The Information System HITRAN on the Web is designed for providing an access via Internet to the information about spectral line parameters of atmospheric gases and pollutants. This information is needed for solving problems of atmospheric optics and for modeling and visualizing molecular absorption spectra.
The System is comprised of the standard database HITRAN  that mainly covers the relatively well-studied infrared spectral region but is extended to wider spectral ranges from microwave to ultraviolet (0.00001-25232.0 cm-1). Besides the mentioned database, supplemental information attributed to molecules is used.
Computation of spectral line profiles of gas molecules is based on the line by line method that is summing of the standard line shapes of single absorption lines. As a rule, lines symbolize rotation-vibrational transitions that are grouped into vibrational bands. Line center positions and intensities are recorded by high-resolution spectrometers under controlled laboratory conditions. Also, in cases where the experimental data are incomplete, theoretical calculations of these quantities are used. When modeling high-resolution spectra, data are selected either via the profile Molecule/Isotope/Band or within a Wavenumber Range selected for a Gas Mixture chosen or composed by the user. To simulate a spectrum actually observed with low-resolution devices, the convolution of the high-resolution spectrum with the apparatus function of the device is computed.
Among problems solved by the System now, one may find modeling of wavenumber profiles of intensity histograms, absorption coefficients, absorption and transmission functions with various parameters followed by possible convolution. The System provides the user with a flexible and convenient tool for selecting these parameters. In particular, adjustable parameters are temperature, pressure, standard line shape of a single line (Voigt, Lorentz, or Doppler), wavenumber increment, extension of line wings (when computing the absorption coefficient, absorption and transmission spectra), path length (when computing absorption and transmission spectra), etc.
All results can be obtained in graphical or tabular forms as well as downloaded on the user computer, by request, in the form of ASCII files for possible treatment by other codes. The data computed may be saved on the server and therefore they can be used in later sessions.
The System is based on the Internet information systems SPECTRA  developed earlier.
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