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Sulphur dioxide

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 -10C (liquid); 2.93 g/l at 20C (gas)
Relative gas density: 2.26
Boiling point: -10C
Melting point: -75.5C
Vapour pressure: 331 kPa at 20C, 462 kPa at 30C, 842 kPa at 50C
Odour threshold: 0.3 - 1 ppm (in air)
Solvolysis/solubility: in water: 112.7 g/l at 20C (1013 mbar);
228.3 g/l at 0C (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 (RMPP, 1988).

Toxicity

Humans: 25 g/m3 (annual average) increased frequency of diseases of lower respiratory tract (acc. UN-ECE, 1984)
  225 g/m3 (annual average) increased frequency of respiratory symptoms; reduced pulmonary function in
    5 year olds (acc. UN-ECE, 1984)
  200 g/m3 (daily max., 30 min) significant increase in pseudocroup in children (acc. AFRL, 1987)
  200 g/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. RMPP, 1988
Flora:
Various species >20 g/m3 (annual average, visible damage) acc. AFRL, 1987
Fir 30-40 g/m3 (annual average, damage) acc. VDI, 1978
Fir 50-70 g/m3 (annual average, severe damage) acc. VDI, 1978
Cultivated plants 50 g/m3 (90 d, damage) acc. DFG, 1988
Pines (Ruhr area) >80 g/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 g/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-Wrttemberg 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 (Lnen-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.


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