Bimodal aerosol distribution in Venus' upper haze from joint SPICAV-UV and -IR observations on Venus Express.

Spectroscopic solar occultation measurements by the Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus/Solar Occultation at Infrared instrument (SPICAV/SOIR) onboard the Venus Express orbiter gave new data about upper haze aerosol properties, its vertical distribution and spatial and temporal variations. Early study with three channels of SPICAV/SOIR instrument using a few selected orbits indicated presence of two aerosol modes in particle size distribution (Wilquet et al., 2009). Analysis of aerosol properties from the SPICAV−IR spectrometer for the whole Venus Express data set obtained from May 2006 till November 2014 has proved it for some occultations (Luginin et al., 2016). In this work, we report retrieval of the upper haze (81–100 km) aerosol properties from 101 simultaneous SPICAV−UV and –IR solar occultation sessions acquired between March 2007 and January 2013. A joint analysis of the data from two spectrometers allowed us to characterize the size distribution ∼10 km higher in the atmosphere compared to previous analysis and to detect bimodal distribution in ∼50% of observations previously believed to be unimodal. At altitudes 81–92 km bimodality is observed in >50% of cases. Mode 2 particles are detected up to 98 km and mode 1 up to 100 km. Mean radius equals 0.14 ± 0.03 μm for mode 1 and 0.78 ± 0.18 μ m for mode 2. Number density profiles for both modes of particles exponentially decrease with altitude, starting from 50 cm−3 and 0.3 cm−3 at 82 km for mode 1 and mode 2, respectively, and reaching 3 cm−3 at 98 km for mode 1 and 0.03 cm−3 at 94 km for mode 2. • At altitudes 81–92 km bimodal distribution was observed in >50% of observations. • The mean effective radii are 0.14 ± 0.03 μm for mode 1 and 0.78 ± 0.18 μm for mode 2. • Number density profiles for both modes exponentially decrease with altitude. • Effective variance of mode 1 reaches 0.4 at <84 km and 0.03–0.15 at higher altitudes. • Effective variance of mode 2 is almost constant with height with mean value 0.04 ± 0.02. [ABSTRACT FROM AUTHOR]