Atmospheric Chemistry and Trace Gases
The chemical reactions and cycles of minor atmospheric constituents that control air quality, oxidizing capacity and climate-active gases.
Definition
Atmospheric chemistry and trace gases is the study of the chemical composition of the atmosphere and the reactions that produce, transform and remove its minor gaseous constituents.
Scope
This area covers the photochemistry of the troposphere, the oxidizing capacity provided by the hydroxyl radical, the formation of ozone and photochemical smog, the sources, sinks and cycles of trace gases such as methane, carbon monoxide, nitrogen oxides and volatile organic compounds, and the chemical kinetics and photolysis processes that govern atmospheric composition.
Sub-topics
Core questions
- What gives the atmosphere its capacity to oxidize and cleanse itself of pollutants?
- How is ozone produced and destroyed in the lower atmosphere?
- What controls the concentrations and lifetimes of climate-active trace gases?
Key theories
- Hydroxyl radical oxidizing capacity
- The hydroxyl radical, produced photochemically, initiates the oxidation of most trace gases and acts as the atmosphere's principal cleansing agent.
- Photochemical ozone production
- In the presence of nitrogen oxides and volatile organic compounds, sunlight drives a catalytic cycle that produces ground-level ozone, the core of photochemical smog.
Mechanisms
Sunlight photolyses ozone and other species to generate the highly reactive hydroxyl radical, which attacks methane, carbon monoxide, volatile organic compounds and other trace gases, setting their atmospheric lifetimes. In polluted air the oxidation of these compounds in the presence of nitrogen oxides drives catalytic cycles that produce ozone and secondary pollutants. Concentrations of trace gases reflect a balance among emission, chemical transformation, transport and deposition.
Clinical relevance
Atmospheric chemistry underpins air-quality management, the prediction of ozone and particulate pollution episodes, and assessment of the budgets of greenhouse gases such as methane.
History
Modern atmospheric chemistry emerged from Haagen-Smit's 1950s identification of photochemical smog in Los Angeles and matured with the recognition of the hydroxyl radical's central oxidizing role in the 1970s, syntheses of which appear in the standard texts by Seinfeld and Pandis and by Finlayson-Pitts and Pitts.
Key figures
- John Seinfeld
- Spyros Pandis
- Barbara Finlayson-Pitts
Related topics
Seminal works
- seinfeldPandis2016
- finlaysonPitts2000
Frequently asked questions
- What is the atmosphere's oxidizing capacity?
- It is the ability of the atmosphere to chemically remove trace gases, governed mainly by the abundance of the hydroxyl radical, which initiates the oxidation of most pollutants.