Lower atmosphere minor gas abundances as retrieved from Venus Express VIRTIS-M-IR data at 2.3µm

Abstract

Minor gas abundances in the lower atmosphere of Venus' southern hemisphere are investigated using spectroscopic nightside measurements recorded by the Visible and InfraRed Thermal Imaging Spectrometer aboard ESA's Venus Express mission in the moderate spectral resolution infrared mapping channel (VIRTIS-M-IR, 1-5 µm, FWHM=17 nm). The entire usable data archive is utilized including only radiation spectra sampled at long detector exposure times (≥3.3 s) during eight Venus solar days between April 2006 and October 2008. Combined radiative transfer and retrieval techniques (Haus et al., 2013; Haus et al., 2014) are applied for a simultaneous determination of total cloud opacity and H 2 O, CO, and OCS abundances from the 2.3 µm atmospheric transparency window that sounds the altitude range between about 30 and 45 km. A wavelength-dependent CO 2 opacity correction is considered. Zonal averages of CO abundances at 35 km increase by about 35% from (22.9 ± 0.8) ppmv at equatorial latitudes to (31.0 ± 2.1) ppmv at 65°S and then decrease to (29.4 ± 2.4) ppmv at 80°S The ± figures refer to the statistical variability of retrieved abundances. In accordance with earlier results, the observed latitudinal variation of tropospheric CO is consistent with a Hadley cell-like circulation. Dawn side CO abundances at high latitudes are slightly smaller than dusk side values by about 7%. The latitudinal distribution of OCS at 35 km is anticorrelated with that of CO, ranging from about (1.15 ± 0.2) ppmv at 65°S to (1.60 ± 0.2) ppmv at low latitudes (poleward decrease of 28%). Zonal averages of H 2 O abundances near 35 km slightly decrease toward the South Pole by about 10%, and the hemispheric average is (32.0 ± 1.3) ppmv. A significant local time dependence of OCS and H 2 O is not observed. Detailed analyses of individual spectrum retrieval errors for different atmospheric models reveal that CO abundance results are reliable (error 4-7%), while H 2 O and OCS results have lower confidence (errors 30-47% and 41-86%, respectively). SO 2 abundances cannot reliably be retrieved from VIRTIS-M-IR spectra.

Sensitivity of net thermal flux to the abundance of trace gases in the lower atmosphere of Venus

“…For our study, any CF is acceptable between CF1.0 and CF4.0, corresponding to a total cloud opacity from 29 to 46 as calculated at 1 μm. This range of cloud opacities is close to the observed variation range from VIRTIS that shows 33–51 [ Haus et al , ]. We decided to use CF2.8 of Crcld as STD because of the good fitting shown in Figure .…”

“…Such a large quantity of water vapor has not been observed by subsequent experiments. Venera 9/10 measured 300 ppmv [ Marov , ], and much less water vapor abundance, 26–32 ppmv, was reported using ground‐based and Venus Express observations [ Marcq et al , , ; Haus et al , ; Arney et al , ]. As a consequence, the reason for the lowest net thermal flux value derived by Revercomb et al [] is not yet understood.…”

“…Very small amounts of thermal energy from the deep atmosphere are leaking out through the so‐called atmospheric transparency windows at several specific wavelengths such as 1.7 μm and 2.3 μm [ Allen and Crawford , ]. These atmospheric window spectra have been used to analyze trace gaseous abundances below the clouds as well as cloud opacities [ Pollack et al , ; Meadows and Crisp , ; Marcq et al , , ; Arney et al , ; Haus et al , ]. But these spectra at the limited range cannot be used to understand thermal energy balance at low altitudes.…”

Error analysis for retrieval of Venus׳ IR surface emissivity from VIRTIS/VEX measurements

“…Expected double standard deviations are set to 2σ H2O = 0.25 = 2σ HCl and correlation lengths to λ H2O = 2000 km = λ HCl . This can be estimated by observing the spatial variation in the results by Tsang et al (2009) (for CO) and Haus et al (2015) (for CO and H 2 O) and is here carried over to other minor gases as first estimate, with the atmospheric convection and its spatial scales as motivation. This leads to C 12 = 0.9973 (Eq.…”

Performance of Akatsuki/IR2 in Venus orbit: the first year

“…On IR2, these observations are made possible by a set of narrow-band filters (1.735, 2.26, and 2.32 μm) installed in a 6-position filter wheel (Satoh et al 2016; hereafter referred to as Paper I). The scientific objectives with these filters include studying the atmospheric dynamics by measuring the cloud motion vectors (Hueso et al 2012), morphology of clouds (McGouldrick et al 2008), aerosol properties (Carlson et al 1993;Haus et al 2015) by combining 1.735-and 2.26-μm images, and mapping the sub-cloud CO (at ~ 35 km altitudes) by differentiating the 2.26-and 2.32-μm quasi-simultaneous image pair.…”