Some decades ago, a cri de coeur went out from foresters around the world. They did not use the words of Tithonus in Tennyson's poem, but their urgent message was identical:
The woods decay, the woods decay and fall,The vapours weep their burthen to the ground.
They had observed that many of their evergreen trees were losing needles, turning brown, and dying, and soon the identity of the culprit was discovered: it was acid rain, a deadly cocktail whose other victims included lakes and streams, the fish that they contained, and many ancient buildings. Rain becomes harmfully acidic when the air is polluted with sulphur and nitrogen oxides, by-products of the combustion of fossil fuels and some industrial processes. Chemical reactions in the atmosphere, often brought about by the effect of sunlight, transform these substances into sulphuric and nitric acids, which are then washed from the air to fall to earth as acid rain.
Although not directly harmful to humans to any significant extent, it plays havoc with our natural and architectural environment.
Once the problem was identified, many governments took remedial action. Measures were introduced to curb the emissions of the harmful oxides, and it was assumed that once these had had time to take effect, a noticeable decrease in acid rain should be apparent. But it was not so, and it has taken a while to find the reasons.
It seems that the persistence of the acid rain may well be due to a deficit of alkaline dust. Dust particles from a variety of sources have a very low acidity. Particles produced, for example, by many industrial processes, by construction works, by soil erosion from the land, and by traffic travelling on untarred roads, have a tendency to be alkaline in nature.
Once in the air, these alkaline dust particles tend to neutralise acid rain in much the same way as antacids counteract the excess acid in an upset stomach. In a sense, when the acids and the alkalis combine, they cancel each other out. Now it seems that in recent years the falling concentrations of the acid rain pollutants have been mirrored by a corresponding fall in the production of "antacid" dust.
Controls have been introduced, for example, to improve air-quality standards by "scrubbing" particulate matters from smokestacks associated with the combustion of fossil fuels; efficiency in the husbandry of agricultural land has been improved; and there are even fewer roadways left unpaved.
We produce, it seems, less dust, and hence the disappointing results, so far, of attempts to reduce the harmful acidity of rain.