2.1 Chemical composition of the atmosphere

The atmosphere's chemical makeup is a complex dance of gases and particles. Nitrogen and oxygen dominate, but trace gases like carbon dioxide play outsized roles in climate. Understanding this composition is key to grasping how our atmosphere functions and changes.

Human activities are altering the atmosphere's delicate balance. From fossil fuel emissions to deforestation, we're increasing greenhouse gases and pollutants. This shift impacts everything from global temperatures to local air quality, making atmospheric chemistry a critical field of study.

Composition of Earth's Atmosphere

Primary Atmospheric Gases

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• Nitrogen (N₂) dominates Earth's atmosphere at 78% by volume
  • Most abundant gas due to its chemical stability and low reactivity
• Oxygen (O₂) comprises 21% of the atmosphere by volume
  • Second most abundant gas, crucial for respiration and combustion processes
• Argon (Ar) accounts for 0.93% of the atmosphere
  • Inert noble gas, does not participate in chemical reactions
• Carbon dioxide (CO₂) makes up 0.04% of the atmosphere
  • Trace gas vital for Earth's climate through the greenhouse effect
• Water vapor (H₂O) averages 0.25% near Earth's surface
  • Highly variable concentration depending on temperature and location
  • Plays a significant role in weather and climate processes

Trace Gases and Other Components

• Neon (Ne), helium (He), methane (CH₄), and ozone (O₃) present in trace amounts
  • Combined concentration less than 0.01% of the atmosphere
• Atmospheric composition varies slightly with altitude and location
  • Troposphere contains most of the water vapor and weather phenomena
  • Stratosphere houses the ozone layer, protecting Earth from UV radiation
• Particulate matter suspended in the atmosphere
  • Includes dust, pollen, soot, and sea salt
  • Affects air quality and can influence climate through interactions with radiation

Trace Gases and Atmospheric Processes

Greenhouse Effect and Climate Impact

• Trace gases play a crucial role in Earth's energy balance despite low concentrations
• Greenhouse gases absorb and emit infrared radiation
  • CO₂, CH₄, and H₂O are primary contributors to the greenhouse effect
  • Trap heat in the atmosphere, leading to global warming
• Positive feedback loops amplify climate change
  • Melting permafrost releases methane, further increasing warming
• Radiative forcing measures the impact of greenhouse gases on Earth's energy balance
  • Expressed in watts per square meter (W/m²)
  • Positive values indicate warming, negative values indicate cooling

Atmospheric Chemistry and Air Quality

• Ozone (O₃) serves different roles depending on its location
  • Stratospheric ozone protects life by absorbing harmful UV radiation
  • Tropospheric ozone acts as a pollutant, causing respiratory issues
• Nitrogen oxides (NOₓ) and volatile organic compounds (VOCs) participate in complex reactions
  • Form secondary pollutants like photochemical smog
  • Affect air quality in urban areas (Los Angeles, Beijing)
• Chlorofluorocarbons (CFCs) and related compounds deplete stratospheric ozone
  • Montreal Protocol phased out CFC production to protect the ozone layer
• Aerosols interact with trace gases and affect atmospheric processes
  • Can scatter or absorb radiation, influencing Earth's energy balance
  • Act as cloud condensation nuclei, affecting cloud formation and precipitation

Sources and Sinks of Atmospheric Gases

Natural Sources and Sinks

• Nitrogen gas (N₂) cycles through biological processes
  • Produced by denitrifying bacteria in soils and oceans
  • Removed by nitrogen-fixing bacteria in root nodules (legumes)
• Oxygen (O₂) primarily generated through photosynthesis
  • Phytoplankton in oceans produce significant amounts of oxygen
  • Consumed by respiration in plants and animals
• Carbon dioxide (CO₂) has multiple natural sources and sinks
  • Volcanic emissions release CO₂ into the atmosphere
  • Oceans act as both source and sink, absorbing and releasing CO₂
  • Photosynthesis removes CO₂ from the atmosphere
• Methane (CH₄) produced by anaerobic decomposition
  • Natural wetlands are a significant source of methane
  • Termites generate methane as part of their digestive process

Anthropogenic Sources and Impacts

• Human activities significantly alter the balance of atmospheric gases
• Fossil fuel combustion releases CO₂ and other pollutants
  • Transportation, electricity generation, and industrial processes are major contributors
• Agricultural practices influence greenhouse gas emissions
  • Rice paddies emit methane due to anaerobic decomposition
  • Livestock (cattle, sheep) produce methane through enteric fermentation
• Deforestation reduces the Earth's capacity to absorb CO₂
  • Tropical rainforests (Amazon, Congo Basin) are crucial carbon sinks
• Industrial processes release various pollutants
  • Cement production emits CO₂ as a byproduct
  • Refrigeration and air conditioning systems can leak potent greenhouse gases (HFCs)

Atmospheric Mixing Ratios

Concept and Measurement

• Mixing ratio represents the proportion of a constituent gas to total air
  • Expressed as parts per million (ppm), parts per billion (ppb), or parts per trillion (ppt) by volume
  • Remains constant with changes in pressure and temperature, unlike number density
• Calculation of mixing ratio uses partial pressure and total atmospheric pressure
  • Mixing ratio = (partial pressure of gas) / (total atmospheric pressure)
• In-situ measurements employ various instruments
  • Gas chromatographs separate and quantify different atmospheric components
  • Mass spectrometers identify gases based on their mass-to-charge ratio
  • Optical analyzers use spectroscopic techniques to measure gas concentrations

Global Monitoring and Variability

• Remote sensing techniques allow measurement from a distance
  • Fourier transform infrared spectroscopy (FTIR) analyzes atmospheric composition using infrared radiation
  • Differential optical absorption spectroscopy (DOAS) measures trace gases using UV and visible light
• Global monitoring networks track atmospheric composition
  • Global Atmosphere Watch (GAW) program operates worldwide measurement stations
  • Mauna Loa Observatory in Hawaii provides long-term CO₂ measurements (Keeling Curve)
• Mixing ratios vary spatially and temporally
  • Long-lived gases (CO₂, CH₄) show relatively uniform global distribution
  • Short-lived species (tropospheric O₃, NO₂) exhibit significant local variations
• Seasonal cycles affect mixing ratios of certain gases
  • CO₂ levels fluctuate annually due to Northern Hemisphere vegetation growth and decay
  • Methane concentrations show seasonal variations linked to wetland emissions and OH radical abundance