Infrared and Millimeter-wavelength Absorption by Atmospheric Water Vapor

The propagation of electromagnetic radiation is affected by three major phenomena: absorption, scattering, and turbulence. Absorption by molecular water vapor is the dominant mechanism of tropospheric attenuation at millimeter and infrared wavelengths, especially in the marine environment. Therefore, the understanding and accurate modeling of absorption by this molecule are important to atmospheric remote sensing, infrared imaging systems, long-path laser propagation, electro-optical systems, radar, and atmospheric meteorology. Figure la shows the low-resolution infrared transmittance of the atmosphere and demonstrates the importance of water vapor over other atmospheric constituents. 1 The H20 absorption bands, along with those of CO2, define the atmospheric window regions. I will concentrate on the water vapor window absorption problem under tropospheric conditions. The main rotational and vibrational bands have been extensively characterized by Benedict and Calfee,2 Gates et al., 3 and Camy-Peyret and Flaud. 4 The work of those investigators has resulted in a compendium of absorptionline parameters, maintained by the Air Force Geophysics Laboratory,5,6 which represents a significant contribution to absorption calculations. The comparatively weak absorption that does occur in the window regions can be described as arising from two distinct sources, local line and continuum absorption, illustrated in the high-resolution spectrum of Fig. 1 b for the 10-JLm window region. Weak absorption bands of CO2 and HDO (hydrogen deuterium oxide), along with other H20 absorption lines in the window regions, compose the local line contribution. The continuum contributes an additional, gradually varying, frequency-dependent background to the total absorption. In 1942, Elsasser 7 recognized in the 13to 8-JLm window region a continuum, which he attributed to the far wings of the strong, nearby rotational and "2 vibrational-rotational bands of H2 O. Further verification of this nonlocalline absorption feature was provided by Yates and Taylor, 8 who studied infrared attenuation along horizontal paths at sea level. Solar spectra studies also indicated continuum absorption in the 13to 8-JLm window. 9-11 The nature of the continuum, judged by