2.1 Chemical composition of the atmosphere

The atmosphere's chemical makeup is a complex blend of gases, each playing a unique role. Nitrogen and oxygen dominate, but trace gases like carbon dioxide and methane are crucial for Earth's climate. Understanding these components is key to grasping atmospheric processes.

From greenhouse gases trapping heat to ozone shielding us from UV rays, atmospheric gases shape our world. Sources and sinks of these gases, along with their mixing ratios, help scientists track changes and predict future climate scenarios.

Chemical Composition of the Atmosphere

Composition of Earth's atmosphere

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• Nitrogen ($N_2$)
  • Most abundant gas in the atmosphere makes up about 78% of the atmosphere by volume
  • Largely inert and does not participate in many chemical reactions
• Oxygen ($O_2$)
  • Second most abundant gas in the atmosphere comprises about 21% of the atmosphere by volume
  • Essential for life on Earth as it is used in respiration by many organisms
• Argon (Ar)
  • Third most abundant gas in the atmosphere makes up about 0.93% of the atmosphere by volume
  • Inert gas that does not react with other substances under normal conditions
• Carbon dioxide ($CO_2$)
  • Trace gas in the atmosphere comprises about 0.04% of the atmosphere by volume
  • Important greenhouse gas that absorbs and emits infrared radiation contributing to the Earth's energy balance

Role of atmospheric trace gases

• Greenhouse gases
  • Absorb and emit infrared radiation trapping heat in the atmosphere (greenhouse effect)
  • Examples: water vapor ($H_2O$), carbon dioxide ($CO_2$), methane ($CH_4$), nitrous oxide ($N_2O$)
  • Increasing concentrations due to human activities lead to global warming and climate change
• Ozone ($O_3$)
  • Absorbs ultraviolet (UV) radiation in the stratosphere protecting life on Earth from harmful UV rays
  • Can be a pollutant in the troposphere contributing to smog and respiratory issues (ground-level ozone)
  • Formed by photochemical reactions involving nitrogen oxides (NOx) and volatile organic compounds (VOCs)
• Aerosols
  • Solid or liquid particles suspended in the atmosphere can have cooling or warming effects on climate
  • Scatter and absorb solar radiation influencing the Earth's energy balance (aerosol direct effect)
  • Act as cloud condensation nuclei affecting cloud properties and precipitation (aerosol indirect effect)
  • Examples: dust, sea salt, volcanic ash, sulfates, black carbon

Sources and sinks of gases

• Carbon dioxide ($CO_2$)
  • Sources: fossil fuel combustion (coal, oil, natural gas), deforestation, cement production, respiration
  • Sinks: photosynthesis by plants and phytoplankton, ocean absorption, weathering of rocks
  • Increasing atmospheric concentrations primarily due to human activities (anthropogenic emissions)
• Methane ($CH_4$)
  • Sources: wetlands, agriculture (rice cultivation, livestock), fossil fuel extraction (natural gas, coal), landfills
  • Sinks: chemical reactions in the atmosphere (oxidation by hydroxyl radicals)
  • Has a stronger greenhouse effect per molecule compared to $CO_2$ but shorter atmospheric lifetime
• Nitrous oxide ($N_2O$)
  • Sources: microbial processes in soils and oceans, agricultural activities (fertilizer use), industrial processes
  • Sinks: photodissociation in the stratosphere where it is broken down by ultraviolet radiation
  • Long-lived greenhouse gas with a global warming potential 298 times that of $CO_2$ over a 100-year period
• Ozone ($O_3$)
  • Sources: photochemical reactions involving NOx and VOCs in the troposphere, stratospheric production from oxygen photolysis
  • Sinks: chemical reactions with other substances (NOx, HOx), deposition to surfaces
  • Beneficial in the stratosphere but harmful to human health and vegetation in the troposphere

Atmospheric mixing ratios

• Mixing ratio
  • Ratio of the number of moles of a particular gas to the total number of moles of air in a given volume
  • Expressed as mole fraction, parts per million by volume (ppmv), or parts per billion by volume (ppbv)
  • Allows for consistent comparison of gas concentrations across different locations and times
• Dry air mixing ratio
  • Excludes water vapor from the calculation as water vapor content can vary significantly
  • Used to compare the composition of air masses with different moisture content
  • Helpful in understanding the behavior of gases in the atmosphere
• Conversion between units
  • ppmv = (mole fraction) × $10^6$
  • ppbv = (mole fraction) × $10^9$
  • Useful for expressing the concentrations of trace gases with very low abundances
• Importance in atmospheric science
  • Used in atmospheric models and climate simulations to represent the composition of the atmosphere
  • Helps monitor changes in atmospheric composition due to natural and anthropogenic factors
  • Provides a standardized way to report and analyze atmospheric measurements from different instruments and platforms