Line Shape Parameters in Support of Remote Sensing of Earth's and Planetary Atmospheres

University Of Massachusetts Lowell, Lowell MA

Investigators

Abstract

This project involves development of a theoretical model to calculate pressure-broadened half-widths and pressure-induced line shifts and their temperature dependences for gases in the terrestrial and planetary atmospheres that meets the needs of the remote sensing community. The half-width remains the major source of error in the spectral parameters used for reducing remote sensing data and in line-by-line models used to calculate radiative forcing from greenhouse gases. Because data are needed for many thousands of ro-vibrational transitions of many different gases a computationally efficient theoretical model, based on physics, is desired that can correctly determine the line shape parameters. The semi-classical Complex Robert-Bonamy formalism is the basis of the theoretical approach. For results to be good for all transitions, not only strong lines, the intermolecular potentials must be expanded to very high order and the molecular dynamics must use Hamilton's equations. For transitions in the near infrared and visible spectral regions better wavefunctions and energies are needed, since the classic effective Hamiltonian determination breaks down. The use of ab initio energies and wavefunctions in the determination of the reduced matrix elements is being explored as a means to remedy the problem. The vibrational state, rotational state, and temperature dependence of the collision-induced parameters are being investigated. Because of the improvement in space borne spectrometers, line shape models need to be improved. While the Voigt model forms the basis for line shape, corrections for Dicke narrowing and speed dependence, and their correlation, must be and are being considered in this work. The resulting data will provide greatly improved line shape parameters and their temperature dependence, aiding researchers doing concentration and temperature profile retrievals and radiative transfer studies of the heat balance of Earth and other planets by reducing the uncertainty of the half-widths and other line shape parameters used in their models. This work will improve confidence in spectroscopic remote sensing results and allow a better determination of the radiative forcing of greenhouse gases. The improvement in understanding the energy balance of the Earth will help make more informed policy decisions. The data will be distributed via the HITRAN, HITEMP, GEISA, and IUPAC (for water vapor) databases and the principal investigator's web site. A postdoctoral scholar and several undergraduate students will work on this project, learning skills that they would not get in a classroom setting. An ongoing collaboration with a local high school allows undergraduate students to routinely visit the school to talk about their work with the aim of creating interest in science among the high schoolers.