Absorption and Emission by Atmospheric Gases

How molecular structure determines which wavelengths atmospheric gases absorb and emit, shaping the spectrum of radiation reaching space and the surface.

Definition

Absorption and emission by atmospheric gases is the wavelength-selective interaction of radiation with gas molecules, governed by quantized energy transitions, that determines how the atmosphere transmits, absorbs and re-emits radiation.

Scope

Covers molecular rotational, vibrational and electronic transitions; the absorption bands of water vapour, carbon dioxide, ozone, methane and nitrous oxide; line shapes and pressure and Doppler broadening; band models and line-by-line calculations; the atmospheric window; and spectroscopic databases used to compute gaseous absorption.

Core questions

• Why do different gases absorb at characteristic wavelengths?
• What sets the strength and width of individual absorption lines?
• Why is the atmosphere relatively transparent in the thermal infrared window near 10 micrometres?

Key theories

Molecular spectroscopy of atmospheric gases

Quantized rotational and vibrational transitions produce discrete absorption lines whose positions and strengths are fixed by molecular structure, organized into databases such as HITRAN for radiative calculations.

Mechanisms

A gas molecule absorbs a photon when its energy matches a transition between quantized rotational, vibrational or electronic states; greenhouse gases such as carbon dioxide and water vapour have strong vibrational-rotational bands in the infrared. Individual lines are broadened by molecular collisions (pressure broadening) at low altitude and by thermal motion (Doppler broadening) at high altitude. Overlapping lines form absorption bands, leaving relatively transparent spectral windows where the surface radiates efficiently to space.

Clinical relevance

Accurate gaseous absorption data underpin radiation schemes in weather and climate models and the retrieval of temperature and trace-gas concentrations from satellite and ground-based spectrometers.

History

Tyndall's nineteenth-century laboratory measurements first revealed the strong infrared absorption of water vapour and carbon dioxide. Twentieth-century spectroscopy resolved individual molecular lines, and compilations such as the HITRAN database, maintained since the 1970s, now provide the line parameters used in modern radiative transfer calculations.

Key figures

• John Tyndall
• Kuo-Nan Liou

Seminal works

• liou2002
• gordon2022

Frequently asked questions

What is the atmospheric window?

The atmospheric window is a band of the thermal infrared near 8 to 12 micrometres where the clear atmosphere absorbs weakly, allowing the surface to radiate heat directly to space.