Atmospheric Chemistry Basics for GCSE AQA

Gases in the Atmosphere

The air around us isn't just empty space—it's primarily a mixture of nitrogen (78%) and oxygen (21%), with smaller amounts of carbon dioxide, hydrogen and other gases. These proportions didn't happen by accident but evolved over billions of years.

Earth's atmosphere today is vastly different from its early composition. Scientists believe that 4.6 billion years ago, our atmosphere resembled those of Mars or Venus—primarily carbon dioxide with very little oxygen.

Remember this! Evidence about Earth's early atmosphere is limited because of the enormous time scale involved (4.6 billion years), and scientific theories have evolved significantly as new discoveries are made.

Early volcanic activity released gases like nitrogen, which gradually built up, along with small amounts of methane and ammonia. As the planet cooled, water vapour condensed to form oceans, which absorbed carbon dioxide and reduced its atmospheric concentration.

The Oxygen Revolution

Oxygen wasn't always abundant in our atmosphere—it was actually produced by living organisms! About 2.7 billion years ago, algae began the process of photosynthesis, converting carbon dioxide and water into glucose and releasing oxygen as a byproduct.

The chemical equation for photosynthesis shows this remarkable transformation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

As plant life evolved over the next billion years, oxygen levels increased, creating conditions that allowed animal life to evolve. At the same time, carbon dioxide levels decreased not only through photosynthesis but also through the formation of carbon-containing sedimentary rocks like limestone and coal.

Did you know? The remains of ancient plants and animals eventually formed the fossil fuels we use today, effectively locking away carbon that was once in the atmosphere.

Greenhouse gases like water vapour, carbon dioxide, and methane play a crucial role in maintaining Earth's temperature. They allow sunlight to pass through the atmosphere but trap the infrared radiation that Earth emits, warming our planet. While this greenhouse effect is essential for life, excessive greenhouse gases cause global warming, leading to climate change, rising sea levels, species extinction, and decreased crop yields.

Atmospheric Pollutants from Fuels

When we burn fuels like coal and petrol, we don't just get energy—we release a cocktail of gases into our atmosphere. Most fuels contain carbon and hydrogen, and sometimes sulphur, which combine with oxygen during combustion.

Complete combustion primarily produces carbon dioxide and water vapour, but the reality is often more complex. When fuels burn at high temperatures, harmful nitrogen oxides form. If there isn't enough oxygen present, incomplete combustion occurs, producing carbon monoxide, soot (carbon particles), and unburned fuel.

These pollutants have serious consequences for both human health and the environment. Carbon monoxide is particularly dangerous—a toxic, colourless, odourless gas that's difficult to detect. Sulphur dioxide and nitrogen oxides contribute to acid rain and cause respiratory problems, while particulates lead to global dimming and various health issues.

Warning! Carbon monoxide is especially dangerous because we can't see, smell, or taste it—making it a silent killer in poorly ventilated spaces where fuels are burned.