Aerosols

Preliminary characterization of the upper haze by SPICAV/SOIR solar occultation in UV to mid-IR onboard Venus Express

SOIR (Solar Occultation in the InfraRed), a echelle grating spectrometer, developed at BISA and flown on board the Venus Express ESA mission, is continuing to deliver high quality spectra from which the vertical concentration profiles of a series of key constituents of the Venus atmosphere can be obtained [1, 2, 3]. SOIR has also proven to be sensitive to the aerosols present in the Venus atmosphere. In Figure 1, are reported the extinction profiles for several observations. From this figure, we can infer a high temporal variability in the aerosol content in the atmosphere. From such profiles number density and radius size of the aerosols can be derived. They are compared in the following to similar results obtained from the two other channels of SPICAV (UV and IR).

 

Fig. 1: Temporal variation of a layer in the extinction profiles observed with the SOIR channel. Extinction coefficients in function of the altitude for: orbit 482 (brown), diffraction orders (o) 111, () 121, () 137, () 171; orbit 484 (blue), diffraction orders (o) 125, () 149, () 172, () 180; orbit 485 (green), diffraction orders (o) 112, () 119, () 149, () 174; orbit 486 (grey), diffraction orders (o) 121, () 149, () 171, () 190 and orbit 487 (orange), diffraction orders (o) 121, () 149, () 171, () 190. (credit: BISA)

Figure 2a shows the vertical size distribution of the particles for the solar occultation performed during orbit 486. The channels cannot discriminate particles with a radius higher than the wavelength at which the data were acquired, i.e. for this particular orbit, a maximal radius of 0.3 µm for SPICAV-UV, of 1.6 µm for SPICAV-IR and of 3.7 µm for SOIR. The data obtained with the UV channel indicate the presence of particles with a radius increasing from ~0.1 µm at 100 km up to ~0.3 µm at 75 km. The vertical distribution for this channel exhibits low variability from one orbit to another. From the SOIR and SPICAV-IR channels, the vertical distribution of particle size is limited to altitudes comprised between 70 km and 90-95 km. The radii determined with both channels are in fairly good agreement and are comprised between 0.4 and 1.0 µm. Particles of ~1 µm were detected at ~74-77 km of altitude during all orbits. The aerosol layers also exhibited temporal and/or spatial variations in terms of the size of the particles within the layer, from 0.4 up to 1.0 µm radius.

Fig. 2: Characterization of the Venus upper haze. (a) Vertical distributions of particle size and (b) Vertical profiles of the particle total number density during orbit 486. Radii derived by fitting normalized extinction to a haze model and allowed to derive N; (o) 2 wavelengths with the SPICAV-UV channel for mode 1 particles, () 10 wavelengths with the SPICAV-IR channel and (*) 4 wavelengths with the SOIR channel for mode 2 particles.. (credit: BISA)

Figure 2b shows the particle number densities derived in the UV spectral range for aerosol particles of about 0.1-0.3 mm size from the SPICAV-UV channel and in the IR for aerosol particles of about 0.4-1.0 mm size from the SPICAV-IR and the SOIR channels depending on the altitude. Number density for the larger particles (mode 2) decreases smoothly from 10-15 cm-3 at 70 km down to less than 1 cm-3 at 90 km. It is important to mention that the results obtained for the 2 IR channels are very comparable although they were calculated from independent data sets, yet from simultaneous measurements.

The present study demonstrated the existence of at least 2 types of particles, one type with a radius comprised between ~0.1 and 0.3 µm as inferred by the UV channel and the second type, detected in the IR, with a radius varying between ~0.4 and 1 µm depending on the altitude. Therefore the model describing the upper haze on Venus should include a bimodal population. To our knowledge, this is the first time that the existence of mode 2 particles at altitudes above 70 km in the atmosphere of Venus is shown. It was found independently for the 2 IR channels of the SPICAV/SOIR instrument, giving strong evidence for its existence.

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Preliminary characterization of the upper haze by SPICAV/SOIR solar occultation in UV to mid-IR onboard Venus Express

 

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