Atmospheric Chemical Kinetics
The rates and temperature dependence of atmospheric reactions and photolysis processes that govern how fast trace gases are transformed.
Definition
Atmospheric chemical kinetics is the quantitative study of the speeds of the gas-phase and photolytic reactions that control the chemical evolution of the atmosphere.
Scope
Covers the formalism of reaction rate constants and the Arrhenius expression, bimolecular and termolecular reactions, pressure dependence of association reactions, photolysis rate calculation from cross sections and quantum yields, laboratory measurement of kinetic parameters, and the evaluated databases used in atmospheric models.
Core questions
- How do reaction rate constants depend on temperature and pressure?
- How are photolysis rates computed from molecular cross sections and sunlight?
- How are kinetic data evaluated and assembled for use in atmospheric models?
Key theories
- Rate-constant formalism
- Reaction rates are expressed through rate constants whose temperature dependence follows the Arrhenius form, with termolecular and association reactions additionally depending on pressure.
Mechanisms
Each elementary reaction has a rate constant relating reaction rate to reactant concentrations; bimolecular constants typically follow an Arrhenius temperature dependence, while three-body and association reactions vary with pressure between low- and high-pressure limits. Photolysis rates are obtained by integrating the product of the actinic flux, the absorption cross section and the quantum yield over wavelength. Laboratory measurements feed periodically evaluated compilations that supply consistent parameters to chemical transport models.
Clinical relevance
Accurate kinetic and photolysis data are indispensable inputs to air-quality and climate-chemistry models that predict pollutant and greenhouse-gas concentrations.
History
Decades of laboratory kinetics measurements have been distilled into community evaluations such as those of the IUPAC subcommittee led by Atkinson and the NASA Jet Propulsion Laboratory panel, which provide the recommended rate and photolysis data underpinning atmospheric modeling.
Key figures
- Roger Atkinson
- John Seinfeld
Related topics
Seminal works
- atkinson2004
- seinfeldPandis2016
Frequently asked questions
- Why do some atmospheric reaction rates depend on pressure?
- Association and three-body reactions require a collision with a third molecule to stabilize the product, so their rates increase with the density of surrounding air until reaching a high-pressure limit.