Keep in mind that CO2 gas dissolves very quickly in water, like less than 5 seconds so the reason you may have seen bigger bubbles is because you were diffusing it even more efficiently and causing larger bubbles of atmospheric gas to form.
The CO2 mist that everyone talks about is only comprised of CO2 near the point of entry. Once the bubbles are on the other side of the tank they are no longer CO2, but atmospheric gasses.
The reason you don't see these using a reactor style difusser is because the atmospheric gases build up inside of the reactor chamber instead of being shot all around the tank.
Conditions such as flow, ambient CO2 concentration, rates of degassing. What about water surface tension on a gas bubble? Hardness/TDS/salts play a large role there. This is WHY we see ultra fine mist in marine systems with aeration/protein skimmers, but hardly any with freshwater. The increase in surface tension allows for the ultra fine mist we see in marine tanks. As a highly soluble gas like CO2 dissolves, the tension for the bubble dramatically increases. Water depth also can play a role, pressure is different.
These are not simplistic conditions.
5 seconds traveling at 1-2 meter/sec also is a long way to go. Traveling from the impeller, that's across the entire length of my tanks(any of them) easily.
Also, given the CO2 gas we add is relatively pure, no one has ever stated what the mystery gas is or even offered a rational hypothesis.
Plants are cleaner and grow better with it, but this may likely just be the mist itself breaking up surface barrier layer around leaves. Not direct main line CO2 gas per se, but this is a rather difficult thing to measure experimentally.
But as far as the gas, till you run the gas mix through a GC etc, or something along those lines and can capture it etc. You really are way out on limb there.
CO2 does dissolve pretty quick, but I can place different currents and see the bubbles travel much farther, and the type of mist also plays a large role, say a Mazzei, vs Needle Wheel vs say a disc. Each produces mist, but the size fractions are very different for the micro bubbles. Medications can cause all sorts of frothy behaviors of gases and mist, surface tensions also.
Still, the Mist itself inside reactors, should be purged IMO/IME. The gas gap inside the reactors reduces flow and reduces the mist from being captured and dissolved(less efficient reactor basically). You want some slight gas mist by pass from a reactor, and only when the gas bubble builds up to some specific level, say 5-10" of the reactor tube volume.
This is the trade off with mist: a little= not an issue and improves reactor efficacy. Too much, ugly haze in the tank, less water clarity. No mist: poor reactor efficacy.
My oldest internal reactor does the self purging at about that level. This can be modified to external type reactors.
My take on what those other gases are: it's the sum total of the gases in the water, not one specific gas. When you have mist entering, this also creates a massive surface area and diffusion and degassing are 2 way streets, correct?
So CO2 is going in while say O2 is coming out, I doubt N2 is changing much. the total gas saturation ppm does change(we add a lot more CO2 and the plants produce more O2 ONLY during the day). The mist is more pronounced later in the day, same with the build up inside reactors. O2 degasses like CO2 would(and we have ample supply above ambient conditions for both during the day cycle).
The higher than ambient ppm CO2 is added and control by the hobbyists, the higher than ambient O2 is controlled by the plants' growth rate. But both gases still degas out of the water into the air if they are at a higher concentration in the water than the air above.
The micro bubble likely acts like the air above on a much smaller scale. CO2 out into the water/O2 into the bubble. I tried to see if I could collect the gas and light a match in the collected flask. But it's pretty wet and was not purged of H2O vapor.
I likely should get around to testing the gas mixture at some point.