This is quite straightforward to explain, so here goes...

Rainwater is naturally acidic even in unpolluted areas. The cause of this is the carbon dioxide in the atmosphere: this dissolves in water to make a weakly acidic solution ('carbonic acid').
As a consequence, rain has a pH close to 5.4 .

BUT, we have added to the acidity of rainwater in two important ways:

1. Burning fossil fuels like coal. Coal contains SULPHUR as an impurity. As the coal burns, so does the sulphur, which is converted into sulphur dioxide gas. This gas dissolves in water to make stronger acids, which then fall, as rain.

2. Vehicle engines get very hot when they are running - so hot that the NITROGEN from the atmosphere is converted to oxides of nitrogen. The story by now should be quite familiar! These oxide gases are released in exhaust fumes and dissolve in water to form acids, which reach us in rain.

So what's the problem?

Acid rain can be carried over large distances by winds affecting places far removed from its source. As lakes acidify, fish and small invertebrates are killed.

The acid rain dissolves nutrients in the soil, which are then leached out, making the soil infertile. This is killing large numbers of trees - conifers in particular.

Acid rain attacks the stonework of buildings and certain metal structures, costing millions of pounds to treat.

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