Atmospheric Chemistry of Earth's Troposphere

Nitrogen (N₂) is the most abundant gas in Earth's atmosphere, including the troposphere. This cartoon also depicts several other tropospheric gases, including oxygen (O₂), carbon dioxide (CO₂), water vapor (H₂O), methane (CH₄), sulfur dioxide (SO₂), and carbon monoxide (CO).
Click on image for full size (52 Kb JPEG)
Image courtesy UCAR, modified by Windows to the Universe staff (Randy Russell).

Chemistry plays an influential role in the behavior of the lowest layer of Earth's atmosphere, the troposphere. The various gases in the troposphere are constantly mixing with and reacting with each other. Gases released by the oceans, emitted by living creatures, and poured into the atmosphere by human activities continually alter the balance of atmospheric chemistry.

Earth's atmosphere consists of about 78% nitrogen, 20% oxygen, and a mixture of small amounts of numerous other ingredients. Some of the minor constituents do, however, have big impacts. For example, greenhouse gases such as carbon dioxide and methane exert a large influence on the temperature of our planet.

The activities of humans play an increasingly important role in tropospheric chemistry. Fossil fuel burning generates sulfur oxides, which create sulfuric acid - a component of acid rain. Exhaust gases from cars and trucks produce nitrogen oxides, which contribute to the formation of smog and of nitric acid - another component of acid rain.

Nature also does its share to alter the chemistry of the troposphere. Volcanic eruptions, wildfires, lightning, and UV radiation from the Sun all add materials to the troposphere or chemically alter those already there. The oceans and the biosphere exchange vast quantities of gases with the atmosphere's lowest layer. The Carbon and Nitrogen Cycles play key roles in these processes.

The table below describes some of the chemical compounds that play important roles in the chemistry of Earth's troposphere. It also lists some of the processes that transform those chemicals.

Chemical Compound	Formula	Role in Tropospheric Chemistry
Carbon dioxide	CO₂	Carbon dioxide in the troposphere is produced by photosynthesis by plants and microbes, by decomposition of organic matter, and by fossil fuel combustion. CO₂ is an important greenhouse gas.
Carbon monoxide	CO	Carbon monoxide comes from wildfires, volcanoes, and incomplete combustion in the exhaust gases from cars and trucks. It is poisonous and can help raise the levels of greenhouse gases via certain chemical reactions. It reacts with oxygen to transform into carbon dioxide.
Hydrocarbons	C_xO_y	Hydrocarbons are released by the combustion of fossil fuels. They are one of the constituents of smog. Hydrocarbons are combinations of hydrogen and carbon atoms.
Hydrogen Peroxide	H₂O₂	Hydrogen peroxide, often present in small quantities in water droplets in the atmosphere, helps create the sulfuric acid in acid rain via reactions with sulfur dioxide.
Methane	CH₄	Methane is a powerful greenhouse gas. Although its concentration in the atmosphere is small, it has a substantial impact on Earth's energy balance.
Nitrogen	N₂	78% of the gas molecules in the troposphere are nitrogen. Nitrogen atoms move through the environment via the nitrogen cycle. Hot combustion, such as in auto exhausts, incorporates nitrogen into nitrogen oxides.
Nitrogen Oxides	NO & NO₂	Nitrogen oxides form during high-temperature combustion in air, such as in automobile exhausts. They help to form smog and mix with water to create nitric acid, a component of acid rain.
Nitric Acid	HNO₃	Nitrogen oxides from pollutants such as car exhaust mix with water to form nitric acid. It is a component of acid rain.
Oxygen & Ozone	O₂ & O₃	About 20% of the gas molecules in the troposphere are oxygen. Although ozone in the stratosphere is good and protects us from UV radiation, ozone in the troposphere is bad and is a health hazard that contributes to the formation of smog.
PAN (Peroxyacytyl nitrate)	C₂H₃O₅N	PAN is a noxious and irritating component of smog. It forms in a reaction involving nitrogen dioxide, oxygen, and substances derived from Volatile Organic Compounds (VOCs).
Photochemical smog	-	Smog was originally a term for a mixture of smoke and fog. Photochemical smog is a toxic "soup" of atmospheric pollutants often found in urban areas. It consists of nitrogen oxides, ozone, VOCs, and PAN.
Photodissociation	-	When molecular bonds break due to energy from a collision with a solar photon.
Sulfur Oxides	SO₂ & SO₃	Sulfur dioxide and sulfur trioxide are produced by coal and oil burning, volcanoes, and other human and natural sources. Sulfur dioxide combines with water droplets in the air to form sulfuric acid, a component of acid rain.
Sulfuric Acid	H₂SO₄	Sulfur dioxide gas from coal and oil burning, volcanoes, and other human and natural sources, combines with water droplets in the air (and the traces of hydrogen peroxide such droplets often contain) to form sulfuric acid - which is a component of acid rain.

Earth's Troposphere

Air Pollution Sources

Ozone in the Troposphere

Atmospheric Chemistry of Earth's Troposphere

This cartoon shows some of the gases in Earth's troposphere. There is more nitrogen (N₂) than anything else. There is also a lot of oxygen (O₂). The cartoon also shows carbon dioxide (CO₂), water vapor (H₂O), methane (CH₄), sulfur dioxide (SO₂), and carbon monoxide (CO).
Click on image for full size (52 Kb JPEG)
Image courtesy UCAR, modified by Windows to the Universe staff (Randy Russell).

When you think of chemistry, do you think about mixing colored liquids in test tubes and maybe making an explosion... or at least a nice puff of smoke? Did you know that a lot of chemistry happens in Earth's atmosphere? There are many different kinds of chemicals in the air. Those chemicals often combine with each other in chemical reactions, making new and different chemicals. This is called "atmospheric chemistry".

Earth's atmosphere has different layers. The lowest layer is called the troposphere. We live in the troposphere. This page explains about atmospheric chemistry in the troposphere.

Most of the gas in our atmosphere is nitrogen. About 4/5ths of the air is nitrogen. What about the other 1/5th? Almost all of it is oxygen, the stuff in the air we need to breathe. There are also very small amounts of a bunch of other chemicals.

Have you heard of greenhouse gases? They are kinds of gases that trap the heat from sunlight in our atmosphere. Earth would be very cold if we didn't have any greenhouse gases. Carbon dioxide and methane are two very important greenhouse gases.

Some of the chemicals in the air come from pollution. When we burn coal in a factory or gasoline in our cars, we make air pollution. Coal and oil have sulfur in them. When they burn, they make chemicals called sulfur oxides. These can turn into sulfuric acid when they mix with water droplets in the air. These droplets of acid can fall to the ground as acid rain. Cars and trucks also give off chemicals called nitrogen oxides. Nitrogen oxides combine with other chemicals to make smog. They also help make nitric acid, which is another acid in acid rain.

Nature also does things to change the chemistry of the troposphere. Volcanoes, lightning, and wildfires all add chemicals to the air or change the ones that are already there. Energy from sunlight can make chemical reactions happen, changing one gas into another. Some chemicals move in cycles between the atmosphere, living creatures, and the oceans. The Carbon Cycle and the Nitrogen Cycles are two important cycles that change the chemistry of the atmosphere.

This table (below) describes some of the chemicals in the troposphere, and some of the chemical reactions that happen in the air:

Chemical	Formula	Role in Tropospheric Chemistry
Carbon dioxide	CO₂	Carbon dioxide is a kind of greenhouse gas. When we breathe, we take in oxygen and breathe out carbon dioxide. Plants and some kinds of microbes use carbon dioxide during photosynthesis to make food. Burning fuels also puts carbon dioxide into the atmosphere.
Carbon monoxide	CO	When things burn, they mostly make carbon dioxide. Sometimes they make carbon monoxide, too. Carbon monoxide is a poisonous gas. Volcanoes and car engines make carbon monoxide.
Hydrocarbons	C_xO_y	Hydrocarbons are chemicals made up of hydrogen and carbon atoms. When fuel burns, it puts some hydrocarbons into the air. Hydrocarbons help to make smog, a kind of air pollution.
Methane	CH₄	Methane is a kind of greenhouse gas.
Nitrogen	N₂	Most of the gas in Earth's atmosphere is nitrogen. About 4/5ths of the air is nitrogen. The nitrogen cycle explains how nitrogen moves around in the environment. When fuel burns hot, like it does in the engine of a car, nitrogen combines with oxygen to make nitrogen oxides.
Nitrogen Oxides	NO & NO₂	Nitrogen oxides are a kind of pollution. Burning fuels like gasoline in air makes nitrogen oxides. Most nitrogen oxides come from cars and trucks. They help to make smog. They also mix with water droplets in the air to make nitric acid. Nitric acid is a part of acid rain.
Nitric Acid	HNO₃	Nitric acid is part of acid rain. Nitric acid forms when nitrogen oxides mix with water droplets in the air. Nitrogen oxides are a kind of pollution that comes from the engines of cars and trucks.
Oxygen & Ozone	O₂ & O₃	About 1/5th of the gas in the atmosphere is oxygen. When you breathe, your body uses the oxygen to keep you alive. Ozone is a special kind of oxygen that has three atoms instead of two.
PAN (Peroxyacytyl nitrate)	C₂H₃O₅N	PAN is a kind of air pollution. Smog has PAN in it. PAN forms when nitrogen dioxide, oxygen, and Volatile Organic Compounds (VOCs) get together.
Smog	-	Smog is a mixture of smoke and fog. Photochemical smog is a kind of air pollution. It has nitrogen oxides, ozone, VOCs, and PAN in it.
Photodissociation	-	When a photon of sunlight breaks apart a molecule.
Sulfur Oxides	SO₂ & SO₃	Sulfur dioxide and sulfur trioxide are types of pollution. People make them when we burn coal and oil. Volcanoes also give off sulfur oxides. Sulfur dioxide mixes with water droplets in the air to make sulfuric acid. Sulfuric acid is in acid rain.
Sulfuric Acid	H₂SO₄	Sulfuric acid is in acid rain. Sulfuric acid in the air is made when sulfur dioxide gas mixes with water droplets. The sulfur dioxide gas comes from volcanoes and from coal and oil that people burn for fuel.

Earth's Troposphere

Air Pollution Sources

Ozone in the Troposphere

Atmospheric Chemistry of Earth's Troposphere

This cartoon shows some of the gases in the lowest layer of Earth's atmosphere. Most of the gas is nitrogen (N₂). There is also a lot of oxygen (O₂). The cartoon also shows carbon dioxide (CO₂), water vapor (H₂O), methane (CH₄), sulfur dioxide (SO₂), and carbon monoxide (CO).
Click on image for full size (63 Kb JPEG)
Image courtesy UCAR, modified by Windows to the Universe staff (Randy Russell).

Earth's atmosphere has different layers. The lowest layer is called the troposphere. We live in the troposphere. This page explains about atmospheric chemistry in the troposphere.

This table (below) describes some of the chemicals in the troposphere, and some of the chemical reactions that happen in the air:

Chemical	Formula	Role in Tropospheric Chemistry
Carbon dioxide	CO₂	Carbon dioxide is a kind of greenhouse gas. When we breathe, we take in oxygen and breathe out carbon dioxide. Plants and some kinds of microbes use carbon dioxide during photosynthesis to make food. Burning fuels also puts carbon dioxide into the atmosphere.
Carbon monoxide	CO	When things burn, they mostly make carbon dioxide. Sometimes they make carbon monoxide, too. Carbon monoxide is a poisonous gas. Volcanoes and car engines make carbon monoxide.
Hydrocarbons	C_xO_y	Hydrocarbons are chemicals made up of hydrogen and carbon atoms. When fuel burns, it puts some hydrocarbons into the air. Hydrocarbons help to make smog, a kind of air pollution.
Methane	CH₄	Methane is a kind of greenhouse gas.
Nitrogen	N₂	Most of the gas in Earth's atmosphere is nitrogen. About 4/5ths of the air is nitrogen. The nitrogen cycle explains how nitrogen moves around in the environment. When fuel burns hot, like it does in the engine of a car, nitrogen combines with oxygen to make nitrogen oxides.
Nitrogen Oxides	NO & NO₂	Nitrogen oxides are a kind of pollution. Burning fuels like gasoline in air makes nitrogen oxides. Most nitrogen oxides come from cars and trucks. They help to make smog. They also mix with water droplets in the air to make nitric acid. Nitric acid is a part of acid rain.
Nitric Acid	HNO₃	Nitric acid is part of acid rain. Nitric acid forms when nitrogen oxides mix with water droplets in the air. Nitrogen oxides are a kind of pollution that comes from the engines of cars and trucks.
Oxygen & Ozone	O₂ & O₃	About 1/5th of the gas in the atmosphere is oxygen. When you breathe, your body uses the oxygen to keep you alive. Ozone is a special kind of oxygen that has three atoms instead of two.
PAN (Peroxyacytyl nitrate)	C₂H₃O₅N	PAN is a kind of air pollution. Smog has PAN in it. PAN forms when nitrogen dioxide, oxygen, and Volatile Organic Compounds (VOCs) get together.
Smog	-	Smog is a mixture of smoke and fog. Photochemical smog is a kind of air pollution. It has nitrogen oxides, ozone, VOCs, and PAN in it.
Photodissociation	-	When a photon of sunlight breaks apart a molecule.
Sulfur Oxides	SO₂ & SO₃	Sulfur dioxide and sulfur trioxide are types of pollution. People make them when we burn coal and oil. Volcanoes also give off sulfur oxides. Sulfur dioxide mixes with water droplets in the air to make sulfuric acid. Sulfuric acid is in acid rain.
Sulfuric Acid	H₂SO₄	Sulfuric acid is in acid rain. Sulfuric acid in the air is made when sulfur dioxide gas mixes with water droplets. The sulfur dioxide gas comes from volcanoes and from coal and oil that people burn for fuel.

Earth's Troposphere

Air Pollution Sources

Ozone in the Troposphere

Page created February 7, 2006 by Randy Russell.
The source of this material is Windows to the Universe, at http://www.windows.ucar.edu/ at the University Corporation for Atmospheric Research (UCAR). © The Regents of the University of Michigan. All Rights Reserved. Site policies and disclaimer