In this thread, Quartz suggested the (to me, entirely unheard-of) possibility that gaseous CO2 “dissolves in water,” thus once it is released into the air by cars, etc., it might be “washed away” by rain rather than staying in the atmosphere indefinitely or until sequestered by plant respiration. Now, I once read in one of Isaac Asimov’s facts & trivia books that it is a myth that rain has any effect on local air pollution or pollen count at all, but I can find no cite for that online, nor can I find a Cecil column that addresses the question. Does rain reduce air pollution? Does it “wash away” CO2?
Oxygen is also soluble in water, which is why I always carry an oxygen tank in a rain storm.
Actually, except for the very rare rain where the water comes from space there is this thing called the water cycle. The water in that rain came from a body of water that was already at equilibrium with the atmosphere with regards to CO[sub]2[/sub]. When that water evaporates, it releases CO[sub]2[/sub] into the atmosphere making the net effect of rain on the concentration of CO[sub]2[/sub] in the atmosphere zero.
Yes, CO2 will disolve in water to form carbonic acid. My uncertaintly in that thread was not can it, but does it (and to a sufficient degree). Of course, it will only disassociate on evaporation if it’s given a chance to evaporate: if it’s been absorbed into the earth, it’s not going to evaporate, which is partly why we get limestone caves and the like.
For other major air pollutants, as far as I’m aware, it’s commonly accepted that rain does indeed wash some particulates out of the air (and wet ground means a little less dust kicked up, but the most dangerous particulates don’t come from dust anyway). If anyone has a cite to the contrary, I’d be very interested.
I don’t know if rain has a direct impact on ozone (smog), but since ozone needs direct sunlight to form, the clouds mean that there probably wouldn’t be much ozone for the rain to effect.
I don’t know if CO or lead would be affected by rain; they’re not a big problem in the industrialized world anymore.
As for CO2, remember the amount down near the ground doesn’t matter – it’s the concentrations higher up that trap heat onto the planet. Any single rainstorm isn’t really going to affect the global equilibrium of CO2 in the stratosphere.
I suppose a change in climate to more or less average global rainfall might affect CO2 levels to some degree, but I also suspect it’s going to be a very small effect compared to things like the effect of more clouds on reflecting sunlight.
What is rain if it is not evaporated water?
Do you mean into the water table, where it eventually finds its way to the surface, or do you mean deep into the earths crust somehow?
I’m not sure where limestone comes from, can you provide me with a cite that confirms that it comes from atmospheric carbon dioxide rather than shellfish?
Natural CO2 is what makes nonpolluted rain water acidic in the first place. It helps break down minerals in the soil that plants can absorb through their roots. If there is more CO2 in the air, water will absorb it to the point that the partial pressures are equal, but even if you triple the partial pressure of CO2, that still leaves you with a very small partial pressure. When SO2 and NO are present in the atmosphere, they too will be absorbed (diffused) into the rain drops, resulting in rain being too acidic. No one has ever shown that this removes any measurable pollution from the atmosphere, but checking the pH of rain water is a great way of checking your air quality. Since it’s the particulates in the air, such as pollen, soot, or dust mixing it around with water vapor that forms a rain drop, it makes sense to me that at least those substances are removed to some degree, but I don’t think it would be possible to test that with any accuracy.
We seem to be having a miscommunication here: I’m wondering if the water that comes down as rain dissolves the low-lying CO2 in the atmosphere. You’ve got your cars and houses pumping out CO2 and other pollutants, forming a blanket over the area, right? So along comes a cloud which drops its rain over the blanket. The air now feels crisp and clear. Does it wash away the emitted CO2?
Limestone is indeed formed by the deposited shells; the caves within the limestone are formed by the limestone being dissolved by weakly acidic water, amongst other processes.
Has it been tested?
Thanks. I think I understand what you are saying.
I have to say I don’t know. We do know that rain drops do indeed equilibrate partial pressures of atmospheric gasses while swirling around at the higher altitudes. Once a rain drop is big enough to fall to earth, does it spend enough time near the ground to equilibrate with the atmosphere? Think of the logistics of such a test. You would have to collect high altitude rain water with low altitude rain water, collect air samples upwind, downwind, before, during and after, and then reproduce it a bunch of times. If such a study has been done, I can’t find it. My hunch is that a typical rain drop would need at least two minutes of exposure to low altitude smog for the PPs of CO2 to equilibrate. If so, is that enough to improve air quality? Cecil?
I was surprised by that as well-but the poster clearly refers to limestone caves-ie the absence of limestone. Carbolic acid is the chemical I have heard referred to most as dissolving the limestone to produce the cave.
What you mean to say is carbonic acid (dissolved CO[sub]2[/sub]), not carbolic acid (phenol). Whether or not the formations in the cave are directly formed from seashells or indirectly formed from seashells is irrelevant to the question. What is needed, is a cite that demonstrates that limestone, or any carbonate rock for that matter, of any formation, comes from atmospheric carbon dioxide dissolved in rain water. I don’t know.
No-one said that limestone comes from atmospheric CO2, so no cite is needed. The limestone itself comes from seashells; the caves are made in that limestone by weak carbonic acid, either rainwater or ground water.
I believe that the limestone in caves comes from acidic rainwater that dissolves the limestone produced by seashells. As you say, it has nothing to do with atmospheric CO[sub]2[/sub]. Unfortunately, despite what you claim no-one has claimed, that is exactly what the premise of the OP is.
Like i said in that other thread, the solubility of CO[sub]2[/sub] in rainwater is fairly low. For ordinary rainwater (not acid rain) which has an average pH of 5.6, the solubility for CO[sub]2[/sub] works out to around 0.65 ppm - this is T dependent, of course (20°C), and varies between 1.1 ppm at freezing point to 0.3 at 40°C.(see Table III here
It’s not the acidity of rainwater that creates karst topography (including limestone caves), it’s the higher acidity of groundwater - partly due to organic activity in the soil leading to a higher CO[sub]2[/sub] concentration, partly due to other reactions.
The OP mentions neither limestone nor caves.
Missed the Edit window: He is merely asking whether rain washes CO2 out of the air at all.
The limestone in caves does not come from “acidic rainwater that dissolves the limestone produced by seashells”. The caves in the limestone are created by acidic water - whether rainwater or groundwater - dissolving limestone from the caves and taking it elsewhere.