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

Abstract

[1] Continuous absorption in the atmospheric window near 2400 cm À1 has been analyzed from infrared solar occultation spectra recorded by the Atmospheric Trace Molecule Spectroscopy (ATMOS) Fourier transform spectrometer at 0.01 cm À1 spectral resolution during its 4 shuttle flights between 1985 and 1994. The spring and autumn measurements were recorded over a wide range of temperatures and sulfate aerosol loadings. We used the ratio of the transmissions in microwindows at 2415 cm À1 and 2501 cm À1 as a measure of the underlying continuum absorption in the spectra. This ratio decreases from 1.0 to 0.0 as the tangent point density increases from 5 Â 10 17 to 100 Â 10 17 molecules cm À3(altitudes $10-30 km). Attenuation in both microwindows is due to overlapping absorptions by the fundamental N 2 collision-induced band and distant sub-Lorentzian wings of CO 2 lines from the strong n 3 fundamental band. Optical depths for both N 2 and CO 2 absorption are proportional to the square of the air density at constant temperature. These microwindows provide high sensitivity to density and low sensitivity to temperature in the lower stratosphere. Stratospheric transmission ratios from all 4 missions are fitted with a single parameter as a function of tangent point density with a root-mean square residual of $1%. Corresponding densities from this fit agree with ATMOS version 3 densities to $2.3% in the region of good sensitivity at 2 Â 10 18 to 8 Â 10 18 molecules cm À3($12-20 km altitude). The measured transmission ratio versus density relation has been compared with calculations from temperature-dependent laboratory measurements for both the N 2 and CO 2 continua. Simulation of the ATMOS spectra based on these laboratory measurements also predict a globally compact relation between the transmission ratio and density but differs from the empirically-determined relation by up to 7% in the lower stratosphere.

DETECTING AND CONSTRAINING N2ABUNDANCES IN PLANETARY ATMOSPHERES USING COLLISIONAL PAIRS

“…The (N 2 ) 2 absorption in this wavelength region is dominated by CIA, but N 2 quadrupole line absorption, and absorption by true (N 2 ) 2 van der Waals molecules also contributes (Long et al 1973). The temperature and wavelength-dependent absorption coefficients for this primarily collisional feature have been measured empirically (Farmer & Houghton 1966;Lafferty et al 1996), and (N 2 ) 2 absorption has been observed in solar occultation (transmission) observations by Earthobserving satellites (Rinsland 2004), but, to our knowledge, it has never before been considered in the context of planet characterization from full-disk or transit transmission observations.…”

Retrievals for the atmospheric chemistry experiment Fourier-transform spectrometer

“…The empirical expression chosen was inspired by the work of Rinsland et al, 25 where it was noted that the baseline in the region of N 2 ϩ CO 2 continua obeyed a consistent function of density for a diverse set of atmospheric conditions. For ACE-FTS processing we consider the ratio of the baseline at 2442.6 and 2502 cm Ϫ1 (denote this ratio as R b ).…”

Carbon dioxide retrievals from Atmospheric Chemistry Experiment solar occultation measurements

“…Collision‐induced absorption coefficients for N 2 (cm −1 atm −1 ) and their dependence with temperature have been reported for 2125 to 2600 cm −1 [ Lafferty et al , 1996, Table 1]. Continuum absorption by CO 2 also occurs in the window region, but its absorption is negligible in the 2500.6–2500.9 cm −1 interval selected for the current analysis [ Rinsland et al , 2004, Figure 2]. The window contains a weak CH 4 line at 2500.7616 cm −1 with a ∼5% line center absorption depth at 6.6 km tangent altitude.…”

“…As pointed out previously [ Foucher et al , 2009], two main difficulties for retrieving CO 2 from ACE spectra are: (a) accurate determination of the instrument pointing parameters (tangent heights) and (b) sensitivity of retrieved CO 2 to pressure/temperature profiles. Studies by Rinsland et al [2004] using atmospheric trace molecule spectroscopy (ATMOS) solar occultation FTS spectra recorded at 0.01 cm −1 resolution indicated that analysis of continuum pressure‐induced absorption by N 2 and CO 2 in the window region 2395–2525 cm −1 could help achieve the goal of precise determination of stratospheric densities from occultation measurements. Thus, we have implemented a procedure to determine the altitude shift to apply to version 2.2 ACE tangent heights as our means to resolve the difficulty (a) mentioned above.…”

“…Continuum absorption by the pressure‐induced fundamental band of molecular nitrogen at 4.3 μ m was studied previously based on FTS measurements [ Rinsland et al , 1989, 2004]. The absorption is significant over a wide altitude range and increases with the square of the density.…”