, natural gas) in power
and heating plants, in industry, in household use and in traffic.
The technical product is made from elemental sulphur, pyrite,
sulphide ores of non-ferrous metals, gypsum, anhydrite and flue
gases (ULLMANN, 1994 for processes involved).
Production figures:
- excluded production from elemental sulphur and pyrites in 1,000 t of sulphur (1982):
| Worldwide: | 5,820 |
| Soviet Union: | 1,700 |
| United States: | 1,380 |
| Japan: | 1,370 |
- production from pyrites in 1,000 t of sulphur (1975):
Worldwide: 11,000
- production from metal ores and sulphur in 1,000 t of sulphur (1992):
Worldwide: 20,000
(all data from ULLMANN, 1994)
Emission figures (estimated):
Total emissions in Germany in 1986 were calculated at 2.3 x
106 tons approximately. Natural emissions in 1982 have
been estimated at 750 x 106 t worldwide, whereas
anthropogenic emissions amounted to about 100 x 106 t
(RÖMPP, 1988).
Toxicity
| Humans: | 25 µg/m3 (annual average) | increased frequency of diseases of lower respiratory tract (acc. UN-ECE, 1984) |
| 225 µg/m3 (annual average) | increased frequency of respiratory symptoms; reduced pulmonary function in | |
| 5 year olds (acc. UN-ECE, 1984) | ||
| 200 µg/m3 (daily max., 30 min) | significant increase in pseudocroup in children (acc. AFRL, 1987) | |
| 200 µg/m3 (24 h values) | increased mortality amongst elderly people (acc. AFRL, 1987) | |
| 1.3 mg/m3 (40 min) | constriction of the respiratory tract amongst people suffering from asthma (acc. AFRL, 1987) | |
| 53.3 mg/m3 (10-30 min) | severe, extremely unpleasant irritation symptoms (acc. DFG, 1988) | |
| 133.2 mg/m3 (60 min) | severe irritation of mucous membranes, pulmonary haemorrhage and oedema, | |
| laryngospasm with danger of asphyxiation (acc. DFG, 1988) |
| Mammals: | ||
| Mouse: | LC50 346 mg/m3 (24 h) | acc. DFG, 1988 |
| Mouse: | LC 1,598 mg/m3 ( 5 h) | acc. DFG, 1988 |
| Mouse: | LC 2,130 mg/m3 (20 min) | acc. DFG, 1988 |
| Rabbit: | LC50 679 mg/m3 (24 h) | acc. DFG, 1988 |
| Rabbit: | LC (after 7 d) 2130 mg/m3 (1 h) | acc. DFG, 1988 |
| Hamster: | LC 1,065 mg/m3 (6 h) | acc. DFG, 1988 |
| Guinea pig: | LC50 1,076 mg/m3 (24 h) | acc. DFG, 1988 |
| Insects: | LC 2 vol% (6 h) | acc. RÖMPP, 1988 |
| Flora: | ||
| Various species | >20 µg/m3 (annual average, visible damage) | acc. AFRL, 1987 |
| Fir | 30-40 µg/m3 (annual average, damage) | acc. VDI, 1978 |
| Fir | 50-70 µg/m3 (annual average, severe damage) | acc. VDI, 1978 |
| Cultivated plants | 50 µg/m3 (90 d, damage) | acc. DFG, 1988 |
| Pines (Ruhr area) | >80 µg/m3 (average, vegetation period, initial damage) | acc. VDI, 1978 |
| Various species | 2.7-5.5 mg/m3 (a few hours, acute damage)1) | acc. ULLMANN, 1984 |
Sensitivity of higher plants (UBA, 1980):
| very sensitive: | Bean | Blackcurrant | Sweet pea | Walnut |
| Douglas fir | Clover | Spinach | ||
| Pea | Lupin | Gooseberry | ||
| Fir | Lucerne | Pine | ||
| sensitive: | Linden | Pine | Oats | Bean |
| Copper beech | Weymouth pine | Rye | Rape | |
| Apple | Larch | Wheat | ||
| Hazelnut | Barley | Lettuce | ||
| less sensitive: | Maple | Potato | Plane | Tomato |
| Beech | Cabbage | Plum family | Juniper | |
| Yew | Leek | Rhododendron | Willow | |
| Oak | Arborvitae | Robinia | Vine | |
| Strawberry | Maize | Turnip | ||
| Aldo | Carrot | False cypress | ||
| Lilac | Poplar | Black pine |
Note: 1) Leaf necrosis, inhibited photosynthesis
Characteristic effects:
Humans/mammals: Keratitis, breathing difficulties, inflammation of respiratory organs and irritation of eyes due to the formation of sulphurous acid on the moist mucous membranes. Disturbances of consciousness, pulmonary oedema, bronchitis, heart failure and circulatory collapse. Similar effects with sulphur trioxide (SO3)
Plants: Visible damage to parts of plants above ground level due to direct action: SO2 enters the leaves via the stomata. It physiologically and biochemically impairs the photosynthesis, the respiration and the transpiration due to its detrimental effect on the pore aperture mechanism. Indirect damage is above all due to soil acidification (damage to mycorrhiza) and results in stunted growth.
ENVIRONMENTAL BEHAVIOUR
Water:
Sulphur dioxide ingresses into surface water and groundwater
through dry and wet deposition. The aqueous solution reacts as a
strong acid. In Germany, SO2 is classed as hazardous
to water as are sulphuric acid and sulphurous acid.
Air:
Sulphur dioxide binds moisture from the air and forms aerosols of
sulphuric and sulphurous acid which are deposited as acid rain.
The aerosol formation and its dwell time in air depend on the
meteorological conditions and on the presence of catalytic
impurities in the air. The average dwell time in the atmosphere
is approx. 3 - 5 days. Thus, sulphur dioxide may also be
transported over long distances.
Soil:
Dry and wet depositions from the atmosphere are the major
sources of sulphate accumulation in soil. Dry deposition
particles chiefly consist of (NH4)2SO4,
(NH4)3H(SO4)2, CaSO4,
MgSO4 with a small percentage of organic sulphur
compounds.
SO2 and its transformation products are the major sources of soil acidification, particularly if the buffer system of the soil is incapable of neutralising the acid which is either directly deposited or produced by the conversion of solid sulphates. The damage is not substance-specific. Almost all reactions in soil depend on the pH: Both the desorption of numerous substances with adverse effects as well as the leaching of nutrients increase with the acidification of the soil.
Degradation, decomposition products, half-life:
As described above (see Air, Soil ), sulphur
dioxide is readily oxidised and very reactive. Sulphuric and
sulphurous acid are the most important reaction products relevant
to the environment.
Synergisms/antagonisms:
Numerous experiments have been performed in this field
generally under standardised conditions. It is however not
possible to provide quantitative information relating to natural
circumstances on account of the complexity of the factors
involved and the courses of action concerned. Nonetheless, it is
certain that the effect of SO2 is more than additively
enhanced in combination with other pollutant gases such as NOx
or HF.
ENVIRONMENTAL STANDARDS
| Medium/acceptor | Sector | Country/organ. | Status | Value | Cat. | Remarks | Source |
| Air: | CDN | (L) |
0.06 mg/m3 |
acc. DORNIER, 1984 | |||
| CDN | (L) |
0.06 mg/m3 |
annual-average | acc. DORNIER, 1984 | |||
| CDN | (L) |
0.3 mg/m3 |
24 h | acc. DORNIER, 1984 | |||
| CDN | (L) |
0.9 mg/m3 |
1 h | acc. DORNIER, 1984 | |||
| CH | (L) |
0.03 mg/m3 |
annual-average | acc. WEIDNER, 1986 | |||
| CH | (L) |
0.4 mg/m3 |
24 h | acc. DORNIER, 1984 | |||
| CH | (L) |
0.26 mg/m3 |
1 month | acc. DORNIER, 1984 | |||
| CH | (L) |
0.7 mg/m3 |
2 h | acc. DORNIER, 1984 | |||
| CS | (L) |
0.15 mg/m3 |
24 h | acc. DORNIER, 1984 | |||
| CS | (L) |
0.5 mg/m3 |
30 min | acc. CES, 1985 | |||
| D | L |
0.14 mg/m3 |
IW1 | 1 y arith. mean | acc. TA Luft, 1986 | ||
| D | L |
0.40 mg/m3 |
IW2 | 1 y 4) | acc. TA Luft, 1986 | ||
| D | L |
1 mg/m3 |
MIK | 30 min | acc. BAUM, 1988 | ||
| D | L |
0.3 mg/m3 |
MIK | 24 h | acc. BAUM, 1988 | ||
| D | L |
0.1 mg/m3 |
MIK | 1 y | acc. BAUM, 1988 | ||
| D | G |
0.05-0.06 mg/m3 | Precautions for low pollution areas | UBA, 1989 | |||
| DDR | (L) |
0.15 mg/m3 |
24 h | acc. DORNIER, 1984 | |||
| DDR | (L) |
0.5 mg/m3 |
30 min | acc. DORNIER, 1984 | |||
| DK | (L) |
0.14 mg/m3 |
1 y | acc. WEIDNER, 1986 | |||
| E | (L) |
0.065 |
1 y | acc. WEIDNER, 1986 | |||
| EC | G |
0.1- 0.15 mg/m3 | 24 h | EC, 1980 | |||
| EC | G |
0.04-0.06 mg/m3 | 1 y | EC, 1980 | |||
| EC | G |
0.08 mg/m3 |
1 y > 403) | EC, 1980 | |||
| EC | G |
0.12 mg/m3 |
1 y <= 403) | EC, 1980 | |||
| EC | G |
0.13 mg/m3 |
1 dwinter > 603) | EC, 1980 | |||
| EC | G |
0.18 mg/m3 |
1 dwinter <= 603) | EC, 1980 | |||
| EC | G |
0.25 mg/m3 |
1 y > 1503)4) | EC, 1980 | |||
| EC | G |
0.35 mg/m3 |
1 y <= 1503)4) | EC, 1980 | |||
| F | (L) |
as EC |
1 y | acc. WEIDNER, 1986 | |||
| GB | (L) |
as EC |
1 y | acc. WEIDNER, 1986 | |||
| GR | (L) |
as EC |
1 y | acc. WEIDNER, 1986 | |||
| H | (L) |
1.15 mg/m3 |
24 h protected areas | acc. DORNIER, 1984 | |||
| H | (L) |
1 mg/m3 |
30 min protected areas | acc. DORNIER, 1984 | |||
| H | (L) |
0.5 mg/m3 |
24 h specially protected areas | acc. DORNIER, 1984 | |||
| H | (L) |
0.5 mg/m3 |
30 min specially protected areas | acc. DORNIER, 1984 | |||
| I | (L) |
as EC |
1 y | acc. WEIDNER, 1986 | |||
| I | (L) |
0.38 mg/m3 |
24 h | acc. DORNIER, 1984 | |||
| I | (L) |
0.75 mg/m3 |
30 min | acc. DORNIER, 1984 | |||
| IL | (L) |
0.26 mg/m3 |
24 h | acc. DORNIER, 1984 | |||
| IL | (L) |
0.78 mg/m3 |
30 min | acc. DORNIER, 1984 | |||
| IRL | (L) |
as EC |
1 y | acc. WEIDNER, 1986 | |||
| J | (L) |
0.11 mg/m3 |
24 h/1 y | acc. DORNIER, 1984 | |||
| J | (L) |
0.29 mg/m3 |
1 h | acc. DORNIER, 1984 | |||
| COL | (L) |
0.07 mg/m3 |
1 y | acc. DORNIER, 1984 | |||
| L | (L) |
as EC |
1 y | acc. WEIDNER, 1986 | |||
| N | (L) |
0.025-0.06 mg/m3 |
1 y | acc. WEIDNER, 1986 | |||
| N | (L) |
0.2 mg/m3 (+2%) |
24 h | acc. DORNIER, 1984 | |||
| N | (L) |
0.4 mg/m3 +2% |
1 h | acc. DORNIER, 1984 | |||
| NL | G |
0.075 mg/m3 |
1 y 50% of 24 h av. | acc. WEIDNER,1986 | |||
| NL | G |
0.20 mg/m3 |
1 y 95% of 24 h av. | acc. UBA, 1980 | |||
| NL | G |
0.25 mg/m3 |
1 y 98% of 24 h av. | acc. WEIDNER,1986 | |||
| NL | 0.15 mg/m3 |
1 y | acc. DORNIER, 1984 | ||||
| NL | 0.3 mg/m3 (+2%) |
24 h +2% | acc. DORNIER, 1984 | ||||
| NL | 0.5 mg/m3 |
24 h +0.3%; 1 d/y | acc. DORNIER, 1984 | ||||
| PL | 0.075 mg/m3 |
24 h specially protected areas | acc. DORNIER, 1984 | ||||
| PL | 0.25 mg/m3 |
20 min specially protected areas | acc. DORNIER, 1984 | ||||
| Emiss. | D | L |
0.5 mg/m3 |
mass flow > 5 kg/h5) | acc. TA Luft, 1986 | ||
| Workp | D | L |
5 mg/m3 |
MAK | acc. DFG, 1994 | ||
| PL | 0.35 mg/m3 |
24 h protected areas | acc. DORNIER, 1984 | ||||
| PL | 0.9 mg/m3 |
20 min protected areas | acc. DORNIER, 1984 | ||||
| RU | 0.25 mg/m3 |
24 h | acc. DORNIER, 1984 | ||||
| RU | 0.75 mg/m3 |
20 min | acc. DORNIER, 1984 | ||||
| S | 0.06 mg/m3 |
1 y | acc. DORNIER, 1984 | ||||
| S | 0.75 mg/m3 |
1 h | acc. DORNIER, 1984 | ||||
| S | 0.10 mg/m3 |
Oct. to March | acc. DORNIER, 1984 | ||||
| S | 0.30 mg/m3 |
24 h | acc. DORNIER, 1984 | ||||
| SF | (L) |
0.04 mg/m3 |
1 y | acc. WEIDNER, 1986 | |||
| SF | (L) |
0.25 mg/m3 |
24 h | acc. DORNIER, 1984 | |||
| SF | (L) |
0.7 mg/m3 |
30 min | acc. DORNIER, 1984 | |||
| SU | (L) |
0.05 mg/m3 |
24 h resid. Areas | acc. DORNIER, 1984 | |||
| SU | (L) |
0.5 mg/m3 |
30 min resid. Areas | acc. DORNIER, 1984 | |||
| TU | (L) |
0.15 mg/m3 |
24 h resid. Areas | acc. DORNIER, 1984 | |||
| TU | G |
0.30 mg/m3 |
24 h industrial areas | acc. DORNIER, 1984 | |||
| USA | (L) |
2 ppm |
TWA | acc. ACGIH, 1986 | |||
| USA | (L) |
5 mg/m3 |
TWA | acc. ACGIH, 1986 | |||
| USA | (L) |
5 ppm |
STEL | acc. ACGIH, 1986 | |||
| USA | (L) |
10 mg/m3 |
STEL | acc. ACGIH, 1986 | |||
| WHO | G |
0.1-0.15 mg/m3 |
24h1) | WHO, 1979 | |||
| WHO | G |
0.04-0.06 mg/m3 |
1 y | WHO, 1979 | |||
| WHO | G |
0.5 mg/m3 |
10 min2) | WHO, 1987 | |||
| WHO | G |
0.35 mg/m3 |
1 h2) | WHO, 1987 | |||
| WHO | G |
0.125 mg/m3 |
24 h2) | WHO, 1987 | |||
| WHO | G |
0.05 mg/m3 |
1 y2) | WHO, 1987 | |||
| YU | (L) |
0.15 mg/m3 |
24 h | acc. DORNIER, 1984 | |||
| YU | (L) |
0.5 mg/m3 |
30 min | acc. DORNIER, 1984 | |||
| Water: | D | G |
WGK 1 |
acc. ROTH, 1989 |
Notes:
1) Mean value, max. 7 exceedings per annum
2) Recommendations for Europe
3) Given suspended-dust content (in µg/m3); median values
4) 98% value of cumulative frequency of all daily mean values in year
5) SO2 and SO3, stated as SO2
VALUES STIPULATED IN REGIONAL SMOG ORDERS IN GERMANY
| State | Advance warning | Stage 1 | Stage 2 |
| B1) | SO2 + 1.3 x Suspended dust > 1.1 mg/m3 or SO2 > 0.60 mg/m3 |
SO2 + 1.3 x Suspended dust > 1.4 mg/m3 or SO2 > 1.20 mg/m3 |
SO2 + 1.3 x Suspended dust >1.7 mg/m3 or SO2 > 1.80 mg/m3 |
| B2) | SO2 + 1.3 x Suspended dust > 1.1 mg/m3 |
SO2 + 1.3 x Suspended dust >1.4 mg/m3 |
|
| HH3) | SO2 + 2.0 x Suspended dust > 1.1 mg/m3 or SO2 > 0.60 mg/m3 |
SO2 + 2.0 x Suspended dust > 1.4 mg/m3 or SO2 > 1.20 mg/m3 |
SO2 + 2.0 x Suspended dust >1.7 mg/m3 or SO2 > 1.80 mg/m3 |
| HH4) | SO2 + 2.0 x Suspended dust > 1.1 mg/m3 |
SO2 + 2.0 x Suspended dust >1.4 mg/m3 |
Lower Saxony 5); North-Rhine Westphalia; Hesse; Rhineland Palatinate; Saarland 5); Baden-Württemberg and Bavaria 5): all values as for Hamburg (HH); Federal States marked with superscripts have different methods of determining the limit values (refer to appropriate notes).
Notes:
1.3 x or 2.0 x = factors by which the suspended dust is multiplied
1) Berlin: averaged over 21 h and in last 3 h
2) Berlin: continuously over 72 h (mean values over 21 h)
3) Hamburg: averaged over 24 h and in last 3 h
4) Hamburg: continuously over 72 h (mean values over 24 h)
5) Averaged over 24 h / continuously over 72 h (mean values over 24 h)
Comparison/reference values
The annual average in Germany is between 0.01 and 0.08 mg/m3. Because of favourable meteorological conditions, the flat lands of Northern Germany only reveal an annual average of 0.01 - 0.02 mg/m3. Similar values are found in the hilly region of Southern Germany and in the Alps. Higher values - between 0.06 and 0.08 mg/m3 - are found in conurbations such as the Ruhr and Rhine/Main areas or in Berlin. On the eastern boundaries of Germany, emissions from regional sources (primarily from Poland and Czechoslovakia) make a considerable contribution to the SO2 concentration in these areas. Occasionally, the concentration in these rural areas reaches up to 2 mg/m3 and thus attains the alarm levels stipulated in the Regional Smog Orders [UBA, 1989].
Approximate values for mean SO2 immissions (annual averages) [SRU, 1988]:
| "Clean-air zones" | 0.005 mg/m3 |
| Rural areas | 0.005 - 0.04 mg/m3 |
| Conurbations | 0.03 - 0.1 mg/m3 |
| Urban areas | 0.14 mg/m3 |
The typical short-term impact (98 percentile of half-hour values) in conurbations is between 0.2 and 0.3 mg/m3. In the most polluted areas, individual stations recorded values of 1.2 mg/m3 (Bottrop, 1982) and even 1.7 mg/m3 (Lünen-Brambauer, 1981).
Assessment/comments
The laboratory animals used to date for toxicity experiments are obviously far less sensitive to sulphur dioxide than humans. The most sensitive animal species (guinea pig) withstands - even over long periods - concentrations which are already intolerable to humans in the short term (DFG, 1988).
Sulphur dioxide is one of the chemicals for which there is a wealth of legislation. Limit and approximate values with differing points of reference are available from numerous countries. In comparison to the values for numerous other substances, the figures for SO2 are subject to relatively rapid change.
When comparing the extensive range of values available, it is important to take account of the method of calculation (median, arithmetic mean, time frame, percentile etc.). The listed values for the Netherlands and the EC are good examples.
UBA (1980) compares technical installations, provides information on the sulphur content of raw materials from various countries and outlines different scenarios.
We have to distinguish between SO2 produced for industrial processes (e.g. production of sulphuric acid) and SO2 which is emitted. Although most of the SO2 is of natural origin, care should be taken to reduce the emissions caused by humans, especially those from combustion processes.