Composition and Structure of the Earth's Atmosphere

Chemical Composition and General Characteristics of the Atmosphere

• Radiant energy from the sun is the primary force that energizes the atmosphere, driving daily weather patterns.
• Earth's atmosphere is a thin gaseous envelope composed primarily of two gases: nitrogen ($N_2$) at approximately $78\%$ and oxygen ($O_2$) at approximately $21\%$.
• The remaining portion of the atmosphere consists of small to trace amounts of other gases, including argon ($Ar$), water vapor ($H_2O$), and carbon dioxide ($CO_2$).
• Approximately $99\%$ of the total atmospheric mass is concentrated within the first $30\,\text{km}$ above the Earth's surface.
• Gases within the atmosphere follow distinct cycles of production, referred to as sources, and removal, referred to as sinks.
• While the total mass of atmospheric gases remains relatively constant, the composition is a dynamic process characterized by continuous cycles of destruction and production.

Permanent and Inactive Gases

• Atmospheric gases are classified based on their reactivity and interaction with radiation as either radiatively and chemically inactive or radiatively and chemically active.
• Nitrogen ($N_2$):
  • Comprises $78\%$ of the lower atmosphere by volume.
  • It is a permanent (or constant) gas because its concentration remains nearly uniform throughout the atmosphere and has been stable over recent history.
  • It is primarily a neutral substance as it does not react easily with other substances.
  • Sinks: Removed from the atmosphere through biological processes involving soil bacteria and tiny ocean-dwelling plants that convert it into nutrients to fortify the ocean food chain.
  • Sources: Returned to the atmosphere mainly through the decay of plant and animal matter.
• Argon ($Ar$):
  • A radiatively and chemically inactive gas that is largely inert.
  • Accounts for just under $1\%$ of the surface atmosphere.
  • It is considered a trace gas due to its small fractional volume.
• Oxygen ($O_2$):
  • Comprises $21\%$ of the atmosphere and is classified as a permanent gas.
  • It is highly reactive and readily combines with other substances through the process of oxidation.
  • Sources: Produced via photosynthesis (plants combining $CO_2$ and $H_2O$ with sunlight to create sugar and oxygen) and through ultraviolet radiation breaking down water and nitrous oxide in the stratosphere.
  • Sinks: Removed via respiration by plants and animals, the weathering of decaying matter, and the growth of shellfish.

Variable Gases and Greenhouse Components

• Variable gases are those whose concentrations fluctuate significantly across different locations and time periods.
• Water Vapor ($H_2O$):
  • An invisible gas and the most abundant variable gas.
  • It is renewed via the hydrologic cycle.
  • Concentration levels vary from near $0\%$ in desert and polar regions to nearly $4\%$ in tropical regions.
  • It is a critical contributor to Earth's energy balance; satellite imagery shows that water vapor distribution is broader than the distribution of visible clouds.
• Carbon Dioxide ($CO_2$):
  • A naturally occurring component of air that traps outgoing radiant energy from the Earth.
  • Sources: Decaying vegetation, volcanic eruptions, animal respiration, the burning of fossil fuels (coal, oil, and natural gas), and deforestation.
  • Sinks: Photosynthesis (carbon is stored in plant roots, branches, and leaves) and absorption by the oceans.
  • Oceans serve as massive reservoirs; phytoplankton fix $CO_2$ into organic tissues, and dissolved $CO_2$ circulates to great depths. Oceans hold more than $50$ times the total atmospheric $CO_2$.
• Methane ($CH_4$):
  • A variable gas present in small concentrations but increasing in recent history.
  • Sources: Fossil fuel activities, livestock digestion (biochemical reactions in cow stomachs), agriculture, and the breakdown of plant material by bacteria in rice paddies or oxygen-poor soil. Termite biological activity also contributes.
  • Role: Functions as a highly effective absorber of terrestrial radiation, playing an active role in warming the air near the surface.

Aerosols and Particulate Matter

• Aerosols (synonymous with particulates) are solid or liquid particles in the atmosphere other than water.
• Sources: Natural sources include desert dust, sea spray (from breaking waves and bursting foam bubbles), and smoke from wildfires (which can reach altitudes of several thousand meters). Anthropogenic sources include combustion and the incomplete burning of fossil fuels, particularly coal.
• Properties: Due to their microscopic size, they can remain suspended in the atmosphere for extended periods.
• Atmospheric Impact:
  • Act as condensation nuclei, which are essential for precipitation processes.
  • Decrease the amount of sunlight reaching the Earth's surface by scattering and absorbing solar radiation.
  • Increase cloud reflectivity, further reducing surface solar intake.

Environmental Lapse Rate and Mountain Vegetation

• In the lower atmosphere, temperature generally decreases as altitude increases.
• Environmental Lapse Rate (ELR): The specific rate of temperature decrease with height, which varies based on atmospheric conditions.
• Average Mid-latitude Lapse Rate: Approximately $6.4\,^\circ\text{C}$ per $1000\,\text{m}$.
• Vegetation Zonation: The ELR is visibly demonstrated by changes in vegetation on mountainsides:
  • Base: Warmer temperatures support lush, green, and large vegetation.
  • Mid-slope: As altitude increases and temperatures drop, vegetation becomes sparser and hardier (smaller plants and bushes).
  • Crest: Near the top, only small bushes and grasses survive.
  • Summit: Above the "tree line," temperatures are too cold to support any vegetation survival.

Characteristics of Atmospheric Layers

• The atmosphere is divided into four primary layers based on temperature trends: the troposphere, stratosphere, mesosphere, and thermosphere.
• Troposphere:
  • The lowest and thinnest layer, yet it contains $80\%$ of the total atmospheric mass.
  • Known for "atmospheric overturning," it is the region where virtually all weather occurs.
  • Warmed by the Earth's surface via solar radiation; temperature decreases steadily with elevation.
  • Tropopause: The upper boundary where cooling stops. It is roughly $16\,\text{km}$ thick over the tropics and $8\,\text{km}$ thick at the poles due to thermal expansion.
• Stratosphere:
  • Characterized by a temperature increase with altitude (temperature inversion).
  • This warming is caused by ozone ($O_3$) absorbing ultraviolet (UV) radiation.
  • The maximum ozone concentration occurs near $25\,\text{km}$, but the maximum temperature occurs near the top ($50\,\text{km}$).
  • This discrepancy is because the highest part of the layer absorbs the most energetic UV wavelengths first; by the time radiation reaches the dense ozone below, there is less energy left to absorb. Low air density also slows energy transfer.
• Mesosphere:
  • Temperatures decrease with height; it is the coldest atmospheric layer.
  • Cooling occurs because there is very little ozone to absorb solar radiation; molecules at the top receive less energy than those at the bottom.
  • Mesopause: Reached at approximately $85\,\text{km}$, where temperatures hit an average minimum of $-90\,^\circ\text{C}$.
• Thermosphere:
  • The uppermost layer that merges with interplanetary space.
  • Temperatures increase with height, reaching up to $1500\,^\circ\text{C}$.
  • While the kinetic energy (temperature) of molecules is high, the air is so sparse that the total heat content is negligible.
  • Along with the mesosphere, it accounts for only $0.1\%$ of the total atmospheric mass.

The Ionosphere and Upper Atmosphere Interactions

• The ionosphere is an electrified region within the upper atmosphere (not a distinct thermal layer) containing high concentrations of ions and free electrons.
• Ions: Atoms or molecules that have gained or lost electrons. This typically happens when they cannot absorb all the energy from the sun or colliding energetic particles, causing them to shed electrons and become positively charged.
• Phenomena:
  • Aurora Borealis (Northern Lights): Caused by interactions between the ionosphere and subatomic particles emitted by the sun, which excite atmospheric gases.
  • Communications: The ionosphere can cause disruptions in communications, particularly for AM radio signals.