Atmospheric Pollutants

You should have already read about carbon dioxide and methane, two of the greenhouse gases leading to global warming. In addition to these two gases there are also a  few other gases you need to know about.

Carbon Monoxide

This gas is produced when carbon based fuels (petrol, coal, wood, gas...) burn without enough oxygen. This process is called incomplete combustion, and is very dangerous to humans. Ordinarily carbon based fuels will produce carbon dioxide (bad) and water vapour (OK), as products of combustion. However, if there isn't enough oxygen available then carbon monoxide is made. This is a colourless, odourless gas that takes the place of oxygen in our red blood cells, leading to loss of consciousness or death. The video below is taken from ITV's Loose Women and highlights the dangers of CO poisoning.

Carbon Particulates

This is the most visible atmospheric pollutant. When there REALLY isn't enough oxygen to combine with the carbon in a carbon based fuel we make just carbon particulates. These are tiny particles of carbon that are most commonly seen as black smoke. Carbon particulates can cause serious health problems by blocking up airways, and are linked to respiratory diseases such as asthma.

In addition to the health problems they also cause a phenomenon called "global dimming" whereby light is reflected off the the particles, causing a dimming. The vidoe below shows this in action.




Sulphur Oxides

Fuels like coal and petrol contain tiny amounts of sulphur (or sulfur if you like that way of spelling it). When fuels are burned the atoms recombine with oxygen atoms to make oxides, so burning methane gas (natural gas), which has the formula of CH4, in oxygen makes carbon dioxide (CO2) and dihydrogen oxide (H2O i.e. water).

Depending upon the amount of oxygen available there can be a number of different oxides of sulphur made. More information about this can be found here:

Sulphur oxides cause a variety of health problems at relatively low levels, and are considered pollutants for this reason. 

In addition sulphur oxides can dissolve in rain water forming acid rain.

The excerpt below gives more details:

"95% of the SOx emitted from the combustion of fossil fuel is sulphur dioxide. SO2 is a toxic gas, which is directly harmful to human health. It is heavier than air and has a suffocating odour at an atmospheric concentration of around 500 parts per billion (ppb), at which level it can be fatal.  At lower levels, chest pains, breathing problems, eye irritation and a lowered resistance to heart and lung diseases can be experienced. At 20 ppb or lower there should be no ill effects to a healthy person. The normal atmospheric background concentration of SO2 is generally less than 10 ppb. [3]

A secondary effect is the formation of sulphates (and nitrates), in the form of aerosols or very fine airborne particles, which can comprise a significant proportion of the particulate matter and have been linked to increased asthma attacks, heart and lung disease and respiratory problems in susceptible population groups. [3]

A third effect can occur further away from the emission source where the sulphur oxides will have converted to acids by aqueous phase reactions in the atmosphere. These acidic aerosols are eventually precipitated as acid rain, snow, sleet or fog but only when they encounter the right meteorological conditions. In the absence of man made pollution rain water would be slightly acidic, around pH 5, due to the presence of carbonic acid from the interaction of water vapour and naturally occurring levels of CO2. Acid rain on the other hand has been measured with pH levels below 3 corresponding to vinegar. [3]"

Taken from

Nitrogen Oxides

Oxides of nitrogen are a mixture of gases that are composed of nitrogen and oxygen. Two of the most toxicologically significant compounds are nitric oxide (NO) and nitrogen dioxide (NO2). Other gases belonging to this group are nitrogen monoxide (or nitrous oxide, N2O), and nitrogen pentoxide (NO5).

Nitrogen dioxide is produced for the manufacture of nitric acid. Most nitric acid is used in the manufacture of fertilisers, while some is used in the production of explosives for both military and mining uses.

Excessive levels of the oxides of nitrogen, particularly nitrogen dioxide (NO2), can cause death in plants and roots and damage the leaves of many agricultural crops. NO2 is the damaging component of photochemical smog. Excessive levels increase the acidity of rain (lower the pH), and thus lower the pH of surface and ground waters and soil. The lowered pH can have harmful effects, possibly even death, on a variety of biological systems.

Entering the environment

Oxides of nitrogen are part of the biogeochemical cycling of nitrogen, and are found in air, soil and water.

In the atmosphere, the oxides of nitrogen are rapidly equilibrated to nitrogen dioxide (NO2), which eventually forms acid rain. In the stratosphere, oxides of nitrogen play a crucial role in maintaining the levels of ozone. Ozone is formed through the photochemical reaction between nitrogen dioxide and oxygen.

Oxides of nitrogen are present in the exhausts of all vehicles, including motor vehicles, lawn mowers, shipping/boating, aeroplanes and railways.

Pollutants in the News

Composition and Evolution of the Earth's Atmosphere

We think that our planet was formed about 4.6 billion years ago, although this is something of an estimate because there isn't enough evidence to be exact.

However, we can use fossil records and complex modelling software to work out what the Earth was like soon after forming. Some of the dates given in the next section are open to question, but are the best approximation we can make.

4.6 billion years ago...

After the Big Bang event it is believed that the universe was essentially made up of dust and tiny amounts of rock, floating around in space. This minute matter coalesced together to form slightly larger bits of matter, which then coalesced again and again and again (ad infinitum) until the masses we now know as planets formed. If you want to think of it like dust then we can quickly use a model that you are (probably) familiar with.

Imagine a room with mostly hard surfaces (tiled or wooden floor, not much furniture), and then imagine what it's like after a week of not cleaning it. There will be a fine layer of dust on all the surfaces, but not really enough to see. Imagine leaving the room for a month, the dust would be more noticeable and there may be some parts of the room where the dust is starting to form dust balls. Now fast forward a year or more.....the dust will have started to coalesce into large dust balls, that now have mass and their own (tiny) field of gravity (all objects with mass possess a gravitational field).

So, that's how our planet formed, just like dust forms under furniture (unless you're really clean and remember to Hoover and polish!)

At this early stage our planet was a molten ball of red hot rock and probably stayed like this for about a billion years. Intense volcanic activity during this period would have made the atmosphere mostly carbon dioxide, with a small amount of water vapour, ammonia and methane. There was probably no oxygen.

3.6 billion years ago

At about this time the Earth started to cool a little, and the water vapour that was in the atmosphere condensed, essentially making it rain for a very long time. This would have caused the planet to flood, with the deepest parts forming the seas. Over the next billion years or so we have to assume that life, in the form of green plants (algae?) appeared in the oceans, and were capable of converting some of the carbon dioxide into oxygen in a process we know as photosynthesis. Plants still do this now so it's reasonable to assume that they are responsible for the fall in levels of carbon dioxide, and rise in levels of oxygen.

Carbon dioxide was also trapped in sedimentary rocks (like limestone), fossil fuels (like coal and gas), and some dissolved in the oceans.

What about the ammonia?

Ammonia has the formula of NH3, with 3 atoms of hydrogen joined to central atom of nitrogen. There is a special process that some plants and bacteria carry out which breaks ammonia down into nitrogen. Denitrifying bacteria (as they're called) would have been responsible for the rise in levels of Nitrogen.

The Current Atmosphere

The atmosphere that we currently have has been pretty much constant for about 200 million years. We have 78% nitrogen, 21% oxygen, small amounts of carbon dioxide (0.04%), argon (0.9%) and "other gases". 

So, lots to think about but make sure you can describe and explain how the atmosphere has changed, and the key events that caused these changes.

Volcanoes, Rain, Plants.

Global Warming

Global Warming is a phenomenon that has, in recent years, become one of the biggest scientific issues in the world. Some people, including high profile political leaders, say that it is a made up thing and that scientists are scare-mongering. Donald Trump has said some pretty dim things about it in the last few years, claiming that it's not happening etc. Unfortunately there is a huge amount of scientific data that backs up the claim that our planet is getting warmer every year, and that this will lead to some catastrophic results if we don't take action.

What is it?

Think about any time you've been in a room with lots of windows (conservatory, greenhouse...) on a sunny day. That room is invariably several degrees warmer than a room without large expanses of glass. This is due to something called the "greenhouse effect". This is shown below:

Certain gases in the atmosphere absorb more infrared radiation than others and "trap" it in the atmosphere. The two most common gases that contribute to this are carbon dioxide and methane.

Carbon Dioxide

Only a tiny percentage of the air is carbon dioxide (about 0.04%) but these levels have steadily risen in the last 200 years, as humans have perfected the burning of fossil fuels to generate electricity. Carbon dioxide is very good at absorbing and re-emitting infrared radiation, so the more of it there is in the atmosphere the more heat is trapped. Our planet has a number of ways to control the levels of carbon dioxide in the atmosphere, all of which have been going on for billions of years. Plants (and trees) take in carbon dioxide from the atmosphere and convert it into oxygen and glucose in a process called photosynthesis. This is why rainforests, such as the Amazon, are called the "lungs of the Earth".

However, mass deforestation and rising carbon dioxide levels mean that this process can't balance out the amount of carbon dioxide in our atmosphere.

The oceans have also played a huge part in absorbing carbon dioxide, having done so since they were first formed billions of years ago. Like plants, the sea has a maximum amount of carbon dioxide that it can process. Normally the carbon dioxide is absorbed by the sea to make carbonates which dissolve. If there is too much carbon dioxide then it can lead to the seas becoming acidified, causing the death of living things. It is also possible for the dissolved carbonates to release carbon dioxide in a process called desorption.

Humans are responsible for producing some of the carbon dioxide in the atmosphere, but much of it is naturally occurring. The pie charts below show how much is man made, and where it comes from:


This is a much misunderstood gas, and one that people tend to blame for global warming. Methane is the gas that comes out of the gas taps in a science lab, and the same gas that heats your home. It is a lot better at trapping heat than carbon dioxide, and this is why it is often blamed for causing global warming. You may well have heard that cows produce enough methane to cause global warming, which isn't exactly true. For more on this follow this link:


Global Warming is a phenomenon in which heat energy is trapped inside the atmosphere by greenhouse gases such as carbon dioxide and methane. Rising levels of carbon dioxide caused by human activity are contributing to global warming. Intensive farming and agriculture is also responsible for producing more methane, which is also responsible for global warming.

Global warming is not, as yet, an exact science. We don't know exactly what will happen because modelling techniques aren't that advanced yet. However we think that raising the average temperature of the planet will lead to rising sea levels and reduced crop production. The videos below show what might happen.