Air Quality Guidelines - World Health Organization Regional Office ...

228 chapter 10
basis of new field studies, it is recommended that an air quality guideline of
10 µg/m 3 annual mean (Table 31) be established for lichens (7–9).
In the 1987 edition, no consideration was given to direct impacts of acid
precipitation on above-ground plant organs. It is now recognized that mists
can contain solute concentrations up to ten times those of rain, and can thus
have a direct impact on vegetation. Since mists and clouds occur most
frequently at high altitudes, and are intercepted with particular efficiency by
forests, trees are likely to be the most sensitive receptors. Experiments on
young trees, backed up by field observations, show significant effects of
acid mists on leaf surface structure at pH 3.5, which is equivalent to
150 µmol/l sulfate. Because of the difficulties of measuring sulfate concentrations
in cloud water, a guideline has been set based on the equivalent
particulate sulfate concentration. A guideline of 1.0 µg/m 3 particulate sulfate
as an annual mean is recommended for trees where ground level cloud is
present 10% or more of the time (Table 31). This guideline only applies,
however, when calcium and magnesium concentrations in cloud do not
exceed hydrogen and ammonium ion concentrations, because no data
exist to establish a guideline under other conditions. This restriction
excludes areas such as the Mediterranean region, eastern Europe and the
Alps.
These guidelines do not take into account that sulfur dioxide increases
sensitivity to other stresses, with the exception of low temperatures for
forests and natural vegetation. Given further knowledge of its effects on
stresses such as drought, pathogens and pests, it is possible that the guidelines
may require further modification in the future. The 24-hour mean
guideline has been abolished, but this is on the basis of knowledge on higher
plants. The inclusion of lichens in these new guidelines may warrant future
considerations of a short-term guideline for these organisms, if knowledge
indicates the necessity for this. The new guideline for acid mists has similarly
been set for forests only, and the effects on other receptors may also
warrant future attention.
REFERENCES
1. Air quality guidelines for Europe. Copenhagen, WHO Regional Office
for Europe, 1987 (WHO Regional Publications, European Series,
No. 23).
2. JÄGER, H.J. ET AL., ED. Effects of air pollution on agricultural crops in
Europe. Brussels, European Commission, 1993 (Air Pollution Research
Report, No. 46).