Most of our atmosphere is made up of nitrogen (78% by volume) and oxygen (21% by volume). The remaining 1% of the atmospheric gases are known as trace gases because they are present in such small concentrations. The most abundant of the trace gases is the noble gas argon (approximately 1% by volume). Noble gases, which also include neon, helium, krypton and xenon, are very inert and do not generally engage in any chemical transformation within the atmosphere. Hydrogen is also present in trace quantities in the atmosphere, but because it is so light, over time much of it has escaped Earth's gravitational pull to space.
Despite their relative scarcity, the most important trace gases in the Earth's atmosphere are the greenhouse gases. Most abundant in the troposphere, these gases include carbon dioxide, methane, nitrous oxide, water vapour and ozone, so-called because they are involved in the Earth natural greenhouse effect which keeps the planet warmer than it would be without an atmosphere. Apart from water vapour, the most abundant greenhouse gas (by volume) is carbon dioxide. Despite being present in only 370 parts per million by volume of air, carbon dioxide and the other greenhouse gases help to keep the Earth 33°C warmer than it would otherwise be without an atmosphere. Through emissions of greenhouse gases however, mankind has enhanced with natural greenhouse effect which may now be leading to a warming of the Earth climate.
Whilst ozone behaves like a greenhouse gas in the troposphere, in the stratosphere where its abundance is most significant within the ozone layer, it helps to filter out the incoming ultraviolet radiation from the Sun, protecting life on Earth from its harmful effects. Air within the stratosphere is thin however. If all the ozone in the stratosphere was compressed to ordinary atmosphere pressure at ground level, it would occupy a layer only 3 mm thick.
Other trace gases in the atmosphere arise from natural phenomena such as volcanic eruptions, lightning strikes and forest fires. Gases from these sources include nitric oxide (NO) and sulphur dioxide (SO2). In addition to natural sources of nitric oxide and sulphur dioxide there are now many man-made sources, including pollutant emissions from cars, agriculture and electricity generation through the burning of fossil fuels. During the 20th century other man-made processes have put completely new trace gases into the atmosphere, for example the chlorofluorocarbons (CFCs) which damage the ozone layer.