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.