Atmospheric Chemistry: Ozone, Greenhouse Gases, and Air
Explore the chemistry of the air: how the ozone layer blocks ultraviolet light, how greenhouse gases trap heat, and how pollutants like smog and acid rain form.
Quick answer
Atmospheric chemistry studies the chemical reactions of gases in Earth's atmosphere — including how the ozone layer blocks harmful ultraviolet light and how greenhouse gases trap heat. Human activities add gases such as carbon dioxide and pollutants that alter these natural cycles.
Example: High in the stratosphere, ultraviolet light splits O2 molecules; the freed oxygen atoms join other O2 molecules to form ozone (O3), which then absorbs more UV — a natural shield maintained by a continuous cycle of formation and destruction.
What you will learn
- Describe the composition and layers of the atmosphere
- Explain how the ozone layer forms and blocks ultraviolet light
- Explain how CFCs deplete stratospheric ozone
- Describe the greenhouse effect and the role of greenhouse gases
- Explain the chemistry of smog and acid rain
The air we breathe: composition and layers
Earth's atmosphere is mostly nitrogen (about 78%) and oxygen (about 21%), with argon and trace gases — including carbon dioxide, water vapor, and ozone — making up the rest. Despite their tiny concentrations, these trace gases run the atmosphere's most important chemistry.
The atmosphere is layered by temperature. Weather happens in the lowest layer, the troposphere. Above it lies the stratosphere, home of the ozone layer. The chemistry of each layer is different because temperature, pressure, and incoming sunlight change with altitude.
The ozone layer: Earth's UV shield
High in the stratosphere, energetic ultraviolet light from the Sun drives the Chapman cycle, a natural loop that maintains the ozone layer:
- UV light splits an oxygen molecule:
- A free oxygen atom joins another oxygen molecule: (ozone)
- Ozone absorbs UV in turn and breaks apart:
Step 3 is the key service: by absorbing ultraviolet light — especially the dangerous UV-B and UV-C bands — ozone prevents most of it from reaching the surface, where it would damage DNA in living cells. The layer is continuously made and destroyed, holding a steady protective concentration.
The same UV light that creates ozone also breaks it down. The ozone layer is not a fixed shell but a dynamic balance — which is why chemicals that tip the destruction side, like CFCs, can thin it.
Ozone depletion and the Montreal Protocol
In the 1970s, scientists discovered that chlorofluorocarbons (CFCs) — chemicals then widely used in refrigerators, air conditioners, and spray cans — were destroying stratospheric ozone. CFCs are extremely stable near the surface, so they drift intact up to the stratosphere, where UV light finally breaks them apart and releases chlorine atoms.
Each chlorine atom then destroys ozone catalytically: it reacts with an ozone molecule, is regenerated in a follow-up step, and goes on to destroy another — one chlorine atom can eliminate a great many ozone molecules before it is removed. This thinned the ozone layer severely over Antarctica each spring, the famous 'ozone hole.'
The world responded with the Montreal Protocol of 1987, which phased out CFCs globally. It is the most successful environmental treaty ever signed: CFC levels are falling and the ozone layer is recovering — proof that understanding atmospheric chemistry can drive effective action.
The greenhouse effect: natural and enhanced
The greenhouse effect works like this:
- Sunlight (mostly visible light) passes through the atmosphere and warms Earth's surface.
- The warm surface radiates energy back upward as infrared radiation (heat).
- Greenhouse gases — water vapor, carbon dioxide, methane, and nitrous oxide — absorb some of that infrared radiation and re-radiate it in all directions, including back toward the surface.
This trapped heat keeps Earth's surface far warmer than it would otherwise be — warm enough for liquid water and life. The effect is entirely natural and essential.
The problem is the enhanced greenhouse effect: burning fossil fuels adds extra carbon dioxide (and other activities add methane and nitrous oxide) on top of the natural baseline, trapping additional heat and shifting the climate. The mechanism is the same physics; only the amount of greenhouse gas has changed.
The natural greenhouse effect keeps Earth habitable — without it the planet would be frozen. Human concern is about strengthening it: more greenhouse gas means more trapped heat, even though the underlying physics is natural.
Smog and acid rain: pollution chemistry
Not all atmospheric chemistry happens high up. Near the surface, pollutants create their own problems:
- Photochemical smog: nitrogen oxides and volatile organic compounds from vehicles and industry react in sunlight to form ground-level ozone — the same molecule as the protective layer, but here a lung irritant that also damages crops.
- Acid rain: sulfur dioxide and nitrogen oxides from burning fossil fuels react with water and oxygen in the air to form sulfuric acid and nitric acid, which fall in rain and snow. Acid rain harms forests and lakes and erodes stone buildings and statues.
Both problems respond to emission controls: when sulfur dioxide emissions were cut, acid rain declined measurably. Atmospheric chemistry does not just diagnose these problems — it points to the fixes.
One atmosphere, connected chemistry
The atmosphere connects everything in this article. Ozone chemistry shows how trace gases control the UV reaching us. Greenhouse-gas chemistry shows how trace gases control the heat escaping us. Pollution chemistry shows how our emissions rewrite both stories near the ground.
The common lesson is that the atmosphere is a chemical system in balance — maintained by cycles of formation and destruction. Human additions, whether CFCs or carbon dioxide, disturb those cycles, but the same chemistry that reveals the disturbance also reveals the way back, as the Montreal Protocol demonstrated.
Common mistakes
They are distinct. Ozone depletion is about UV-blocking ozone destroyed by chlorine from CFCs; global warming is about heat trapped by greenhouse gases like carbon dioxide. Different gases, different mechanisms — though some gases, like CFCs, unfortunately contribute to both.
Stratospheric ozone is a vital UV shield, but ground-level ozone is a pollutant that harms lungs and crops. Whether ozone helps or harms depends entirely on where in the atmosphere it is.
The natural greenhouse effect predates humans and keeps Earth habitable. What humans have changed is its strength, by adding extra greenhouse gases — mainly carbon dioxide from fossil fuels — on top of the natural baseline.
Real-world applications
Air-quality forecasting and pollution alerts
Designing low-emission industrial processes
Protecting the ozone layer through the Montreal Protocol
Acid-rain reduction programs
Climate and weather modeling
Frequently asked questions
What is the difference between the ozone layer and ground-level ozone?
Location and effect. The ozone layer sits high in the stratosphere and absorbs harmful ultraviolet light — beneficial. Ground-level ozone forms in polluted air near the surface from car exhaust and sunlight, where it irritates lungs and damages crops — harmful. Same molecule, opposite roles depending on altitude.
Why is the greenhouse effect not entirely bad?
Without any greenhouse effect, Earth would be a frozen world — the natural effect keeps the surface warm enough for liquid water and life. The concern is the enhanced effect: adding extra greenhouse gases from burning fossil fuels traps additional heat, shifting the climate. The effect itself is essential; strengthening it too fast is the problem.
What caused the ozone hole?
In the 1970s scientists discovered that chlorofluorocarbons (CFCs) — then widely used in refrigerators and spray cans — drift up to the stratosphere, where UV light releases chlorine atoms from them. Each chlorine atom can catalytically destroy many ozone molecules. The 1987 Montreal Protocol phased out CFCs worldwide, and the ozone layer is now recovering.
How does acid rain form?
Sulfur dioxide and nitrogen oxides released by burning fossil fuels react with water and oxygen in the atmosphere to form sulfuric and nitric acids, which then fall as acid rain. It damages forests, lakes, and stone buildings — and cutting those emissions has measurably reduced the problem in many regions.
Do greenhouse gases trap heat like a blanket?
The blanket metaphor is only partly right. Greenhouse gases do not block heat like a blanket blocks wind; they absorb infrared radiation rising from Earth's surface and re-radiate it in all directions, including back downward. A better image is a very slight dimming of Earth's ability to cool itself to space.