Q:

What is the atmospheric composition of Venus at the 34 mile (50 km) altitude of the proposed HAVOC program?

Various people over the past 60ish years have suggested floating habitats in the high-altitude Venusian atmosphere, as the pressure, gravity, and temperature are comparable to Earth. The premise is that there's a high CO₂ concentration which makes an Earth-like atmosphere lighter-than-air, allowing the habitat to float just by virtue of being filled with breathable air.
One such proposal is the HAVOC concept by NASA's Langley Research Center, proposed to float at 50 km / 32 mi altitude, where the pressure is ½ to 1 atmosphere, gravity is about 0.9 gees, and temperature is 27-75 C / 81-167 F, depending on the exact altitude and what source I'm reading.
Facts about the surface are easy enough to find, but I'm having no luck finding useful information about the high-altitude air and its composition. Some sources mention droplets of sulfuric acid and a general lack of oxygen, but that's not very specific.
It seems most people answering questions about the feasibility of such habitats are just assuming the relative compositions are the same at any altitude, but this seems unlikely to me. This plot of Earth shows very different compositions at different altitudes, which leads me to think Venus would have similar differences at higher altitudes.

Volume fraction of the main constituents of the Earth's atmosphere as a function of height.
From Wikipedia, published under Creative Commons 1.0.
It's worth noting that the differences really only start around 100 km / 62 mi, where pressures are very low and we're essentially in space. But given the 200:1 pressure differential between the surface and the cloud layers in question, and the various layers of weather phenomenon, I'm not sure that means Venus would also have the same compositions out to the 250 km / 155 mi Kármán line.
Is there any available data on the actual atmospheric composition of Venus at altitudes being considered for the proposed habitats? (50 km / 32 mi) Or is it just the same everywhere and I'm overthinking this?

A:

You're not overthinking it, but it's not a problem.  The important concept is that of the 'homosphere`, which is the part of the atmosphere where it is well-mixed by turbulence.  This is the part to the left of the diagram you show, where you can see that the proportions of the various constituent gases in the atmosphere don't change with height.
(It's not entirely the case that the proportions don't change: for instance ozone concentration varies with height: this happens because ozone is only produced at certain heights & has a rather short half-life, so it never has time to become well-mixed.  Water vapour also varies with height.)
There's a place in the atmosphere called the homopause, which is the place where diffusion due to turbulence in the atmosphere becomes small enough that molecular diffusion becomes more important.  Above this point the proportions of various species in the atmosphere depend on molecular diffusion & on their atomic weight.  The homopause in Earth's atmosphere is roughly 100km up, and in fact you can read it off the diagram you included.
The important thing for this is that the homopause happens when the atmosphere has low enough density that turbulence becomes so relatively weak that it stops mattering.  By definition this is above the point at which you could float some object full of air that you could breathe!
I don't know the height of the homopause on Venus: this abstract talks about it being between 129 and 140 km up, but that's an old paper and also I only can find the abstract: there is probably more current information.
However the important thing is: if you can get some kind of balloon to a given height, that height is below the homopause, pretty much by definition.

See also Tom Spiker's answer which has a lot more detailed references than this.

A:

This is not a stand-alone answer but is an addition to tfb's answer; but since I provide multiple references, one with a rather lengthly URL, it wouldn't work well as a comment.
The homopause altitude at Venus isn't a constant. It varies with latitude, "time of day" (longitude with respect to the subsolar point), and solar activity. This abstract shows an example of some of that variability. Note that this is only one sample of the variability; it can go outside the range shown.
But as tfb says, in all cases it is well above the proposed altitudes for balloon-suspended vehicles. The observed variability in Venus's atmospheric composition is a result of multiple phenomena, such as the increase in temperature with depth below the tropopause and chemical reactions resulting from solar UV and IR in the stratosphere and above. The Wikipedia article has some good information, and the list of references includes a lot of scholarly papers that delve into the details.
One example of temperature-driven variation of composition is the sulfuric acid that GdD alludes to. It is stable at the altitudes proposed, but the atmosphere gets hotter as you go downward from the tropopause. This abstract (and a lot of others!) and the Wikipedia article indicate that below about 35 km altitude the temperature is high enough to break down the sulfuric acid to SO3 and H2O.
At cloud altitudes there's a lot we don't know about the composition of Venus's atmosphere. For instance, at or near the cloud-tops there's something that absorbs UV but its composition is still unknown — though there have been some suggestions, including biology (!), none are universally accepted.
All this said, the proposed altitudes are in the turbulently-mixed region of the atmosphere, so the relative proportions of the major constituents — CO2 and a bit of N2, with everything else being also-rans — are pretty much constant.

