Chemical reactions in the atmosphere shape our world. From ozone formation to pollutant breakdown, these processes impact air quality, climate, and life on Earth. Understanding them is key to grasping atmospheric chemistry's role in our environment.

Four main reaction types drive atmospheric chemistry: photochemical, oxidation-reduction, acid-base, and free radical chain reactions. These processes occur in gas phase and on particle surfaces, influenced by factors like temperature, pressure, and humidity.

Atmospheric Chemical Reactions

Types of Atmospheric Reactions

• Chemical reactions in the atmosphere fall into four main categories
  • Photochemical reactions initiated by solar radiation absorption
  • Oxidation-reduction reactions transferring electrons between species
  • Acid-base reactions contributing to acid rain formation
  • Free radical chain reactions propagating through the atmosphere
• Gas-phase reactions dominate atmospheric chemistry
• Heterogeneous reactions occur on aerosol and cloud droplet surfaces
• Reaction rates and mechanisms influenced by environmental factors
  • Temperature
  • Pressure
  • Humidity
  • Presence of catalysts or inhibitors

Key Atmospheric Processes

• Photochemical reactions form reactive species (hydroxyl radicals, ozone)
• Oxidation-reduction reactions transform atmospheric pollutants
• Acid-base reactions neutralize atmospheric particles
• Free radical reactions alter trace gas and aerosol concentrations

Photochemistry in the Atmosphere

Fundamentals of Atmospheric Photochemistry

• Study of light-initiated chemical reactions in the atmosphere
• Solar radiation (especially UV) provides energy to break chemical bonds
• Photolysis breaks chemical bonds forming reactive species
  • Atomic oxygen
  • Hydroxyl radicals
• Influences Earth's energy balance and atmospheric chemistry
• Affects lifetimes and distributions of trace gases (natural and anthropogenic)

Important Photochemical Processes

• Chapman cycle describes natural ozone formation/destruction in stratosphere
• Photochemical smog forms in urban areas
  • Reactions between nitrogen oxides and volatile organic compounds
  • Driven by sunlight
• Hydroxyl radical (OH) production crucial for atmosphere's self-cleaning
  • OH primary oxidant for many pollutants
• Examples of key photochemical reactions:
  • $\ce{NO2 + hv -> NO + O}$ (leads to ozone formation)
  • $\ce{O3 + hv -> O2 + O(^1D)}$ (leads to OH production)

Ozone Formation and Destruction

Stratospheric Ozone Processes

• Ozone formation through oxygen photolysis and recombination
  • $\ce{O2 + hv -> O + O}$
  • $\ce{O + O2 + M -> O3 + M}$ (M stabilizing molecule)
• Chapman cycle balances natural ozone formation and destruction
• Catalytic destruction cycles enhance ozone depletion
  • Involve chlorine, bromine, nitrogen oxides, hydrogen oxides
• Chlorofluorocarbons (CFCs) release reactive halogens accelerating depletion

Ozone Depletion Phenomena

• Antarctic ozone hole results from unique conditions
  • Polar vortex isolation
  • Polar stratospheric cloud formation
  • Spring sunlight return
• Natural influences on stratospheric ozone levels
  • Volcanic eruptions (inject sulfur dioxide)
  • Solar cycles (affect UV radiation intensity)
• Human activities primary driver of observed depletion
• Montreal Protocol implementation leading to ozone layer recovery
  • Phased out ozone-depleting substance production

Oxidation Reactions in the Troposphere

Tropospheric Oxidation Processes

• Hydroxyl radicals (OH) drive primary oxidation reactions
  • Known as atmospheric "detergent" for pollutant removal
• Volatile organic compound (VOC) and nitrogen oxide (NOx) oxidation
  • Forms secondary pollutants (ozone, secondary organic aerosols)
• Sulfur dioxide oxidation to sulfuric acid
  • Contributes to acid rain and sulfate aerosol formation
• Oxidation influences greenhouse gas lifetimes (methane)
• Night-time chemistry dominated by nitrate radicals (NO3)
  • Oxidizes certain pollutants
  • Forms secondary organic aerosols

Air Quality Impacts

• Tropospheric ozone formation through complex NOx-VOC reactions
  • Major component of photochemical smog
  • Impacts human health and vegetation
• Secondary organic aerosol formation affects particulate matter levels
• Acid rain from sulfuric and nitric acid deposition
  • Damages ecosystems and infrastructure
• Oxidative capacity of troposphere key for air quality maintenance
  • Determined largely by OH concentrations
  • Removes both natural and anthropogenic pollutants
• Examples of key tropospheric oxidation reactions:
  • $\ce{CH4 + OH -> CH3 + H2O}$ (methane oxidation)
  • $\ce{SO2 + OH -> HOSO2}$ (first step in sulfuric acid formation)