The Greenland and Antarctic ice cores contain a record of the composition of the atmosphere every year for at least eighty thousand years, the whole period of the existance of Modern Man. You can read about these ice cores on another Page. From these ice cores we know that for the past three hundred years Man has been releasing many different substances into the atmosphere.Some of these chemicals, such as mercury (present in the smoke from burning coal), and 1940s insecticides such as DDT, have now passed into the soil and water and every food chain, so almost all animals, from penguins in the Antarctic to polar bears in the Arctic, and including Man, now contain measurable amounts of mercury. However over the past fifty years there have been great advances in controlling these chemicals, and provided we are all vigilant they need not continue to be a long-term global threat. There are many other web sites which give more information, including campaigning environmental groups such as RSPB and Friends of the Earth.
There are however three main groups of substances with which Man is still continuing to pollute the atmosphere on a large scale.
The ozone layer is a layer of ozone very high up in the Earth's atmosphere. It absorbs much of the harmful radiation reaching us from the Sun and from space, preventing it from reaching the Earth's surface.
Certain chemicals, mainly CFCs (chloro-fluoro-carbons), destroy this high-level ozone, and the destruction of this layer is leading to higher levels of harmful radiation at the surface of the Earth. This in turn is leading to an increase in skin cancers and other conditions in Man, and a similar adverse effect on many other living organisms.
CFCs are a group of very stable gaseous and mostly non-toxic compounds of carbon, chlorine and fluorine. Because they are stable and non-toxic, since the 1930s they have been very widely used as the refrigerating agent in air conditioning units, refrigerators and freezers, and also in aerosols and in many other ways. A commonly used refrigerant has the trade name Freon, although modern forms of Freon do not contain CFCs. Their effect on the ozone layer was first discovered in the 1970s and since then their use has been almost completely phased out, throughout the whole World, but very large numbers of older refrigerators and freezers still contain them. In many countries, even Great Britain, many of these old units are being dumped without the refrigerant being removed and made safe.
CFC levels in the ozone layer are now falling, although because of their great stability (and also because of the amount that may yet be released from abandoned refrigerators) it is likely to be at least a hundred years before the ozone layer is free of them.
It should be noted that low-level ozone, that is, ozone in the lower levels of the atmosphere, is very poisonous to Man and most other organisms. Low-level ozone is sometimes produced by electrical machinery, and also by the action of sunlight on industrial haze and heavy concentrations of exhaust fumes from motor vehicles.
Acid rain is produced by oxides of nitrogen and sulphur reacting with water in the atmosphere under the influence of sunlight to produce nitric and sulphuric acids. Acid rain has a pH of about 2 or 3 and does very great damage to plants, lakes and rivers, and soil, and therefore indirectly to animals and other organisms, and also of course to buildings and other Man-made objects.
Oxides of sulphur are produced by the burning of substances containing sulphur, particularly coal. British coal-fired power stations currently produce very large quantities of sulphur dioxide which is carried by the prevailing winds to Scandinavia and Central Europe. It is possible to remove the sulphur dioxide from power station exhaust gases, although this is not always done. Natural gas and propane and butane (LPG) do not contain sulphur; petroleum (and therefore the various fuels and other substances made from it) contains some sulphur although the amount depends upon where the petroleum comes from. Oil shales contain low-grade oil deposits with high levels of sulphur. In the past these have not been developed but today the United States and Canada, and other countries, are developing them, even those in Wilderness Areas and National Parks, mainly in order to reduce their dependence on imported oil.
Nitrogen does not normally react with oxygen, but it may do so at very high temperatures, such as are produced in the cylinders of internal combustion engines. The oxides of nitrogen present in atmospheric pollution are produced almost entirely by motor cars and lorries, both petrol and diesel.
Small amounts of carbon dioxide, and also water vapour, are always naturally present in the atmosphere, and therefore even totally unpolluted air has a pH of about 6 - but this is not a cause of acid rain. This is also why, although pH indicator solution in a stoppered bottle is green (pH 7), pH paper is yellow (pH 6) because it is measuring the pH of the moist air surrounding it. This is further discussed on the Page on Calcium carbonate and the lime cycle.
Burning fossil fuels is raising the level of carbon dioxide in the atmosphere - this is discussed in the next Section. Whatever the other effects of this, there is no suggestion that it is changing the pH of the atmosphere in a way which is likely to have any environmental impact. However there are also vast amounts of carbon dioxide dissolved in the oceans and the oceans are becoming more acidic: this is affecting the ability of many marine organisms, particularly corals, to make their shells, and most of the World’s coral reefs are now under threat.
The Earth's atmosphere affects the way in which the Earth receives radiant heat energy from the Sun and radiates heat energy into space - this is discussed on another Page - and changes in the Earth's atmosphere change these. Over the past two hundred years carbon dioxide levels have been rising, and this is the major cause of global warming. We call carbon dioxide a greenhouse gas.
Many industrial processes produce carbon dioxide, for example making steel and other metals, and cement and concrete and chemical fertilisers, and there is much we can do to reduce this. But the biggest factor in rising carbon dioxide levels is the burning of fossil fuels such as oil and natural gas and particularly coal.
Another greehouse gas is methane. This is not normally present in the atmosphere but can be produced by many biological processes, particularly anaerobic respiration, when things decompose in places where there is very little air - in our gut for example, or in the mud at the bottom of a pond. We produce small amounts of methane every time we fart or belch, but Man has always done both and it has never made a contribution to global warming! But modern methods of farming, particularly modern methods of keeping cattle, are producing much more significant amounts of methane.
Rubbish dumps containing food and garden and agricultural waste produce large quantities of methane, but today more and more of this is being collected and used as a fuel.
In the atmosphere methane is oxidised to carbon dioxide but this may take several weeks and while it is in the atmosphere it is a far more potent greenhouse gas than carbon dioxide.
Huge amounts of methane are trapped under the Antarctic ice cap and in the permafrost (areas where the subsoil is frozen all the year round), and if the Earth’s temperature rises to the point at which this is released then the rate of global warming will increase dramatically.
© Barry Gray June 2016