<p>Most likely, the rain would still be acidic, at least from thunderstorms. The reactions involved are fairly well-known and their documentation has crossed over into nontechnical sources such as <a href="https://wxguys.ssec.wisc.edu/2018/07/09/lightning/" rel="nofollow noreferrer">The Weather Guys (from the University of Wisconsin)</a>:</p>
<blockquote>
<p>Each bolt of lightning carries electrical energy that is powerful enough to break the strong bonds of the nitrogen molecule in the atmosphere. Once split, the nitrogen atoms quickly bond to oxygen in the atmosphere, forming nitrogen dioxide.</p>
<p>Along with the lightning in the cloud are cloud droplets and raindrops. Nitrogen dioxide dissolves in water, creating nitric acid, which forms nitrates. The nitrates fall to the ground in raindrops and seep into the soil in a form that can be absorbed by plants.</p>
</blockquote>
<p>Thus the predominant nitrogen compound dissolved in rainwater (using only constituents in an unpolluted atmosphere) is not ammonia but nitric acid, which is neutralized on contact with soil but in the meantime, the rainwater is weakly acidic from the nitric acid as well as carbon dioxide/carbonic acid.</p>

CC BY-SA 4.0
2024-06-20T06:30:47.730Z
I struggle to envision lightning having much impact at all on typical rain... just because the area/duration each bolt impacts to that degree would be crazy tiny, so even with thousands of bolts in a storm, it would be a tiny tiny fraction of the air (to consider it another way, how often does lightning strike within your yard in a storm???)  Unless nitrogen dioxide is incredibly stable and resident, so the atmosphere has a built up supply over many many years???  Otherwise I'd think CCN would be the much larger factor?
JeopardyTempest
CC BY-SA 4.0
2024-06-18T23:24:44.950Z
2024-06-19T00:06:04.413Z
2024-06-19T00:06:04.413Z
Oscar Lanzi
<p>If the rain is pure water (H2O), then it would have a pH of 7 (give or take a small variation with temperature ; but between STP and NTP (freezing point and room temperature) that doesn't make much difference.
At nanomolar concentrations the presence of ammonia might make the rainfall basic (pH&gt;7) but that's not a realistic scenario. Free ammonia in the atmosphere is very short-lived.</p>

CC BY-SA 4.0
2024-06-17T13:21:58.540Z
What about calcium compounds? Wouldn't them make it basic?
Flamethrower
CC BY-SA 4.0
2024-06-17T14:24:25.630Z
Calcium compounds in the vapour phase ? That implies a temperature in the vicinity of 1000 degrees. Celsius, Kelvin, meh - but obviously not that American unit.
Rockdoctor
CC BY-SA 4.0
2024-06-19T18:32:14.103Z
Magnesium is known to form some more volatile organic compounds and it too can make water basic, but again such compounds would not get very far before reacting in our atmosphere.
Oscar Lanzi
CC BY-SA 4.0
2024-06-20T06:34:17.540Z
@Rockdoctor why would Flamethrower make you think it'd be in vapor phase??  $CO_2$ is of course, but many other compounds are particulates that then will dissolve into liquid, as I'd think calcium compounds would??
JeopardyTempest
CC BY-SA 4.0
2024-06-20T20:35:27.183Z
Most calcium-bearing rock is compounds that don't readily dissolve in water; you have to have some acidification. So I would not suppose calcium-bearing rock (or any rock) would spontaneously render rainwater basic.
Oscar Lanzi
CC BY-SA 4.0
2024-06-24T02:28:20.577Z
@JeopardyTempest : the subject raised was the composition of "rain" - which means "as it is falling", not "while it is laying in puddles/ streams" (that would be "rainwater"). So yes, somehow the putative calcium compounds would need, somehow, to get up into the air. Meaning, practically, as dust.
As Oscar says, most calcium-rich minerals have low solubility, so the dust effect is going to be pretty minor, at most.
Rockdoctor
CC BY-SA 4.0
2024-06-17T12:34:16.240Z
2024-06-17T12:34:16.240Z
Rockdoctor
<p>Rain is acidic because carbon dioxide dissolves in water droplets to form carbonic acid. However, if there was less of it, would other factors end up causing rain to be basic instead of acidic?</p>

CC BY-SA 4.0
2024-06-16T13:29:52.987Z
(Looks like carbon dioxide causes rain to have pH around 5.6, but the term acid rain is more often used for special cases when sulfur dioxide and nitrogen oxides are involved, with pH nearer 4.  But that doesn't take away from your question, rain is acidic because of CO2 indeed)
JeopardyTempest
CC BY-SA 4.0
2024-06-17T06:19:58.807Z
That's true, I've edited the question to clarify it.
Flamethrower
<atmosphere><co2><atmospheric-chemistry><rain><acid-rain>
If there was less carbon dioxide, would rain be basic?
Flamethrower
# If there was less carbon dioxide, would rain be basic?

(Asked by: username_0 on 2024-06-16)

Rain is acidic because carbon dioxide dissolves in water droplets to form carbonic acid. However, if there was less of it, would other factors end up causing rain to be basic instead of acidic?

**Tags:** atmosphere, co2, atmospheric-chemistry, rain, acid-rain

> **Comments:**
> **username_1**: (Looks like carbon dioxide causes rain to have pH around 5.6, but the term acid rain is more often used for special cases when sulfur dioxide and nitrogen oxides are involved, with pH nearer 4.  But that doesn't take away from your question, rain is acidic because of CO2 indeed)  
> **username_0**: That's true, I've edited the question to clarify it.  

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## 2 answers

### Answer by username_2 on 2024-06-18. Score: 2

Most likely, the rain would still be acidic, at least from thunderstorms. The reactions involved are fairly well-known and their documentation has crossed over into nontechnical sources such as [The Weather Guys (from the University of Wisconsin)](https://wxguys.ssec.wisc.edu/2018/07/09/lightning/):

> Each bolt of lightning carries electrical energy that is powerful enough to break the strong bonds of the nitrogen molecule in the atmosphere. Once split, the nitrogen atoms quickly bond to oxygen in the atmosphere, forming nitrogen dioxide.
>
> Along with the lightning in the cloud are cloud droplets and raindrops. Nitrogen dioxide dissolves in water, creating nitric acid, which forms nitrates. The nitrates fall to the ground in raindrops and seep into the soil in a form that can be absorbed by plants.

Thus the predominant nitrogen compound dissolved in rainwater (using only constituents in an unpolluted atmosphere) is not ammonia but nitric acid, which is neutralized on contact with soil but in the meantime, the rainwater is weakly acidic from the nitric acid as well as carbon dioxide/carbonic acid.

> **Comments:**
> **username_1**: I struggle to envision lightning having much impact at all on typical rain... just because the area/duration each bolt impacts to that degree would be crazy tiny, so even with thousands of bolts in a storm, it would be a tiny tiny fraction of the air (to consider it another way, how often does lightning strike within your yard in a storm???)  Unless nitrogen dioxide is incredibly stable and resident, so the atmosphere has a built up supply over many many years???  Otherwise I'd think CCN would be the much larger factor?  

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### Answer by username_3 on 2024-06-17. Score: 0

If the rain is pure water (H2O), then it would have a pH of 7 (give or take a small variation with temperature ; but between STP and NTP (freezing point and room temperature) that doesn't make much difference.
At nanomolar concentrations the presence of ammonia might make the rainfall basic (pH>7) but that's not a realistic scenario. Free ammonia in the atmosphere is very short-lived.

> **Comments:**
> **username_0**: What about calcium compounds? Wouldn't them make it basic?  
> **username_3**: Calcium compounds in the vapour phase ? That implies a temperature in the vicinity of 1000 degrees. Celsius, Kelvin, meh - but obviously not that American unit.  
> **username_2**: Magnesium is known to form some more volatile organic compounds and it too can make water basic, but again such compounds would not get very far before reacting in our atmosphere.  
> **username_1**: @username_3 why would username_0 make you think it'd be in vapor phase??  $CO_2$ is of course, but many other compounds are particulates that then will dissolve into liquid, as I'd think calcium compounds would??  
> **username_2**: Most calcium-bearing rock is compounds that don't readily dissolve in water; you have to have some acidification. So I would not suppose calcium-bearing rock (or any rock) would spontaneously render rainwater basic.  
> **username_3**: @username_1 : the subject raised was the composition of "rain" - which means "as it is falling", not "while it is laying in puddles/ streams" (that would be "rainwater"). So yes, somehow the putative calcium compounds would need, somehow, to get up into the air. Meaning, practically, as dust. As username_2 says, most calcium-rich minerals have low solubility, so the dust effect is going to be pretty minor, at most.  

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