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Since when $\ce{CO2}$ goes into water it makes carbonic acid, then what happens when it rains? Some of the $\ce{CO2}$ in the air joins the rain drop as it falls and while the rain drop falls and changes shape, it releases then regains $\ce{CO2}$? Is there a critical point or point of saturation within the raindrop for $\ce{CO2}$? It’s an interesting dynamic considering the complexity of storms and weather systems.

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Short answer

It does form carbonic acid, as rain drops are water and carbon dioxide is soluble in water. But the content of carbonic acid is minimal as you can guess from $\mathrm{pH=5.6}$

Longer answer

Majority of dissolved $\ce{CO2}$ does not form carbonic acid, 99.83% stays as hydrated oxide.

The equilibrium $\ce{CO2}$ concentration in water follows the Henry's law and is proportional to the $\ce{CO2(g)}$ partial pressure.

Therefore, as a drop falls, the partial pressure of $\ce{CO2(g)}$ grows together with the atmospheric pressure and $\ce{CO2}$ solubility in water would increase as well, if temperature did not raise with decreasing altitude, typically $\mathrm{ 0.0065 K/m}$. The total effect is general decreasing $\ce{CO2}$ solubility when a drop is falling.

Sure, some drops may evaporate during their fall ( seen as precipitation patterns without raining ) and the dissolved $\ce{CO2}$ becomes gas again.

Natural rain has $\mathrm{pH=5.6}$ because of dissolved $\ce{CO2}$ and equilibrium concentration of $\ce{H2CO3}$.

If all CO2 converted to H2CO3, the natural rain and soda water would be much more acidic, because carbonic acid is quite strong acid with pKa=3.6, instead of formal 6.3 if 100% conversion is supposed.

In carst areas, there is ongoing reversible reaction of dissolving calcium carbonate by surface rain water going down the soil and precipitating it again in caves :

$$\ce{CaCO3(s) + H2O + CO2(aq) <=> Ca(HCO3)2}(aq)$$

The calcium bicarbonate comes also to surface water and makes frequently the majority of the calcium content.( and similarly for magnesium l

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  • $\begingroup$ Just saw on "A strange rock" that rain is one important mechanism for CO2 regulation. That wasn't very clear but I think that is how CO2 enters the soil besides long time sequestration. This is just to say that in spite of the little 0.17, it can be an important path. Plus one. $\endgroup$ – Alchimista Sep 21 at 9:54
  • $\begingroup$ Surely it is, I have not say it is not. If all CO2 converted to H2CO3, the natural rain and soda water would be much more acidic, because carbonic acid is quite strong acid with pKa=3.6, instead of formal 6.3 if 100% conversion is supposed. $\endgroup$ – Poutnik Sep 21 at 10:04
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    $\begingroup$ That means, I take gas solubility as the maximum amount of gas soluble in a unit volume at given conditions, in whatever equivalent units. $\endgroup$ – Poutnik Sep 21 at 12:23
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    $\begingroup$ I would not say so, otherwise all but small drops would be hails, as near all rain drops, especially big and fast ones, have ice origin. $\endgroup$ – Poutnik Sep 21 at 17:04
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    $\begingroup$ Plus, the absolute solubility depends on both temperature and pressure, and temperature vertical gradient varies from -0.01 K/m to +0.0x K/m. $\endgroup$ – Poutnik Sep 21 at 17:16

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