Stratospheric infrared continuum absorptions observed by the ATMOS instrument

Abstract

A quantitative analysis of continuum absorptions in the lower stratosphere is reported based on infrared solar absorption spectra recorded in the occultation mode at 0.01 cm−1 resolution. The spectra were obtained during seven sunrises near 48°S latitude and 12 sunsets near 28°N latitude with the atmospheric trace molecule spectroscopy (ATMOS) Fourier transform spectrometer operating April 29 to May 1, 1985, on board the shuttle/Spacelab 3 platform. Continuous absorption produced primarily by the collision‐induced fundamental vibration‐rotation band of O2 and to a lesser extent by the superposition of H2O far line wings has been observed in the 1400‐ to 1800‐cm−1 interval below tangent heights of about 25 km. Continuum optical depths measured in microwindows nearly free of atmospheric line absorption are 0.78±0.06 times those calculated with the O2 absorption coefficients of Timofeyev and Tonkov (1978). This difference is consistent with previous measurements, but more than a factor of 2 improvement in accuracy has been achieved. Transmittance measurements in microwindows between 2395 and 2535 cm−1 have been used to study continuous absorption from the collision‐induced fundamental vibration‐rotation band of N2 and the far wings of strong CO2 lines (primarily those of the ν3 fundamental). Below about 2430 cm−1 there is fair agreement between the measurements and calculations based on published absorption coefficients for both N2 and CO2; at higher wave numbers, where absorption by N2 is dominant, the calculated absorption exceeds the measured absorption. The measured transmittances have been analyzed to derive best fit absorption coefficients for the N2 pressure‐induced band at lower stratospheric temperatures (∼210 K). An absorption peak has been detected near 1200 cm−1 and is believed to result from extinction by the H2SO4 stratospheric aerosol layer.

Technical Note: An assessment of the accuracy of the RTTOV fast radiative transfer model using IASI data

“…However, we carried out our computations using version 2.4 of the CKD model (hence after referred to as CKD v2.4) that became available at the time the regression coefficients were about to be generated. The pressure-broadened band of N 2 at 2350 cm −1 (Menoux at al., 1993) and that of O 2 at 1550 cm −1 (Timofeyev and Tonkov, 1978;Rinsland et al, 1989) is included as broadband continuum contribution to the absorption and heavy molecules can be modeled using high-resolution cross-sectional data. In our computations, CO 2 Q-branch line mixing has been fully accounted for using first order line-mixing coefficients from Strow et al (1994).…”

The natural greenhouse effect of atmospheric oxygen (O2) and nitrogen (N2)

“…Rinsland et al [1981] confirmed these observations by analysis of balloon-borne limb solar occultation spectra. A further detailed analysis of the mid-infrared continuum signals of O 2 and N 2 has been performed on basis of space-borne observations by Rinsland et al [1989].…”

Lower stratospheric densities from solar occultation measurements of continuum absorption near 2400 cm−1

“…Continuum absorption in this region is produced by the superposition of absorption from collision‐induced fundamental vibration‐rotation band of N 2 and distant wings of lines from the strong CO 2 ν 3 fundamental band at 2349 cm −1 . The continuum has been studied using ground‐based, aircraft‐borne, and balloon‐borne spectra (see the references cited by Rinsland et al [1989], section 5, for early measurements). More recent experimental and theoretical studies of the 4.3 μm N 2 continuum include the papers by Lafferty et al [1996] and Moreau et al [2001].…”