Diluting black rain
THERE has been a reduction in the emissions of the oxides of sulphur and nitrogen, but the observed decrease in the acidic content of precipitation is less than expected.
According to a recent study, the decrease hasn't come about because of the decline in the atmospheric concentration and deposition of "cations" -- positively charged atoms -- of calcium, magnesium, potassium, and sodium, which chemically neutralise acid rain (Nature, Vol 367, No 6461).
Acid rain occurs when the oxides of sulphur and nitrogen, which are emitted when fossil-fuels like oil are burned, dissolve in the atmospheric moisture which condenses into rain. Acid rain leads to the acidification of streams and lakes, affecting aquatic plants and animals, sometimes even killing off fish stocks.
It has also been implicated in the destruction of forests, although acid rain expert Eville Gorham of the University of Minnesota holds that the deleterious effects of acid rain "on terrestrial ecosystems are less clear, but may be substantial".
The effects of acid rain are diluted by cations present in flyash from urban and industrial smokestacks, particles of wind-borne soil and cement dust, all of which are alkaline in nature. Ammonia from nitrogenous fertilisers also finds its way into the atmosphere, and this too can neutralise acid rain.
But according to Lars O Hedin and his colleagues from Cornell University who undertook an analysis of cation concentrations, there has been a sharp decline in concentrations over the past 10 to 26 years. In the Netherlands, for example, there was a 32 per cent decrease in cation concentration from 1978 to 1987; in Sweden, the decrease was as much as 74 per cent from 1971 to 1989. In the US, at the Hubbard Brook Experimental Forest where atmospheric chemical constituents are measured, a decline of 48 per cent from 1965 to 1989 was observed.
Hedin and his colleagues found that the decrease in cation concentrations is linked to a decline in the emissions of particulate matter from regional urban and industrial sources, and emissions from non-point sources such as unpaved roads. In Sweden, such emissions were reduced by as much as 80 per cent from 1960 to 1980, and in the Netherlands, concern for pollution led to a 63 per cent reduction in dust emissions from 1970 to 1990.
Ironically, efforts to check particulate emissions, says Gorham, have lead to an increase in acid rain, and environmentalists will now have to call for a yet more stringent regulation of sulphur dioxide -- the main acid rain culprit.