HookThe five days of fog that rewrote Britain's air
On the evening of 5 December 1952, a cold, still anticyclone settled over London and trapped the smoke of a million coal fires and the city's power stations under a lid of motionless air. For five days a yellow-black fog — the Great Smog — cut visibility to a few metres; buses were led by men on foot with torches, and prize cattle at Smithfield market suffocated in their pens. The official death toll was about 4,000, though later analysis put it closer to 12,000. The killers were sulfur dioxide and soot from burning coal, and the disaster forced the Clean Air Act of 1956 — the first time a British government legislated the chemistry of the air its citizens breathed.
C9 tells two stories about that air, on two utterly different clocks. The slow story is how the atmosphere came to be roughly four-fifths nitrogen and one-fifth oxygen over 4.6 billion years. The fast story is how, in barely two centuries of burning the carbon that took those aeons to bury, humans are changing the levels of greenhouse gases and pumping out the pollutants that once killed Londoners in their beds. Both stories are chemistry with consequences you can measure.
ModelToday's air and the early atmosphere
Today's atmosphere has been remarkably stable for about 200 million years: roughly 78% nitrogen (about four-fifths) and 21% oxygen (about one-fifth), with the last one per cent made up mostly of argon, a small but crucial 0.04% of carbon dioxide, plus water vapour and traces of the other noble gases. It has not always looked like this. The Earth formed about 4.6 billion years ago, and for its first billion years its surface was violently volcanic.
The leading theory is that this intense volcanic activity released the gases that made the early atmosphere, and that it was dominated by carbon dioxide, with water vapour, some nitrogen and smaller amounts of methane and ammonia, and little or no oxygen. It is thought to have resembled the atmospheres of Mars and Venus today, which are largely carbon dioxide. As the young Earth cooled, the water vapour condensed to form the oceans. Because there are no samples of air from four billion years ago, all of this is pieced together from indirect evidence, so the models carry genuine scientific uncertainty — a point examiners want you to acknowledge rather than state the early atmosphere as settled fact.
A quick proportion check keeps the numbers honest. If nitrogen is about four-fifths of the air, that is \(\frac{4}{5}=0.80\), or 80%, close to the measured 78%; oxygen at one-fifth is \(\frac{1}{5}=0.20\), or 20%, close to the measured 21%. The remaining slice — a little under 2% — is argon plus that vital 0.04% of carbon dioxide. Keeping the fractions and the percentages lined up is what lets you answer both fraction-style and percentage-style questions from one memorised pair of figures.
MechanismHow oxygen rose and carbon dioxide fell
Two of the biggest changes in Earth's history were the rise of oxygen and the fall of carbon dioxide, and both come back to life. About 2.7 billion years ago, primitive algae and cyanobacteria evolved photosynthesis, taking in carbon dioxide and water and releasing oxygen. For a long time the new oxygen was mopped up by dissolved iron and rock, but eventually it began to accumulate in the air, and over roughly a billion years it reached the levels that made animal life possible.
Carbon dioxide fell for four connected reasons. It dissolved into the newly formed oceans; it was consumed by that ever-growing population of photosynthesising organisms; marine organisms used the dissolved carbon to build carbonate shells and skeletons that sank and were compressed into sedimentary rock such as limestone; and enormous quantities of carbon were locked away in fossil fuels — coal formed from buried plants, and oil and gas from buried plankton — and in other sediments. In other words, the carbon that had once filled the sky was slowly filed away into rock and fuel. That is the reserve humans are now digging up and burning.
Photosynthesis is the reaction that reshaped the atmosphere, and it is worth being able to write: \[\mathrm{6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2}\] Read it as a bookkeeping entry for the whole planet — every glucose molecule an alga builds removes six carbon dioxide molecules from the air and adds six of oxygen. Run that reaction across countless organisms and billions of years and you have simultaneously drained the early atmosphere's carbon dioxide and filled it with the oxygen you are breathing now.
MechanismThe greenhouse effect — why the planet is not frozen
Three gases in the atmosphere are the important greenhouse gases: carbon dioxide, methane and water vapour. The greenhouse effect is the mechanism by which they keep the Earth warm, and it works in three steps. Short-wavelength radiation from the Sun passes easily through the atmosphere and is absorbed by the Earth's surface, warming it. The warmed surface then re-emits energy as longer-wavelength infrared radiation. Greenhouse gases absorb this outgoing infrared and re-radiate it in all directions — including back down to the surface — trapping heat that would otherwise escape to space.
This natural greenhouse effect is not the villain; it is essential. Without it the Earth's average temperature would be roughly 33°C colder — below freezing — and the planet would be a frozen rock. The problem is one of degree: adding more greenhouse gas thickens the blanket, so more infrared is absorbed and the surface warms further. That extra, human-driven warming is called the enhanced greenhouse effect, and separating the natural effect (good, and necessary for life) from the enhanced effect (the cause of modern climate change) is exactly the distinction an exam answer needs to draw.
DataThe Keeling Curve, human activity and the carbon footprint
In March 1958 a young geochemist named Charles David Keeling began measuring carbon dioxide from an observatory near the summit of Mauna Loa in Hawaii, far from any city. The resulting Keeling Curve is the most important graph in climate science: a small annual saw-tooth (the northern hemisphere's forests breathing in each summer) riding on a relentless upward line. Carbon dioxide has climbed from about 280 parts per million before the Industrial Revolution to over 400 ppm — a threshold first crossed in May 2013 — and around 420 ppm today.
The rise tracks human activity. Burning fossil fuels for electricity, transport and industry releases carbon dioxide; deforestation both burns carbon and removes the trees that would absorb it; and agriculture adds methane — from cattle and from flooded rice paddies — while landfill waste adds still more. Judging this evidence, an examinable skill, means separating peer-reviewed measurements from opinion: the data on rising greenhouse gases are firm, but climate is a hugely complex system and the models that project the future are simplifications carrying uncertainty, which is why careful scientists give ranges rather than single numbers, and why media reports — often simplified or biased — should be read critically.
The likely consequences of continued warming include rising sea levels from melting ice and expanding water, more frequent extreme weather, and shifts in rainfall that stress farming and habitats. A carbon footprint is the total greenhouse gas emitted over the whole life cycle of a product, service or event, expressed as carbon dioxide equivalent. It can be cut by using renewable energy, improving efficiency, capturing and storing carbon, and taxing or capping emissions — but each runs into real barriers of cost, technology, and the political and lifestyle changes people are willing to make.
A worked comparison sharpens the idea. Suppose a petrol car emits about 180 grams of carbon dioxide per kilometre and an electric car charged on low-carbon electricity emits an equivalent of about 60 grams per kilometre over its use. Across a 15,000 km year that is \(180\times15000=2\,700\,000\) g, or 2.7 tonnes, against \(60\times15000=900\,000\) g, or 0.9 tonnes — a saving of 1.8 tonnes a year from one switch. The honest exam point is that a full carbon footprint must also count the emissions of making each car and generating the electricity, which is why life-cycle figures, not just tailpipe figures, are the fair comparison.
CasePollutants from burning fuels
Return to the Great Smog. When a fuel burns in plenty of oxygen you get clean complete combustion — carbon dioxide and water. But real furnaces, engines and open fires often burn in a limited oxygen supply, giving incomplete combustion and a set of dangerous by-products. Not enough oxygen leaves carbon only partly oxidised as carbon monoxide (CO), a toxic, colourless, odourless gas that binds to haemoglobin in place of oxygen and can kill without warning — the silent killer behind faulty boilers. Even less oxygen leaves unburnt carbon as particulates, or soot, which cause global dimming by reflecting sunlight, blacken buildings, and lodge in lungs to cause respiratory disease.
Two more pollutants come not from the carbon but from what rides along with it. Fuels such as coal and diesel contain sulfur impurities, which burn to sulfur dioxide, \(\mathrm{SO_2}\). And in the fierce heat of an engine, nitrogen and oxygen from the air itself react to form oxides of nitrogen, \(\mathrm{NO_x}\). Both sulfur dioxide and the nitrogen oxides dissolve in rain to make acid rain, which kills trees, acidifies lakes and eats away limestone buildings and statues, and both irritate the lungs and worsen breathing. The Great Smog was this exact chemistry at lethal concentration — sulfur dioxide and soot from coal, trapped over a city — and it is why modern power stations remove sulfur and cars carry catalytic converters.
Show, with equations, how burning methane can produce carbon monoxide or soot instead of carbon dioxide. With a good oxygen supply combustion is complete: \(\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}\). With a restricted supply the carbon is only partly oxidised to carbon monoxide: \[\mathrm{2CH_4 + 3O_2 \rightarrow 2CO + 4H_2O}\] and with even less oxygen it is left as solid carbon, or soot: \[\mathrm{CH_4 + O_2 \rightarrow C + 2H_2O}\] The hydrogen still burns to water in every case; it is the carbon whose fate — \(\mathrm{CO_2}\), \(\mathrm{CO}\) or \(\mathrm{C}\) — depends on how much oxygen is available.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
Quote the numbers: the atmosphere is about 78% nitrogen and 21% oxygen, carbon dioxide is roughly 0.04%, and it has risen from around 280 ppm to over 400 ppm. A common, costly error is to say oxygen came from volcanoes — it came from algae and photosynthesis, while volcanoes supplied the early carbon dioxide, so keep those two mechanisms apart.
When the question is about pollutants, sort them by cause: incomplete combustion gives carbon monoxide and soot; sulfur impurities give sulfur dioxide; engine heat gives oxides of nitrogen; sulfur dioxide and nitrogen oxides give acid rain. Keep greenhouse gases (carbon dioxide, methane, water vapour) firmly separate from toxic pollutants — mixing the two lists is a frequent lost mark. Balance any combustion equation and let the amount of oxygen decide whether carbon ends up as \(\mathrm{CO_2}\), \(\mathrm{CO}\) or \(\mathrm{C}\).
For evaluate-the-evidence or discuss-climate-change questions, credit comes from handling uncertainty like a scientist: distinguish peer-reviewed measurements from media opinion, acknowledge that climate models are simplifications, and give consequences (sea-level rise, extreme weather, shifting rainfall) alongside the actions and their barriers, rather than writing a one-sided answer.