Chemistry of the Atmosphere: Air Composition, Nitrogen and Oxygen Cycles, and Air Pollution

Chemistry of the Atmosphere

Air Composition

The Earth's atmosphere is a mixture of gases, primarily nitrogen, oxygen, argon, and carbon dioxide. Understanding the composition of air is fundamental to chemistry and environmental science.

• Nitrogen (N2): Makes up about 78% of the atmosphere by volume.

• Oxygen (O2): Constitutes about 21% of the atmosphere.

• Argon (Ar): Accounts for approximately 0.9%.

• Carbon Dioxide (CO2): Less than 0.04%.

Table: Approximate Composition of Clean Dry Air near Sea Level

Component	Percent by Volume
Nitrogen (N2)	78.08
Oxygen (O2)	20.95
Argon (Ar)	0.93
Carbon Dioxide (CO2)	0.04

Atmospheric Layers

The atmosphere is divided into four main regions, each with distinct characteristics and roles in Earth's climate and chemistry.

• Troposphere: Closest to Earth's surface; contains most living things and weather phenomena. Temperature decreases with altitude.

• Stratosphere: Above the troposphere; contains the ozone layer. Temperature increases with altitude.

• Mesosphere: Above the stratosphere; temperature decreases with altitude.

• Thermosphere: Outermost layer; temperature increases with altitude.

Nitrogen Cycle and Fixation

Nitrogen in the Atmosphere

Nitrogen is essential for life but must be 'fixed' (converted to usable forms) for biological processes. Most organisms cannot use atmospheric N2 directly.

• Nitrogen Fixation: Conversion of N2 to compounds like ammonia (NH3) or nitrate (NO3-).

• Lightning can fix nitrogen by providing energy for the reaction:

$\mathrm{N_2 + O_2 + energy \rightarrow 2NO}$

Nitrogen oxides can further react to form nitric acid:

$\mathrm{2NO + O_2 \rightarrow 2NO_2}$ $\mathrm{3NO_2 + H_2O \rightarrow 2HNO_3 + NO}$

Industrial Nitrogen Fixation

The Haber-Bosch process is an industrial method to fix nitrogen, producing ammonia for fertilizers.

$\mathrm{N_2 + 3H_2 \rightarrow 2NH_3}$

Impact on Population

The invention of the Haber-Bosch process greatly increased food production and world population.

Oxygen Cycle and Ozone Formation

Oxygen in the Atmosphere

Oxygen is vital for respiration and forms ozone in the stratosphere, which protects life by absorbing ultraviolet radiation.

• Ozone Formation: O2 molecules react to form O3 (ozone).

$\mathrm{O_2 (g) + O (g) \rightarrow O_3 (g)}$ $\mathrm{O_3 (g) + UV \rightarrow O_2 (g) + O (g)}$

Temperature Inversion and Air Pollution

Temperature Inversion

A temperature inversion occurs when cold air is trapped near Earth's surface by a layer of warmer air, preventing pollutants from dispersing and increasing air pollution.

• Pollutant: Any substance in the wrong place at the wrong time.

Pollution Through the Ages

Air pollution has been present throughout history, caused by natural events (wildfires, dust, volcanoes) and human activities (land clearing, fire use).

Industrial Smog and Its Chemistry

Definition and Sources

Smog is a combination of smoke and fog, often resulting from industrial activity.

Chemistry of Industrial Smog

Oxides of Carbon

Burning coal produces carbon monoxide, carbon dioxide, and soot.

$\mathrm{C(s) + O_2(g) \rightarrow CO_2(g)}$ $\mathrm{2C(s) + O_2(g) \rightarrow 2CO(g)}$

Oxides of Sulfur

Coal containing sulfur produces sulfur dioxide, which can further oxidize and form sulfuric acid.

$\mathrm{S(s) + O_2(g) \rightarrow SO_2(g)}$ $\mathrm{2SO_2(g) + O_2(g) \rightarrow 2SO_3(g)}$ $\mathrm{SO_3(g) + H_2O(l) \rightarrow H_2SO_4(l)}$

Particulate Matter (PM)

PM consists of solid and liquid particles larger than individual molecules, emitted from various sources and chemical reactions.

• Sources include construction sites, unpaved roads, fields, smokestacks, and fires.

• PM can result from reactions of chemicals such as sulfur dioxide and nitrogen oxides.

Types of Smog

Smog can be classified as photochemical or industrial, each with distinct sources and chemical characteristics.

Type	Main Sources	Main Pollutants	Weather
Photochemical Smog	Vehicles, Los Angeles	NO, hydrocarbons, O3, aldehydes	Warm or sunny
Industrial Smog	Industry, London	SO2, particulates	Cold or damp

Health and Environmental Effects of Industrial Smog

Health Effects

Particle pollution can impair the immune system, increase susceptibility to infections, and raise the risk of chronic lung and cardiovascular diseases.

Environmental Effects

Acidic precipitation and smaller particulates can damage plants, including farm crops.

Controlling Industrial Smog

Electrostatic Precipitators

Electrostatic precipitators remove particulates from emissions by inducing electrostatic charges, causing particles to deposit on plates.

Bag Filtration and Cyclone Separators

Bag filtration uses filters to remove particulates, while cyclone separators use centrifugal force to separate heavier particles.

Wet Scrubbers and Sulfur Dioxide Removal

Wet scrubbers remove particulates by passing gases through water. Sulfur dioxide can be reduced by removing sulfur from coal or adding limestone.

$\mathrm{CaCO_3(s) + heat \rightarrow CaO(s) + CO_2(g)}$ $\mathrm{CaO(s) + SO_2(g) \rightarrow CaSO_3(s)}$

Automobile Emissions

Gasoline and Octane Combustion

Gasoline is a mixture of hydrocarbons; octane (C8H18) is a typical representative. Complete combustion produces carbon dioxide and water.

$\mathrm{2C_8H_{18} + 25O_2 \rightarrow 16CO_2 + 18H_2O}$

Carbon Monoxide

Incomplete combustion produces carbon monoxide (CO), a toxic, colorless, odorless gas that binds with hemoglobin, preventing oxygen transport.

Emission Trends

U.S. emissions of CO have varied over time, with efforts to reduce idling and improve air quality.

Nitrogen Oxides

High temperatures in engines produce nitrogen oxides (NO and NO2), which contribute to smog and acid rain.

$\mathrm{N_2(g) + O_2(g) \rightarrow 2NO(g)}$ $\mathrm{2NO(g) + O_2(g) \rightarrow 2NO_2(g)}$

Volatile Organic Compounds (VOCs)

VOCs are major contributors to smog, produced by fuel exhaust, paints, and consumer products. Alkenes in VOCs can react with oxygen or ozone to form aldehydes.

Peroxyacetyl Nitrate (PAN)

PAN is produced by the reaction of hydrocarbons with oxygen and nitrogen oxides, responsible for many harmful effects of smog.

$\mathrm{CH_3COONO_2}$

Photochemical Smog

Formation and Components

NO2 components of smog react with sunlight to form an amber haze known as photochemical smog.

Summary Table: Types of Smog

Type	Main Pollutants	Weather
Photochemical Smog	NO, hydrocarbons, O3, aldehydes	Warm or sunny
Industrial Smog	SO2, particulates	Cold or damp

Additional info: These notes expand on the original slides by providing definitions, chemical equations, and context for atmospheric chemistry, air pollution, and emission control methods. All images included are directly relevant to the adjacent content and reinforce key concepts for introductory chemistry students.