Search

This book develops both spectroscopy and radiative transfer for planetary atmospheric composition in a rigorous and quantitative sense for students of atmospheric and/or planetary science. Spectroscopic field measurements including satellite remote sensing have advanced rapidly in recent years, and are being increasingly applied to provide information about planetary atmospheres. Examples include systematic observation of the atmospheric constituents that affect weather, climate, biogeochemical cycles, air quality on Earth, as well as the physics and evolution of planetary atmospheres in our solar system and beyond. Understanding atmospheric spectroscopy and radiative transfer is important throughout the disciplines of atmospheric science and planetary atmospheres to understand principles of remote sensing of atmospheric composition and the effects of atmospheric composition on climate. Atmospheric scientists need an understanding of the details, strength and weaknesses of the spectroscopic measurement sources. Those in remote sensing require an understanding of the information content of the measured spectra that are needed for the design of retrieval algorithms and for developing new instrumentation.

Concepts and formulas that are generally useful in atmospheric radiative transfer are gathered here for later use. These include units for light wavelengths and frequencies. Optical elements for instrument description and characterization (solid angle descriptions, étendue, and the diffraction limit) are presented. Lambertian reflectance and emission properties are described, and the bi-directional reflectance distribution function, BRDF, introduced.

Line shapes describe how absorption and emission are spectrally distributed around the line positions formed by rotational, vibrational, and electronic transitions. Line shapes arise from the different processes that spectrally broaden the absorption and emission of radiation. Optical thickness and equivalent width are shown to be fundamentally related to line shape. The fundamental line shape functions for atmospheres including the Gaussian line shape due to molecular motion and the Lorentzian line shape from lifetime broadening, including collision (pressure) broadening are described. Their convolution, the Voigt line shape, which is important in some atmospheric conditions is also described. The standard HITRAN database of spectroscopic parameters of molecules for use in calculation of radiative transfer in planetary atmospheres, from radiofrequencies to the near ultraviolet, is introduced.

This chapter provides a broad overview of the spectroscopic principles required in order to perform quantitative spectroscopy of atmospheres. It couples the details of atmospheric spectroscopy with the radiative transfer processes and also with the assessment of rotational, vibrational, and electronic spectroscopic measurements of atmospheres. The principles apply from line-resolved measurements (chiefly microwave through infrared) through ultraviolet and visible measurements employing absorption cross sections developed from individual transitions. The chapter introduces Einstein coefficients before in turn discussing rotational spectroscopy, vibrational spectroscopy, nuclear spin, and electronic spectroscopy.

Radiative transfer is the process of energy transfer during the propagation of electromagnetic radiation through a medium. The processes of extinction, due to absorption and scattering, and thermal emission are described. It is shown how they can be represented by wavelength-dependent optical thickness, due to absorption or emission cross sections and the number of absorbers, emitters, or scatterers. Cloud optical thickness and conservative scattering are described. The scattering phase function is introduced. Next, the general form of radiative transfer is given, and its applicability to the details of planetary atmospheric radiation shown.

Global climate is controlled by an energy balance between incoming solar radiation and outgoing terrestrial radiation. An energy balance is first developed using a simple one-layer model of the atmosphere and then made more realistic by distributing the atmospheric optical depth smoothly in a Gray Atmosphere Model. Wavelength-specific and altitude-dependent absorption and emission for the ultraviolet through long-wave infrared are described. Knowledge is combined into an overall Earth energy budget. The sensitivity of the climate to radiative forcing is examined.

This chapter describes elastic scattering events, where the wavelength of the scattered light is unchanged from that of the incident light and conservative scattering, scattering without absorption, sometimes closely approximated in clouds. The scattering regime, scattering versus wavelengths and scatterer size are introduced. Polarization in scattering is described by the Stokes vector and the polarization ellipse. Molecular (Rayleigh) scattering is presented and its atmospherically-important inelastic component, Raman scattering (the Ring effect) quantified. Mie scattering for spherical particles is described as is the commonly-used Henyey-Greenstein Mie phase function approximation. Non-spherical scatterers are introduced. The Ångstrom exponent and the expansion of phase functions in Legendre polynomials are described.

This chapter provides an overview of atmospheric remote sensing measurement types, with examples. Measurements are made from a variety of locations, platforms, and geometries, including ground-based measurements, aircraft measurements, balloon measurements, and satellite measurements. The chapter discusses spectral sampling, vertical sensitivity including air mass factors, and noise, which all affect measurements in fundamental ways. An overview of the state of trace gas and aerosol measurements from nadir-viewing satellites is given, and the upcoming geostationary constellation of air pollution/air quality satellites is introduced.

Basic concepts and definitions of radiative transfer modeling are introduced. The applicability of single scattering to aerosol retrievals is demonstrated. A two-stream formulation of radiative transfer is introduced. The two streams, upwelling and downwelling radiation, are selected to angularly represent average atmospheric properties: Relatively simple, two-stream calculations form the basis for much practical calculation, particularly of hemispherical fluxes in stratified atmospheres. Following this development, the most usual method for replacing the general integrodifferential equations obtained when setting up a scattering scenario by a system of linear equations is demonstrated.

This chapter explores several of the most common and useful approaches to atmospheric data fitting as well as the process of using air mass factors to produce vertical atmospheric column abundances from line-of-sight slant columns determined by data fitting. An atmospheric spectrum or other type of atmospheric sounding is usually fitted to a parameterized physical model by minimizing a cost function, usually chi-squared. Linear fitting, when the model of the measurements is linear in the model parameters is described, followed by the more common nonlinear fitting case. For nonlinear fitting, the standard Levenberg-Marquardt method is described, followed by the use of optimal estimation, one of several retrieval methods that make use of a priori information to providing regularization for the solution. In the context of optimal estimation, weighting functions, contribution functions, and averaging kernels are described. The Twomey-Tikhonov regularization procedure is presented. Correlated parameters, with the important example of Earth’s atmospheric ozone, are discussed.

Basic properties of the Sun, the Earth and its atmosphere, other solar system atmospheres, and extrasolar planetary atmospheres are introduced here to provide background and context for the detailed study of the spectroscopy and radiative transfer of planetary atmospheres. Solar structure is described, including the solar cycle and variability, and a reference solar irradiance is presented. The Earth’s orbit, the seasons, and the ecliptic plane are introduced. The properties of hydrostatic equilibrium, albedo, and spectral reflectance are described. Earth’s atmospheric composition, including aerosols and gases, is summarized. Other atmospheres in the solar system are described and the growing field of extrasolar planets detection and characterization introduced.