Over the last 6 months or so injection of CO2 into a planted tank has evolved to something like this I believe:

For stability, best use pressurized CO2.
Connect that to a regulator and a bubble counter.
Use an atomizer style diffusor such as the Rhinox.
Place diffusor in an area of the tank with good water movement - i.e. near filter intake. Distribute that mist of CO2 all over the tank!
Leave CO2 on all the time for ultra-stability (but costs more $$$).
This leads to potential O2 problems, hence encourage good surface movement - put a lilly tube from outlet of filter 1/2 in & 1/2 out of the water.

Now this all makes good sense, but I note that things have come away from the traditional thinking that you should aim for minimal surface disturbance as this 'drives off CO2' - who cares now with such a fine mist of CO2 going everywhere....

So this got me thinking (be warned, I am no chemist so I lack the foresight of knowledge here):
The atmosphere contains something like 300ppm to 600ppm of CO2? - in other words the 'ariel advantage' automatically afforded to all non-aquatic plants. This is a free source of CO2, so my thinking is to replace the pressurized CO2 cannister with a simple air-pump (as those used for air-stones) - simply pump naturally occuring atmospheric CO2 & O2 into the tank via a Rhinox atomizer diffuser (no change to the rest of the setup mentioned above).

All I am stuck on really is, is this a viable idea? Would the naturally occuring CO2 in the atmosphere, if pumped into the tank in this manner, really increase the CO2 ppm in the tank water by any signficant amount do you think? We all know Excel is no replacement for direct CO2 injection, so even if this idea makes the use of Excel redundant, there may be some mileage in my lateral thinking.... (thus can be used for EI / El natural hybrid tanks mentioned here: Non CO2 methods )

Andy

PS, my fingers are crossed. Hopefully this is not a completely dumb-ass idea.

The water will try to equillibrate itself with the atmosphere. Hence only water movement will do the trick. What we do is force CO2 into the water making higher CO2 there rather in the air so the plants can do luxury uptake. We do this with concentrated CO2.
If you have really low light, then you hinder the photosynthesis process and the plants wouldnt demand so much CO2. Then it would work.. All is depending on what type of aquarium you want. High or lowtech.

I think the equilibrium of CO2 in water (mixing with just air) is about 4 ppm, which is far too low for the high-tech methods.

Air is about 0.05% CO2, so to get one milligram of CO2 in the water you need to add 1/.0005 mg of air, or 2 kg of air to the water! If all of that CO2 dissolved, which it probably would, you would have in one liter of water, 1 ppm of CO2. In a typical 100 liter tank you would then need to release 4000 kg of air to reach 20 ppm of CO2! Other than that, it is a great idea. (This is a crude calculation, so don't go shopping for an air pump based on these numbers.)

Edit: I think I'm off by three decimal places! It is 2 grams of air to yield one miligram of CO2, so it takes only 4 kg of air to reach 20 ppm of CO2 in a 100 liter tank. That is still an awful lot of air.

But even if you release 4000 kg of air into the water, it wouldn't hold on to 20 ppm of CO2. Most of it would go back into the atmosphere.

Yes, even though it is actual 4 kg of air needed, it is still so much that the surface of the water would be virtually boiled away by the air flow, and the CO2 would leave as fast as it arrived.