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absorption--fractional amount of incident radiation absorbed by a gas. Absorption = 1 – transmittance. In general, absorption varies with the frequency of the radiation.

absorption line-- The decrease in a transmittance spectrum corresponding to the radiation absorbed by a particular transition between quantum states of a molecule. Early spectra were recorded on strips of film, where molecular absorption features appeared as dark lines across the continuous spectrum, hence the name "absorption line".

abundance--The relative amount, or concentration, of a gas in a volume. This is typically specified by the gas' volume mixing ratio.

blackbody--a hypothetical object that absorbs all incident radiation while maintaining thermal equilibrium. No light is reflected from or passes through a blackbody, but radiation is emitted, and is called blackbody radiation. The prefix "black" is used because at room temperature such an object would emit almost no visible light, appearing black to an observer. No physical object exactly fits this definition, but most behave at least in part as blackbodies. Planck's law gives the spectral distribution of radiation emitted by a blackbody.

cross sections--normalized molecular absorption spectra, given in units of cm2 per molecule at each wavenumber. These are used to model spectra where line parameters are insufficient or unavailable.

Doppler effect--the change in frequency of observed radiation resulting from motion of the source relative to the observer.

emissivity--a unitless spectral quantity equal to the ratio of energy emitted by an object to that emitted by a blackbody at the same temperature.

emission (see radiance)

GEISA--a compilation of spectroscopic line parameters, similar to HITRAN (GEISA = Gestion et Etude des Informations Spectroscopiques Atmosphériques.)

HITEMP--the high-temperature analogue of HITRAN. The current compilation contains parameters for water vapor, carbon dioxide and carbon monoxide.

HITRAN--High-resolution transmission molecular absorption database. HITRAN is a compilation of spectroscopic parameters widely used for simulating the transmission and emission of radiation of gases in the atmosphere. See www.hitran.com for more details.

The HITRAN data can be downloaded to your computer (go to Line List Browser, Extract Data Tab). Subscribers can also upload their own line list files. The required format is the HITRAN format -- a fixed-field text file with the following fields

Field width FORTRAN format
molecule number 2 I2
isotopologue number 1 I1
transition frequency (cm-1) 12 F12.6
line Intensity 10 1PE10.3
Einstein A-coefficient 10 E10.3
air-broadened width 50PF5.4
self-broadened width5 F5.4
lower-state energy 101PF10.4
temperature dependence (of air width) 4 0PF4.2
pressure shift 8 F8.6
upper vibrational quanta 15A15
lower vibrational quanta 15A15
upper local quanta 15A15
lower local quanta 15A15
error codes 66I1
reference codes12 6I2
flag for line-mixing 1A1
upper statistical weight 7F7.1
lower statistical weight 7F7.1

Isotopologue--Isotopologues of a molecule share the same molecular structure, but their individual atoms are different isotopes. For example, 16O12C16O and 16O13C16O are different isotopologues of CO2. Any sample of air will contain a mixture of CO2 isotopologues in fixed proportions. 98.4% of the CO2 molecules will be the primary isotopologue, 16O12C16O, while the next most abundant isotopologue, 16O13C16O, will account for about 1.1%. The remaining fraction is made up by even less common isotopologues. These fixed percentages are sometimes referred to as natural or terrestrial abundances, and are listed here for all the species in our database. Under certain circumstances, there are slight variations in these isotopic fractions, but for most atmospheric applications it is sufficient to assume the fractions are constant.

Note: The name isotope applies only to atoms, but is commonly (albeit incorrectly) used as a synonym for isotopologue.

Isotope--An isotope is a variant of an atom with a specific atomic number (i.e. number protons plus neutrons). For example, the most common isotope of hydrogen, 1H, contains one proton and no neutrons. A less common isotope is 2H, called deuterium, which contains one proton and one neutron.

Note: The name isotope applies only to atoms, but is commonly (albeit incorrectly) used as a synonym for isotopologue.

LinePakTM--The GATS library of radiative transfer software used by SpectralCalc. LinePak performs detailed "line-by-line" modeling of molecular absorption and emission. Efficient and flexible, LinePakTM is at the heart of analysis systems for many major atmospheric remote sensing missions including HALOE, LIMS, SABER, CRISTA, CLAES and SOFIE. More details.

Line List--A list of parameters used in modeling molecular absorption spectra. Each molecular absorption line is described by a set of parameters including its center frequency, halfwidth, and intensity. These parameters allow the radiative transfer code (LINEPAK) to accurately model the absorption line shape at any temperature, pressure and gas concentration. See HITRAN for a more complete description of the parameters in a line list.

micron--10-6 meters. microns are a common unit for the wavelength of infrared radiation.

millibar (mbar)--a unit of pressure equal to 100 Pascals. Other common conversions are: 1 atmosphere = 1013.25 mbar, 1 lb/in2 = 68.9476 mbar

radiance--emitted power per unit surface area per solid angle per spectral interval. Common units are (W m-2 sr-1 cm-1) or (photons s-1 m-2 sr-1 m-1). Also referred to as emission.

Torr-- A unit of pressure equal to 1/760 atmosphere, or 101325/760 Pa.

transmittance--ratio of the intensity of radiation exiting a medium to the incident intensity. In general, transmittance varies with the frequency of radiation. Absorption = 1 – transmittance.

volume mixing ratio (vmr)--the fractional number of molecules of a species in a mixture. vmr = M/N where M is the number of molecules of the species in a volume and N is the total number of molecules. For trace gases, vmr is often given in parts per million or parts per billion.

wavenumber--the number of cycles of electromagnetic radiation per cm. The wavenumber, in cm-1, is equal to 10000 / (wavelength in microns).