Tuesday, August 26, 2008

Something's in the air


Something's in the air

Importance of the invisible gases all around you


Our atmosphere comprises the gases and water vapour that extend from the Earth's surface all the way into space and is an essential for life on Earth.

Our atmosphere is held around the Earth by the Earth's gravity. The fact that we have an atmosphere makes the Earth's temperature fairly stable. Without it, Earth would either be extremely hot or extremely cold. The atmosphere (mainly carbon dioxide) traps some of the sun's heat reflected off the Earth's surface, meaning that, at night, although it does get cooler, it doesn't become inhospitable.

It also manages to block out many of the sun's harmful ultraviolet rays that would cause irreparable damage to living tissues. It also protects us from the constant bombardment of meteorites that mostly burn up as they enter the atmosphere.

The reason they do this is because of pressure and heat. If you have ever put your thumb over the top of a bicycle pump and pumped, you will notice that it gets hot very quickly. This is because you are compressing the air in the tube.

The same thing happens with meteorites. As they are moving so fast, often tens of times faster than a bullet, the air beneath them cannot get out of the way and it quickly becomes compressed. As it becomes compressed, the pressure builds up, which, in turn, produces enormous heat, and the meteorite breaks up. Thankfully!

Composition of atmosphere

It may surprise many of you to know that oxygen does not make up the majority of the Earth's atmosphere. Nitrogen is in the highest concentration in the atmosphere, making up approximately 78 percent of the mixture of gases we call air. Oxygen is second in concentration, at about 21 percent. Argon, an inert (non-reactive) gas, makes up about one percent, and carbon dioxide makes up 0.04 percent.

With all the controversy about the greenhouse effect, it may seem strange that there is only a small quantity of carbon dioxide, but about a hundred years ago, it was approximately 0.03 percent, and the rate of rise to 0.04 percent was enough to potentially cause a global catastrophe. You also have to consider that 0.04 percent of a quantity the size of the atmosphere is also quite a lot!

As you may have noticed, the above figures add up to over 100 percent, and I haven't even mentioned the other gases, such as sulphur dioxide, methane, nitrous oxide, ozone and carbon monoxide, which are all present, but in extremely low concentrations. The above concentrations of the main gases are rounded to their nearest whole number, but they also fluctuate slightly, depending on varying factors.

Life on Earth has evolved to use these gases in our atmosphere, with all higher organisms (plants and animals) using oxygen for respiration, and plants, protists and certain bacteria using the carbon dioxide for photosynthesis. Strangely though, no higher organisms have properly been able to utilise the nitrogen component of air, despite it being essential to making DNA and proteins. For that, we rely on bacteria to convert it into usable compounds to be absorbed by plants.

Like layers in a cake

Essentially, the atmosphere has four main layers. The only one you and I will ever experience, unless we start holidaying in space, is the troposphere. This layer comprises about 80 percent of the mass of the atmosphere and extends from the Earth's surface to about seven to 20km above it.

Due to the heating of the air from the Earth's surface, hot air rises as cold air sinks, which creates pressure and temperature differences, and this, coupled with moisture variation, creates weather, meaning the troposphere is where predominantly all our weather happens.

Above the troposphere is the stratosphere, which stretches from the upper limit of the troposphere to about 50km. The stratosphere is where an allotrope (different form) of oxygen exists, known as ozone (O3). Although very scarce compared to the other gases in the atmosphere, in the stratosphere it gathers in "relative" abundance and forms the ozone layer.

The use of chlorofluorocarbons (CFCs) in aerosol cans since the 1970s led to a major depletion of ozone in our atmosphere. This led to ozone holes forming above the poles. Ozone is responsible for blocking much of the ultraviolet rays from the sun reaching Earth, but there is still a hole over Antarctica and Australia.

Obviously, Antarctica is not inhabited, but Australia has the highest rate of skin cancer in the world, and this is probably because of the amount of harmful UVB rays passing through the Earth's atmosphere where there is no ozone preventing them.

Stretching into space

Above the stratosphere is the mesosphere, which extends from 50km to about 80km. This is the layer where most meteorites burn up in our atmosphere, which we see as "shooting stars". In the mesosphere, temperature decreases with altitude and this is where the lowest temperatures of our planet are recorded: about -1000C.

The furthest atmospheric level from the Earth's surface is the thermosphere, which, from 80km to 550km, contains the ionosphere. Here, the sun's energy is intense enough to break air atoms and molecules into ions (charged particles).

Due to this ionization, it is where auroras take place, and it allows radio waves to be "bounced" off it, allowing radio communication to take place over long distances. It also protects the Earth from solar wind emitted from the sun, especially during periods of high solar activity. These solar winds can have dramatic effects on our radio communication systems.

The upper part of the thermosphere, from 550km to about 10,000km, is known as the exosphere and is where the atmosphere thins out completely and merges into space.

The concentration of molecules here is so limited, however, that it all would seem like space. In reality, you are in space in the lower thermosphere, above the Ka'rma'n line, at about 100km above the Earth's surface, which is commonly regarded as the official boundary of atmosphere and space. I really hope I get there one day!

David Canavan has an MSc in Behavioural Ecology and teaches science, maths and ICT at Garden International School. David is fascinated by science and loves animals, especially the dangerous kind; the more dangerous the better. You may contact David at davidc@gardenbangkok.com

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