Earth's Atmosphere: Composition and Evolution

• A planet's climate is determined by its size, distance from the Sun, and atmospheric composition
• The atmosphere is a layer of gases surrounding a planet, retained by gravity
• The atmosphere protects life by absorbing UV radiation and moderates temperature extremes
• Earth is a system with a fixed amount of stable chemical elements
• Elements cycle between the solid earth, oceans, and atmosphere

Earth’s First Atmosphere

• The early history of Earth's atmosphere is not well understood
• Earth likely formed as a spin-off of the Sun, primarily hydrogen and helium
• Primitive Earth may have lacked an atmosphere due to the loss of hydrogen and helium from solar heat
• The first atmosphere likely formed from the release of ammonia, bromine, chlorine, fluorine, and sulfur from Earth's interior.
• Volcanic eruptions and tectonic activity still contribute to this process
• UV radiation sparked chemical reactions in this atmosphere, leading to the development of life

Earth’s Second Atmosphere

• About 3.5 billion years ago, Earth's surface cooled to form a crust
• Volcanoes released steam, carbon dioxide, and ammonia
• Resulted in a second atmosphere of carbon dioxide and water vapor, with some nitrogen and almost no oxygen
• The second atmosphere had roughly 100 times more gas than the current atmosphere
• A strong greenhouse effect, from high carbon dioxide levels, kept Earth from freezing
• As Earth cooled, water vapor condensed into rain and formed oceans
• Oceans absorbed approximately 50% of the carbon dioxide released by volcanoes or reacted with calcium to create limestone
• Photosynthetic bacteria emerged around 3.6 billion years ago
• These bacteria converted carbon dioxide and water into oxygen and sugars
• These organisms released oxygen into the oceans and atmosphere, which helped to bind atmospheric hydrogen
• Initial oxygen release was minimal; iron and sulfur in oceans/land oxidized to rust and sulfates
• Oxygen was consumed in these oxidation reactions, keeping atmospheric oxygen at ≤1% of present levels
• Around 1 billion years ago, oxidizable rock reservoirs became saturated, allowing oxygen to persist in the air
• This led to mass extinction events and further evolution
• Photosynthesizing plants converted carbon dioxide into oxygen, providing food and enabling the evolution of life
• Excess carbon became trapped in fossil fuels, sedimentary rocks, and animal shells
• Oxygen reacted with ammonia in the atmosphere to form nitrogen
• Atmospheric oxygen led to the formation of an ozone layer
• The ozone layer protected life from ultraviolet radiation, enabling life to move out of the oceans and evolve respiration
• The resulting oxygen-nitrogen atmosphere is considered to be the "third atmosphere"

Earth’s Third Atmosphere

• The modern atmosphere is referred to as the "third atmosphere"
• The composition of the earth's modern atmosphere includes: - 78.08% Nitrogen (N2) - 20.95% Oxygen (O2) - 0.93% Argon (Ar) - 0.03% Carbon dioxide (CO2) - 0.002% Neon (Ne) - 0.008% Other gases
• Oxygen was only 1% of the atmosphere throughout most of Earth's history.
• It drastically increased with the emergence of large plants and animals
• Water and carbon dioxide are minor components but are important in temperature control and sustaining life
• Atmospheric carbon dioxide was about 20 times higher than current levels 500 million years ago
• Levels dropped and then rose to 4-5 times present levels 200 million years ago
• Carbon dioxide levels declined until the Industrial Revolution due to carbon becoming trapped in sediment (coal and hydrocarbons)
• As carbon dioxide was trapped, the planet cooled
• Methane hydrate releases from the sea floor might explain temperature spikes
• The Earth's climate has alternated between ice ages and interglacial periods over the last few million years
• Current trends indicate a move into a new ice age
• Global warming since the Industrial Revolution is linked to increased carbon dioxide from pollution, fossil fuels, and deforestation
• Approximately 10^16 tonnes of carbon are extracted from the earth and oceans each year
• The greenhouse effect warms Earth when gases in the atmosphere absorb solar energy (converted to heat)
• Absorbed heat warms the Earth's surface
• Greenhouse effect sustains life
• Increased greenhouse gases (like carbon dioxide) enhance the greenhouse effect, causing global warming

Studying Atmospheric Gases

• Early Greeks believed everything was made of earth, air, fire, and water
• Joseph Black isolated carbon dioxide in 1752
• John Dalton analyzed the atmosphere in the early 1800s - 71% nitrogen (isolated by Daniel Rutherford in 1772) - 21% oxygen (isolated by Carl Wilhelm Scheele in 1773 and named by Antoine-Laurent Lavoisier) - Small amount of incombustible gas (argon, isolated by William Strutt and Sir William Ramsay in 1894)
• The spectroscope (1920s) allowed scientists to discover trace gases like ozone
• The concentrations of these gases varied widely by location

Units of Pressure

• Pressure describes a burdensomeness on a person's mind
• Scientists viewed pressure as a force applied over an area
• Common pressure units include those used for tire pressure, weather reports, and blood pressure
• The S.I. unit of pressure is the Pascal (Pa), named after Blaise Pascal
• 1 Pa is the pressure exerted by a postage stamp on an envelope
• Kilopascals (kPa) are more convenient
• Engineering uses pounds per square inch (psi)
• Barometers/manometers measure in millimeters of mercury (mm Hg) or Torrs (after Evangelista Torricelli)
• Millibars (mbar) are used in meteorology
• Atmosphere (atm) is standard atmospheric pressure at sea level (0°C)
• Atmospheric pressure is from air mass and Earth's gravity
• 1 atm = 1013 mbar = 1.013 Bar = 760 Torr = 760 mm Hg = 14.7 psi = 101.325 kPa = 101325 Pa

Figures in measuring pressure

• Aristotle (384-322 BC): - Believed vacuums are impossible due to matter filling the world - Did not recognize that air had weight to exert pressure
• Archimedes (c. 287-212 BC): - Studies in hydrostatics helped scientists understand air pressure in the 1500s
• Galileo Galilei (1564-1642): - Patented the suction pump for irrigation using air to draw underground water, up to a limit of 10m (34 ft)
• Evangelista Torricelli (1608-1647): - Student of Galileo, later Court Mathematician - Argued pump height limits caused by atmospheric pressure - Invented the barometer in 1643, using mercury in an inverted tube and the unit "Torr" named after him
• Otto von Guericke (1602-1686): - Created a vacuum pump in 1644, strong enough that teams of horses could not pull two hemispheres apart - Proposed pressure inside was not equal to the pressure outside
• Blaise Pascal (1623-1662): - In 1648, observed that atmospheric pressure increased as a barometer was taken down the Puy-de-Dome mountain - Proposed that pressure acts uniformly in all directions
• Robert Boyle (1627-1691): - Improved Guericke's pump - In 1661, stated that the volume of a gas varies inversely with its pressure (temperature and amount of gas constant)
• Christiaan Huygens (1629-1695): - Developed the manometer to study elastic force in gases (1661) - Manometer is a U-shaped tube with mercury; height difference signifies pressure difference
• John Dalton (1766-1844): - Stated that total pressure in a gas mixture equals the sum of individual partial pressures (1801)
• Joseph Louis Gay-Lussac (1778-1850): - Proposed that pressure is proportional to temperature for fixed gas amount and volume (1802) - In 1808, he presented Gay-Lussac's law of combining volumes
• Count Lorenzo Romano Amedeo Carlo Avogadro di Quaregna e Cerreto (1776-1856): - Suggested the Avogadro's hypothesis, using Gay-Lussac’s experiments, that pressure is directly proportional to the number of particles in a container (1811)