Journal of Quantitative Spectroscopy & Radiative Transfer 64 (2000) 439 } 456 Argon-induced halfwidths and line shifts of water vapor transitions R.R. Gamache*, R. Lynch Department of Environmental, Earth, and Atmospheric Sciences, The University of Massachusetts Lowell, 1 University Avenue, Lowell, MA 01854, USA Columbia University, Goddard Institute for Space Studies, 2880 Broadway, New York, NY 10025, USA Abstract Pressure-broadened halfwidths and pressure-induced line shifts are determined for water vapor transitions with argon as the perturbing gas. Calculations based on the complex Robert}Bonamy (CRB) formalism are made for a number of vibrational bands, for which there are experimental data to compare with. The intermolecular potential is taken as a sum of Lennard}Jones (6}12) atom}atom, isotropic induction, and dispersion components. The dynamics of the collision process are correct to second order in time. The calculations demonstrate that the atom}atom potential for this system should be expanded to 12th order. A new feature in the CRB approach is that the real and imaginary components of the Liouville S matrix a!ect both the halfwidth and the line shift. It is shown here that the imaginary parts of the S matrix strongly a!ect the calculated H  O}Ar halfwidths. The calculated values are compared with those obtained in a number of experimental studies. In general, good agreement is observed between the CRB calculations and the measured values for both halfwidths and line shifts. It is also clear that some parameters describing the intermolecular potential need to be better determined.  1999 Elsevier Science Ltd. All rights reserved. 1. Introduction Water is the principle absorber of longwave radiation in the terrestrial atmosphere, responsible for some 80% of `greenhousea warming of the Earth's surface [1]. The study and comprehension of the radiative-forcing e!ects and the interpretation of remote-sensing data of water vapor requires the use of atmospheric radiative-transfer models, which require high-precision parameters describ- ing line positions, intensities, pressure-broadened halfwidths and line shifts. Although laboratory measurements can and do supply such parameters, it is di$cult to exhaustively cover the vast * Corresponding author. E-mail address: robert } gamache@uml.edu (R.R. Gamache) 0022-4073/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 2 - 4 0 7 3 ( 9 9 ) 0 0 1 0 1 - 6