What is the typical size of the sulfuric acid droplets in the atmosphere of Venus?

Question

According to Wikipedia:

Venusian clouds are thick and are composed mainly (75-96%) of sulfuric acid droplets. These clouds obscure the surface of Venus from optical imaging, and reflect about 75% of the sunlight that falls on them. The geometric albedo, a common measure of reflectivity, is the highest of any planet in the Solar System. This high reflectivity potentially enables any probe exploring the cloud tops sufficient solar energy such that solar cells can be fitted anywhere on the craft. The density of the clouds is highly variable with the densest layer at about 48.5 km, reaching 0.1 g/m^3 similar to the lower range of cumulonimbus storm clouds on Earth.

Can the typical size of the sulfuric acid droplets in the atmosphere of Venus be estimated from observation, or from calculation based on models of the atmosphere? Would the average size be expected to vary with altitude?

Answer 1

TL;DR: Since no one is answering, I am taking take the responsibility. Yes, just like @uhoh said, there are two size modes with mean radii ~0.2 µm (mode 1) and ~1 µm 60 (mode 2), along with a third, controversial mode with radius ~3.5 µm.

Long answer

The main cloud deck extends from about 48 km up to ∼70 km. It can be subdivided in three layers according to the behaviour of extinction coefficient and particle population.

1. The upper cloud (57–70 km) is populated by submicron (r1∼0.2 μm) and micron size (r2∼1 μm) particles. This is the altitude range where the photochemical “factory” producing sulphuric acid from SO₂ and H₂O is located.
2. The middle and lower clouds are separated from upper clouds a 1–2 km gap with reduced extinction located at ∼56 km. Below that level the cloud density gradually increases with depth reaching its maximum at ∼50 km. This region is characterized by tri-modal particle distribution with typical radii of 0.15-0.2 μm (mode 1), 1-1.25 μm (mode 2) and 3.5-4.0 μm (mode 3). Sulphuric acid was found to be the major aerosol constituent in the middle clouds, although significant elemental abundances of chlorine and phosphorous were also found at these altitudes.

Extended layers of fine aerosols are observed both above and below the main cloud level. The upper haze fills the mesosphere up to ∼100 km altitude with evidences of detached layers. The haze is presumably composed of very very fine particles of sulphuric acid. The lower haze extends down to ∼33 km, far below the level of sulphuric acid thermal decomposition. Descent probes also provided some evidence for thin aerosol layers near the surface.

References

1. Titov, D.V., Ignatiev, N.I., McGouldrick, K. et al. Clouds and Hazes of Venus. Space Sci Rev 214, 126 (2018). DOI: 10.1007/s11214-018-0552-z
2. https://arxiv.org/ftp/arxiv/papers/1312/1312.3750.pdf