This project involves the development of a theoretical framework to calculate pressure--broadened halfwidths and pressure-induced line shifts for trace gases in the terrestrial atmosphere. The halfwidth is the major source of error reducing remote sensing data and in line-by-line models used to calculate radiative forcing from greenhouse gases. Recent work also shows that the inclusion of the line shift in profile retrievals reduces the error in the procedure. To address these problems, data is needed for thousands of ro-vibrational transitions. Hence a theoretical model is needed that is computationally efficient and capable of determining these parameters accurately. The complex Robert-Bonamy (CRB) formalism is the basis of the theoretical approach. The calculations use realistic molecular dynamics, all relevant terms in the interaction potential, and no cutoff procedure. The effects of explicit velocity averaging on the halfwidth and line shift will be studied. The intermolecular potential and the dependence of the halfwidth and line shift on the parameters describing this potential will be investigated. The potentials will be optimized by nonlinear least-squares fits to experimental and theoretical data. Initial calculations will be done for species for which there are experimental measurements for several vibrational bands to compare with. These are water and ozone perturbed by molecular nitrogen. The theory will also be extended to consider self-broadening and shifting of these species and other developments of the wavefunctions. The vibrational state, rotational state, and temperature dependence of these collision-induced parameters will be investigated. This project is supported jointly by the Atmospheric Chemistry and the Experimental Physical Chemistry Programs.
