|Austrian Bio-Indicator Grid – Chlorine|
Hydrogen chloride forms during thermal recycling of residual materials (e.g. waste incineration, cement mills, paper manufacture…), in the metalworking industry (e.g. galvanising plants, printed circuit board manufacture, steel production and –coating) and in the chemical industry. HCl is less phytotoxic than hydrogen fluoride and sulphur dioxide, but ten times as toxic as the NaCl-Aerosol.
HCl acts as an acid and causes a lowering of the cytosolic pH. This leads to a damage of the chloroplasts, to plasmolysis, as well as to impaired assimilation and disturbed metabolism. Furthermore, the following symptoms occur: cell wall deformation, destruction of the cytoplasmic membrane, destruction of the mesophyll after uptake through the stomata (in conjunction with a deep brown discolouration), marginal chlorosis as well as apical and marginal necroses of the leaves, discolourations and acid damage, and in the case of conifers damage to the needle tips. The detection is carried out via plant analysis, threshold values are specified in the Austrian forest law “Zweite Verordnung gegen Forstschädliche Luftverunreinigungen, BGBl. Nr. 199/1984” for the tree species spruce (0,1% Cl) and beech (0,1% Cl).
Chlorine is not measured at all plots in the Austrian Bio-Indicator Grid, but the plots of the Bio-Indicator Grid are integrated into local measuring grids close to emitters. By means of that integration, the temporal development of the effects related to the chlorine impact in the investigated area can be extrapolated, using current data. At the moment, the chlorine impact around 30 Austrian emitters is monitored with local bio-indicator grids. Due to the increased thermal recycling of waste materials in the last few years, plant-analytical methods gained special importance for chlorine assay in the monitoring of industrial processes. These emitters mostly belong to the groups building materials (cement mills, clay production, brick works…), waste incineration/landfills and paper industry.
Results of the analyses of leaves and needles in 2005 (1010 samples)
Samples with an exceedance of the threshold value are highlighted in red
Chloride-containing road salts are used for keeping roads ice-free during the winter months. The uptake of chloride can occur directly by leaves and needles when a chloride-containing aerosol reaches the plant. Also, chloride-containing snowmelt can seep into the ground and is then taken up by roots, whereby it is transported to the assimilation organs by the sap flow. Excess chloride inhibits transpiration and above all, disturbs the osmotic balance. Marginal leaf necrosis and premature leaf abscission occur in deciduous trees, coniferous trees tend to develop apical needle necrosis. Because of the high mobility of chloride in the ground it can easily be washed out, which is why the best time for sample collection is in spring, during shoot development. In the young shoots, the highest contents are found, so that values up to 0.5-1% chloride are possible.
The samples taken from the plots of the Austrian Bio-Indicator Grid are not suitable for a representative assessment of the influence of road salts. That is because the relevant impact is limited to a very small area (only a few 100 meters on both sides of the road) and because the sample collection period of the Bio-Indicator Grid is in autumn. To assess the influence of road salts, local measuring grids are necessary. At the moment around 30 cases per year are investigated with the aid of such locally gathered data.
Salt damage along the road