Biogeochemical Cycles of Trace Gases

The sources, sinks and atmospheric budgets of climate- and chemistry-relevant trace gases such as methane, nitrous oxide and carbon monoxide.

Definition

The biogeochemical cycles of trace gases are the linked flows of minor atmospheric gases among the atmosphere, biosphere, oceans and lithosphere, balancing their emission, transformation and removal.

Scope

Covers the natural and anthropogenic sources of major trace gases, their removal by chemical oxidation and deposition, the concept of atmospheric lifetime, the coupling of atmospheric chemistry to terrestrial and marine biogeochemistry, and the construction of global budgets such as the methane budget.

Core questions

• What are the main natural and human sources of major trace gases?
• How is the atmospheric lifetime of a trace gas determined?
• How are global budgets such as the methane budget constructed and balanced?

Key theories

Source-sink budget framework

The atmospheric burden of a trace gas reflects the balance of emissions against chemical and depositional sinks, with the steady-state lifetime equal to burden divided by removal rate.

Mechanisms

Trace gases enter the atmosphere from biological activity, combustion, agriculture and geological sources, and are removed by chemical oxidation, mainly by the hydroxyl radical, and by surface deposition. The ratio of the atmospheric burden to the total removal rate defines the lifetime, which ranges from hours for reactive species to a decade for methane. Bottom-up inventories and top-down inversions using observed concentrations are reconciled to close global budgets.

Clinical relevance

Quantifying trace-gas budgets is essential for attributing changes in greenhouse-gas concentrations, projecting future climate, and designing mitigation of methane and other emissions.

History

Systematic measurement of trace-gas concentrations from the mid-twentieth century, beginning with carbon dioxide at Mauna Loa, expanded to methane and other gases, enabling the global budget syntheses coordinated by international projects such as the Global Carbon Project.

Key figures

• Marielle Saunois
• Paul Crutzen

Seminal works

• saunois2020
• seinfeldPandis2016

Frequently asked questions

What is the atmospheric lifetime of a gas?

It is the average time a molecule remains in the atmosphere before removal, calculated as the atmospheric burden divided by the rate of loss; methane's is about a decade.