13.1 Atmospheric chemical composition and reactions

The atmosphere's chemical composition is a complex mix of gases and particles. Nitrogen and oxygen dominate, but trace gases like carbon dioxide and methane play crucial roles in climate and chemistry. Understanding these components is key to grasping atmospheric processes.

Chemical reactions in the atmosphere shape air quality and climate. Photochemical reactions, driven by sunlight, and oxidation processes, often initiated by the hydroxyl radical, are fundamental. Factors like temperature, pressure, and solar radiation influence reaction rates and outcomes.

Atmospheric Chemical Composition

Components of atmospheric composition

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• Nitrogen ($N_2$) most abundant component makes up about 78% of the atmosphere
• Oxygen ($O_2$) second most abundant component makes up about 21% of the atmosphere essential for life and combustion processes
• Argon (Ar) third most abundant component makes up about 0.93% of the atmosphere inert gas does not participate in chemical reactions
• Water vapor ($H_2O$) highly variable concentration ranging from 0.1% to 4% depending on location and weather conditions (humidity) plays a crucial role in the Earth's energy balance and hydrological cycle
• Carbon dioxide ($CO_2$) trace gas with a concentration of about 0.04% (400 ppm) important greenhouse gas contributes to climate change
• Other trace gases collectively make up less than 0.1% of the atmosphere
  • Methane ($CH_4$) potent greenhouse gas produced by natural and anthropogenic sources (wetlands, agriculture, fossil fuel extraction)
  • Nitrous oxide ($N_2O$) long-lived greenhouse gas produced by microbial processes in soils and oceans and anthropogenic activities (fertilizer use, industrial processes)
  • Ozone ($O_3$) secondary pollutant formed by photochemical reactions protects life from UV radiation in the stratosphere but harmful to health and vegetation in the troposphere

Trace gases and aerosols

• Trace gases play critical roles in atmospheric chemistry and climate
  • Greenhouse gases ($CO_2$, $CH_4$, $N_2O$) absorb and emit infrared radiation contributing to the greenhouse effect and climate change
  • Ozone ($O_3$) in the stratosphere absorbs harmful ultraviolet (UV) radiation protecting life on Earth but acts as a pollutant in the troposphere harming human health and vegetation
• Aerosols are solid or liquid particles suspended in the atmosphere can be natural (dust, sea salt) or anthropogenic (sulfates, black carbon)
  • Affect the Earth's radiative balance by scattering and absorbing solar radiation leading to cooling or warming effects depending on their properties and altitude
  • Act as cloud condensation nuclei (CCN) and ice nuclei (IN) influencing cloud formation and precipitation (indirect effect on climate)
  • Can have adverse effects on human health when inhaled particularly fine particulate matter (PM2.5) linked to respiratory and cardiovascular diseases

Atmospheric Chemical Reactions

Key atmospheric chemical reactions

• Photochemical reactions are initiated by the absorption of solar radiation
  • Example: Photolysis of nitrogen dioxide ($NO_2$) to form nitric oxide (NO) and atomic oxygen (O) $NO_2 + hν → NO + O$
  • Atomic oxygen can then react with molecular oxygen to form ozone $O + O_2 + M → O_3 + M$ where M is a third molecule (usually $N_2$ or $O_2$) that absorbs excess energy and stabilizes the ozone molecule
• Oxidation processes involve the transfer of electrons from one species to another
  • Hydroxyl radical ($OH$) is the primary oxidant in the troposphere formed by the reaction of excited atomic oxygen with water vapor $O(^1D) + H_2O → 2OH$
  • $OH$ initiates the oxidation of many trace gases such as methane and carbon monoxide $CH_4 + OH → CH_3 + H_2O$ $CO + OH → CO_2 + H$ these reactions help to remove pollutants and greenhouse gases from the atmosphere but can also lead to the formation of secondary pollutants like ozone and aerosols

Factors in atmospheric reaction dynamics

• Temperature higher temperatures generally increase reaction rates by providing more kinetic energy for collisions and can shift equilibrium constants affecting the balance between reactants and products
• Pressure higher pressures increase the frequency of molecular collisions enhancing reaction rates and can shift equilibrium constants in reactions involving changes in the number of moles of gas
• Concentration of reactants higher concentrations lead to more frequent collisions and faster reaction rates
• Solar radiation photochemical reactions depend on the intensity and wavelength of available solar radiation with diurnal and seasonal variations affecting reaction rates
• Presence of catalysts lower the activation energy of reactions increasing rates without being consumed
  • Examples include heterogeneous reactions on aerosol surfaces (ice, mineral dust) and homogeneous reactions involving trace metals (iron, copper)