Welcome to Science Questions Answered! A reader contacted me, asking about acid rain, a threat that was very well known in the 1980s. They said:

“Twenty or thirty years ago there was much concern about acid rain and whether it would destroy our forests (particularly, if I remember right, in the northern hemisphere). It occurred to me the other day that we have not heard anything about acid rain for a very long time.  Do you think you could find out if this problem has gone away? I should be most interested.”

What is acid rain?

Acid rain happens when chemical emissions – chiefly sulphur dioxide (SO2) and nitrogen oxides (NOx) from the burning of fossil fuels in factories, power stations (two thirds of SO2 and one quarter of NOx come from electric power generators) and cars – react with water, oxygen and other substances that are present in the atmosphere to create sulphuric and nitric acids which then fall back down as acidic rain, snow, sleet, hail or fog.

The effects of acid rain

Acid rain leaches aluminium (and calcium, meaning that plants are deprived of this crucial nutrient) from clay particles in the soil. If this aluminium enters lakes or streams, it and the acid combined can kill wildlife. In addition, aluminium can sometimes be harmful to plants and trees, and, in high places, acidic fog and clouds can strip trees’ foliage of nutrients, creating areas of devastation where trees have brown and dead leaves and needles. This makes the trees less capable of absorbing sunlight, which causes them to be vulnerable and less able to endure icy temperatures.

So, is acid rain still a problem? The short answer is yes, but there is more to it than that. Although acid rain continues to be problematic, according to the Swedish NGO Secretariat on Acid Rain, land-based sulphur emissions have plummeted from 53 million tonnes in 1980 to 14 million tonnes in 2003 – possibly because people have become more aware of the fact that too much sulphur dioxide and nitrogen oxides are being emitted into the atmosphere, and are taking measures to address the problem. However, China, Russia and the USA (all very industrial places) still suffer from acid rain. Unfortunately, air pollution can travel long stretches of land and water, carried by the wind, so other countries are sometimes affected as well. Acid rain hasn’t quite been solved because of three key reasons:

  • The revitalisation of lakewater acidity levels has been gradual, and in cases has been going slower than had been hoped by some researchers;
  • The recovery of ecosystems that have been affected by acid rain is progressing extremely slowly, and this has been exacerbated by other pressures such as climate change;
  • There have been unexpected effects on the environment.

So, the problem of acid rain hasn’t been solved yet, and still occurs in some countries, although much less often.

I hope you have found this post interesting, and have a merry Christmas!


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