Tropospheric Photochemistry and Oxidation

How sunlight drives the production of reactive radicals that oxidize and remove trace gases throughout the lower atmosphere.

Definition

Tropospheric photochemistry and oxidation is the suite of sunlight-driven radical reactions that initiate and propagate the chemical removal of trace gases in the lower atmosphere.

Scope

Covers photolysis reactions that generate radicals, the production of the hydroxyl radical from ozone photolysis and water vapour, the oxidation chains of methane and carbon monoxide, the cycling of hydroperoxy and organic peroxy radicals, and the concept of the atmosphere's self-cleansing oxidizing capacity.

Core questions

• How is the hydroxyl radical produced in the troposphere?
• Why is the hydroxyl radical the dominant oxidant despite its tiny concentration?
• How do radical chains control the lifetimes of methane and carbon monoxide?

Key theories

Hydroxyl-driven oxidation chains

Photolysis of ozone yields excited oxygen atoms that react with water vapour to form the hydroxyl radical, which initiates radical chains that oxidize most trace gases.

Mechanisms

Ultraviolet sunlight photolyses ozone to produce excited atomic oxygen, a fraction of which reacts with water vapour to form two hydroxyl radicals. Hydroxyl reacts rapidly with carbon monoxide, methane and volatile organic compounds, producing hydroperoxy and organic peroxy radicals that react with nitric oxide and recycle hydroxyl, sustaining the oxidizing chain. The steady-state hydroxyl concentration, though only about a million molecules per cubic centimetre, sets the atmospheric lifetimes of most reactive trace gases.

Clinical relevance

The hydroxyl radical's oxidizing capacity determines how long pollutants and greenhouse gases such as methane persist, a key control on air quality and climate.

History

Hiram Levy's 1971 calculation that the clean troposphere sustains substantial hydroxyl radical concentrations, building on insights from Crutzen and others, established the modern understanding of the atmosphere's oxidizing capacity.

Key figures

• Hiram Levy
• Paul Crutzen

Seminal works

• levy1971
• seinfeldPandis2016

Frequently asked questions

Why is the hydroxyl radical called the detergent of the atmosphere?

Because it reacts with and initiates the breakdown of nearly all reactive trace gases, cleansing the atmosphere of pollutants despite being present only in trace amounts.