tokens: be generated at levels within the cloud deck, where the background continuum emission originates. It must be generated well above this layer, where the absorbing gas is cooler and therefore absorbs more efficiently than it emits. The narrow width of the absorption line also suggests that it was formed at pressures substantially less than those of the cloud top (70 km, ∼3000 Pa). Figure 4. Radiance mixing ratio Jacobians ( dI ν / dr mix ) as a function of layer pressure for the radiance streams at 21 degrees zenith angle, for the 266.94 GHz feature for SO 2 (left) or PH 3 (right). Absorption line strengths (lower panels in each case) are given in units of cm −1 /(molecule cm −2 ). The continuum originates from SO 2 at ∼6 × 10 4 Pa (∼54 km, within the cloud deck), while the line cores for either species do not originate in the clouds (48–70 km) but at over 400 Pa (over 80 km) in the mesosphere. In the right panels in both plots, the temperature structure is given as a black line, while the colored lines denote SO 2 (green) or PH 3 (purple) mixing ratios. On the right, the evenly mixed 20 ppb PH 3 profile is shown with a solid line and the photochemical PH 3 profile is shown with a dashed line. 3.3. ALMA Line Dilution While the nondetection of prominent SO 2 spectral features in the ALMA wideband data could indicate a low abundance, as argued by Greaves et al. ( 2020a ), the estimation of this abundance was done without correcting for line dilution as a result of the ALMA observing geometry (Greaves et al. 2020a ). Significant line dilution is likely, especially considering the global distribution of SO 2 in the Venus atmosphere, and the exclusion of the short baseline ALMA measurements. Greaves et al. ( 2020a ) estimated line dilution (filtering losses) of 60%–92% depending on position on the disk. To determine the disk-averaged line dilution for the SO 2 reference line search, we simulated observations of Venus using the ALMA configuration of Greaves et al. ( 2020a ) by imposing an appropriate resolution spectrum (0.00003 cm −1 , 1 MHz) of our Case D atmospheric model over a limb-darkened disk model. The Fourier Transform of this model was resampled to match the ALMA configuration and reimaged using the imaging routines of Greaves et al. ( 2020a ), as provided in their Supplementary Software 3. As shown in Figure 5 , line dilutions on the order of 95% at the line core are observed for the full disk. We observe similar dilutions when the spectrum is only imposed on one hemisphere (∼8′′ extent at the time of observation). This line dilution suggests that the SO 2 reference features produced by our best-fit SO 2 distribution (Case D) would be heavily suppressed by line dilution in the ALMA data, which would cause them to mimic smaller features below the ripple detection limit of −0.0006. Figure 5. Modeled line dilution for the ALMA observations: disk-averaged line/continuum ratios for our nominal Case D atmosphere model containing SO 2
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