Air Chemistry: Composition, Pollution, and Environmental Effects

Air Composition and Structure

Major Components of Air

The Earth's atmosphere is a mixture of gases that is essential for life and environmental processes. Understanding its composition is fundamental to chemistry and environmental science.

• Nitrogen (N2): Makes up approximately 78% of the atmosphere. It is relatively inert and serves as a buffer gas.

• Oxygen (O2): Comprises about 21% of the atmosphere. It is vital for respiration and combustion.

• Carbon Dioxide (CO2) and Other Gases: Account for roughly 1%. These include argon, neon, helium, methane, and trace gases.

Example: The pie chart shows the approximate percentages of major atmospheric gases: 78% nitrogen, 21% oxygen, 1% carbon dioxide and other gases.

Layers of the Atmosphere

The atmosphere is divided into layers based on temperature and composition, which affect chemical reactions and pollutant behavior.

• Troposphere: The lowest layer, where weather occurs and most pollutants are found.

• Stratosphere: Contains the ozone layer, which absorbs harmful UV radiation.

• Mesosphere and Thermosphere: Higher layers with decreasing air density.

Example: Aircraft and balloons operate at different altitudes within these layers.

Air Pollutants and Their Effects

Definition of Pollutants

A pollutant is a substance that appears in the environment where it exhibits toxicity or is present at unsafe levels. Persistent pollutants do not readily break down in the environment.

• Persistent Pollutant: Remains in the environment for long periods, increasing risk of harm.

Major Air Pollutants

Air pollutants originate from various sources and have significant health and environmental impacts.

Pollutant	Formula/Symbol	Major Sources	Health Effects	Environmental Effects
Carbon monoxide	CO	Motor vehicles, industry	Reduces oxygen delivery, toxic at high levels	Slight
Hydrocarbons	CxHy	Motor vehicles, industry	Some are carcinogenic, respiratory irritation	Precursor of ozone, smog formation
Sulfur oxides	SOx	Power plants, industry	Respiratory problems	Acid rain, damage to plants and buildings
Nitrogen oxides	NOx	Power plants, motor vehicles	Respiratory irritation	Acid rain, ozone formation
Particulate matter	PM	Industry, vehicles, dust	Respiratory issues, cancer	Visibility reduction, health impacts
Ozone	O3	Secondary pollutant	Respiratory problems	Damages crops, materials
Lead	Pb	Motor vehicles (historically), industry	Neurological damage	Soil and water contamination

Sources of Indoor Air Pollutants

Indoor air pollution can arise from household products, building materials, and activities such as cooking and cleaning.

• Examples: Pesticides, cleaning agents, tobacco smoke, radon from soil.

Photochemistry of Tropospheric Pollutants

Photochemical Reactions

Photochemical reactions in the troposphere are driven by sunlight and lead to the formation of secondary pollutants such as ozone and smog.

• Hydrocarbons and nitrogen oxides react under sunlight to produce ozone and other photochemical oxidants.

• Concentration of pollutants varies throughout the day, peaking during periods of high sunlight.

Example: Graphs show the diurnal variation of hydrocarbons, NO, NO2, and ozone.

Photochemical Smog Formation

Photochemical smog is a complex mixture of pollutants formed by the reaction of sunlight with nitrogen oxides and volatile organic compounds.

• Key reactions involve NOx, hydrocarbons, and ozone.

• Smog formation is most intense in urban areas with high vehicle emissions.

Ozone in the Stratosphere

Ozone Formation and Destruction

Stratospheric ozone is crucial for absorbing ultraviolet (UV) radiation and protecting life on Earth.

• Formation: $O_2 + UV ightarrow O + O$ $O + O_2 ightarrow O_3$

• Destruction: $O_3 + UV ightarrow O_2 + O$

• Average ozone concentration in the stratosphere is about 10 ppm.

Mechanism of Ozone Destruction by CFCs

Chlorofluorocarbons (CFCs) catalyze the destruction of ozone in the stratosphere, leading to ozone depletion.

• Key reactions: $CF_2Cl_2 + UV ightarrow CF_2Cl + Clullet$ $Clullet + O_3 + UV ightarrow ClOullet + O_2$ $ClOullet + O ightarrow Clullet + O_2$ Net Reaction: $O_3 + O ightarrow 2O_2$

Effects of Stratospheric Ozone

Ozone in the stratosphere absorbs harmful UV radiation, reducing the risk of skin cancer and other biological damage.

• Graphs show the absorption of UV by ozone and the biological effects of different UV wavelengths.

Reasons for Concern About Stratospheric Ozone

Depletion of stratospheric ozone increases exposure to UV radiation, leading to health risks and environmental damage.

UV Index	Minutes for Skin Damage (Never Tan)	Minutes for Skin Damage (Tan Easily)
0-2 (Minimal)	30+	120+
3-4 (Low)	18-29	90-119
5-6 (Moderate)	15-22	60-89
7-9 (High)	7-14	30-59
10+ (Very High)	<4	20-29

Example: Sunscreens with broad spectrum SPF protect against UV damage.

Acid Rain

Formation of Acid Rain

Acid rain is produced when sulfur oxides (SOx) and nitrogen oxides (NOx) react with water vapor in the atmosphere to form acids.

• Key reactions: $SO_2 + H_2O ightarrow H_2SO_3$ $SO_3 + H_2O ightarrow H_2SO_4$ $NO_2 + H_2O ightarrow HNO_3$

• Sources include power plants, vehicles, and industrial processes.

pH of Acid Rain

Normal rainwater has a pH of about 5.6 due to dissolved CO2. Acid rain has a lower pH, often below 5, due to stronger acids formed from pollutants.

• pH scale ranges from acidic (0) to basic (14).

• Acid rain can be as low as pH 4 or lower.

Problems Caused by Acid Rain

Acid rain causes environmental damage, structural deterioration, and health impacts.

• Aquatic ecosystems: Toxic to fish and aquatic life.

• Forests and soils: Leaches nutrients, damages plants.

• Structures: Corrodes buildings and monuments.

• Human health: Respiratory problems, contaminated water.

Greenhouse Effect and Global Warming

Greenhouse Effect

The greenhouse effect is a natural process where certain gases trap heat in the atmosphere, maintaining Earth's temperature.

• Greenhouse gases: CO2, CH4, H2O vapor, N2O, and others.

• These gases absorb infrared radiation and re-emit it, warming the atmosphere.

Global Warming

Global warming refers to the increase in Earth's average temperature due to rising concentrations of greenhouse gases from human activities.

• Major sources: Fossil fuel combustion, deforestation, industrial processes.

• Consequences: Climate change, sea level rise, extreme weather.

Comparing Planets: The Goldilocks Paradox

Earth's temperature is 'just right' due to its atmospheric composition and greenhouse gas content, compared to Venus (too hot, thick atmosphere) and Mars (too cold, thin atmosphere).

• Greenhouse gases absorb infrared radiation, warming the planet.

Drivers of Climate

Climate is influenced by both natural and anthropogenic (human-caused) factors, including greenhouse gas emissions, solar radiation, and volcanic activity.

Carbon Dioxide Emissions and Mitigation

Vehicle Emissions

Vehicles emit pollutants such as CO, NOx, hydrocarbons, and CO2, contributing to air pollution and climate change.

Carbon Capture and Sequestration

Carbon capture and storage (CCS) is a technology to reduce the impact of CO2 emissions by capturing and storing it underground, but it does not affect CO2 already released.

Summary

• The atmosphere's composition and structure are crucial for life and environmental chemistry.

• Air pollutants have significant health and environmental impacts, including acid rain, smog, and ozone depletion.

• Greenhouse gases drive the greenhouse effect and global warming.

• Mitigation strategies such as CCS are being developed to address rising CO2 levels.

Additional info: Some explanations and table entries have been expanded for clarity and completeness based on standard introductory chemistry textbooks.