HITRAN (an acronym for High Resolution Transmission) molecular spectroscopic database is a compilation of spectroscopic parameters used to simulate and analyze the transmission and emission of light in gaseous media, with an emphasis on planetary atmospheres. The knowledge of spectroscopic parameters for transitions between energy levels in molecules (and atoms) is essential for interpreting and modeling the interaction of radiation (light) within different media. For half a century, HITRAN has been considered to be an international standard which provides the user a recommended value of parameters for millions of transitions for different molecules. HITRAN includes both experimental and theoretical data which are gathered from a worldwide network of contributors as well as from articles, books, proceedings, databases, theses, reports, presentations, unpublished data, papers in-preparation and private communications. A major effort is then dedicated to evaluating and processing the spectroscopic data. A single transition in HITRAN has many parameters, including a default 160-byte fixed-width format used since HITRAN2004. Wherever possible, the retrieved data are validated against accurate laboratory data. The original version of HITRAN was compiled by the US Air Force Cambridge Research Laboratories (1960s) in order to enable surveillance of military aircraft detected through the terrestrial atmosphere. One of the early applications of HITRAN was a program called Atmospheric Radiation Measurement (ARM) for the US Department of Energy. In this program spectral atmospheric measurements were made around the globe in order to better understand the balance between the radiant energy that reaches Earth from the sun and the energy that flows from Earth back out to space. The US Department of Transportation also utilized HITRAN in its early days for monitoring the gas emissions (NO, SO2, NO2) of super-sonic transports flying at high altitude. HITRAN was first made publicly available in 1973 and today there are a multitude of ongoing and future NASA satellite missions which incorporate HITRAN. One of the NASA missions currently utilizing HITRAN is the Orbiting Carbon Observatory (OCO) which measures the sources and sinks of CO2 in the global atmosphere. HITRAN is a free resource and is currently maintained and developed at the Center for Astrophysics | Harvard & Smithsonian, Cambridge MA, USA.
HITRAN is the worldwide standard for calculating or simulating atmospheric molecular transmission and radiance from the microwave through ultraviolet region of the spectrum. The HITRAN database is officially released on a quadrennial basis, with updates posted in the intervening years on HITRANonline.There is a new journal article published in conjunction with the most recent release of the HITRAN database, and users are strongly encouraged to use the most recent edition. Throughout HITRAN's history, there have been around 50,000 unique users of the database and in recent years there are over 40,000 users registered on HITRANonline. There are YouTube tutorials on the HITRANonline webpage to answer frequently asked questions by users.
Line-by-Line The current version of the database, HITRAN2024, contains 61 molecules in the line-by-line portion of HITRAN along with some of their most significant isotopologues (156 isotopologues in total). These data are archived as a multitude of high-resolution line transitions, each containing many spectral parameters required for high-resolution simulations.
Absorption Cross-Sections In addition to the traditional line-by-line spectroscopic absorption parameters, the HITRAN database contains information on absorption cross-sections where the line-by-line parameters are absent or incomplete. Typically HITRAN includes absorption cross-sections for heavy polyatomic molecules (with low-lying vibrational modes) which are difficult for detailed analysis due to the high density of the spectral bands/lines, broadening effects, isomerization, and overall modeling complexity. There are 644 molecular species in the current edition of the database provided as cross-section files. The cross-section files are provided in the HITRAN format described on the official HITRAN website.
Collision-Induced Absorption The HITRAN compilation also provides collision-induced absorption (CIA) that was first introduced into HITRAN in the 2012 edition. CIA refers to absorption by transient electric dipoles induced by the interaction between colliding molecules. Instructions for accessing the CIA data files can be found on HITRAN/CIA.
MT_CKD Water Vapor Continuum The MT_CKD (Mlawer-Tobin-Clough-Kneizys-Davies) Water Vapor Continuum Model provides water vapor absorption coefficients that should be added to the local contributions of water vapor transitions, such as those available in the HITRAN line-by-line section, in order to obtain the total absorption due to water vapor. Compared to line absorption, the continuum absorption varies more slowly across the spectrum. The MT_CKD Water Vapor Continuum provides absorption coefficients for both the self continuum (arising from interactions between water molecules) and the foreign continuum (arising from interactions between water vapor and other atmospheric gases). These coefficients are distributed in a netCDF file at a reference density corresponding to a pressure of 1013 mbar and temperature of 296 K for both the self and foreign continuum. This file also contains the temperature-dependence coefficients for the self continuum, allowing users to adjust the reference values for temperatures other than 296 K. The accompanying code performs the appropriate scaling and interpolation to compute the continuum absorption coefficients at arbitrary pressures, temperatures, and wavenumbers. The MT_CKD Water Vapor Continuum Model is constrained to remain consistent with high-quality analyses of atmospheric and laboratory measurements. Keeping the MT_CKD continuum consistent with current observational studies requires periodic updates to the continuum coefficients. A record of revisions to the MT_CKD Water Vapor Continuum Model can be found on the model's GitHub page. The associated code and data are available through the HITRAN website at hitran.org/mtckd and at the AER GitHub site at github.com/AER-RC/MT_CKD_H2O.
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