Sulphur Dioxide :: Neutral Grassland

Impact Type: Direct exposure to pollutant

Key Concerns:

Relatively little information is available on exposure effects.

Key concerns are:

  • Visible decline symptoms for example, leaf discolouration.
  • Stimulated growth at low concentrations of S potentially changing community composition.
  • The vulnerability to direct damage of mosses, liverworts and lichens which are sensitive to lower concentrations than those causing injury to higher plants.

Grass spp. have been shown to evolve tolerance to SO2 in a short period of time at polluted sites. This however has led to reduced growth in clean air compared with sensitive genotypes (UKCLAG 1996).

Risk Areas

The concentrations of SO2 in the UK are well below the critical level with the largest concetrations in the urban and suburban areas of the country, with values of a few ppb, and in the rural Midlands and home counties, with concentrations of 1 to 2 ppb (2.7 - 5.4 ug  m-3). Elsewhere, in rural areas and especially in the west and north of the UK, SO2 concentrations are low at below 0.5 ppb (<1.3 ug SO2 m-3). Emissions from UK shipping are highest close to major ports where elevated concentrations may occur locally (ROTAP, 2012).

See also Acid deposition :: Neutral grasslands

Critical Load/Level: 
Habitat/ Ecosystem Type Critical Load/ Level Status Reliability Indication of exceedance Reference
Forests and semi-natural vegetation

20 µg SO2 m-3 annual mean and half-year(Oct-March) mean

 UNECE, 2004 quite reliable i.e. the results of some studies are comparable

Low temperature appears to enhance the negative effects of SO2, and the lower critical level of 15 µg SO2 m-3 is used where the effective temperature sum (ETS) (i.e. the sum of temperatures) above 5oC is below 1000 degree days (d.d) (Ashmore et al., 1994).

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Cyanobacterial lichens

10 µg SO2 m-3 annual mean

 UNECE, 2004 quite reliable i.e. the results of some studies are comparable

SO2 dissolves in water to produce acidic ions which are readily absorbed through the lichen thalli disrupting photosynthesis. SO2 has also been shown to inhibit the activity of nitrogenase, which is used by cyanobacterial photobionts to fix atmospheric nitrogen.

Reference: Gries, C. (2008). Lichen sensitivity to air pollution, Chapter 13 in Nash, TH, III (ed.) Lichen Biology (2nd. ed.), Cambridge University Press, Cambridge.

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References: 
Ashmore, M.R.; Wilson, [Eds.] R.B. 1994 Critical levels of air pollutants for Europe Report of the Egham workshop
Defra, 2012 RoTAP: Review of Transboundary Air Pollution: Acidification, Eutrophication, Ground Level Ozone and Heavy Metals in the UK
Gries, C. 2008 Lichen sensitivity to air pollution Lichen Biology
UKCLAG, 1996 Critical levels of air pollutants for the United Kingdom