Condensation Nuclei and Carbon Dioxide
In this article we address the curious issue that you will not likely read about in climate science. It is the natural relationship between carbon dioxide and water in the context of the natural atmosphere. Perhaps we need many more chemical scientists to prod climate scientists into addressing this issue more diligently?
In his 1966 book—Weather and Climate—R. C. Sutcliffe, a meteorologist, wrote:
It has been necessary to labour over this image of the processes in terms of molecular movements in order to appreciate the difficulties which arise when the vapour exists in the atmosphere far removed from any liquid surface. The air might be supersaturated, in the sense that if there were liquid present the vapour would quickly be captured by it, but in the absence of any liquid there is no obvious reason why condensation should ever begin and experiment proves that the argument is a valid one. If air is carefully purified by filtering, it will not produce cloud droplets even if cooled by expansion far beyond its normal saturation point or dew point. C. T. R. Wilson, working with his famous expansion cloud chamber, was able to show this quite conclusively late in the last century. His method of purifying the air was to allow the droplets produced during cloud formation to settle out of the chamber and to repeat the process several times with the same sampler of air. Ultimately four-fold supersaturations, that is humidities of 400 per cent, were necessary to produce condensation in the purified air.
These results, obtained first by Wilson and broadly confirmed by many later experimenters, have a very important bearing on natural meteorology, not because supersaturation occurs in the atmosphere but because it does not occur: why is it that in the atmosphere condensation to clouds invariably happens as soon as normal saturation is reached? The answer is that the natural atmosphere, however clean it may appear to be, is always supplied with a sufficient number of minute particles of salts, acids, or other substances which serve just as well as liquid water in capturing water molecules from the vapour. These are the ‘nuclei of condensation’, and are effective as soon as the air becomes even slightly supersaturated. As a matter of fact, there are many observations of clouds in air whose relative humidity is considerably below 100 per cent, evidence of nuclei which are hygroscopic, but methods of measurement within natural cloud are not sufficiently refined to prove that even slight supersaturation ever occurs. If for practical purposes we assume that cloud will always form in the atmosphere when ordinary saturation is attained (that is relative to a flat surface of pure water), we shall not go far wrong. Microscopic counting shows that the droplets forming in an ordinary cloud are measured in hundreds or even thousands to the cubic centimetre, millions to the litre, numbers which may strike one as incredibly large until we become familiar with the minuteness of the particles with which cloud physics has to deal. We shall need to return to these problems of nuclei and droplets when the process of raindrop formation is considered, but mean while we may note that although the nuclei are extremely numerous they are generally quite invisible and utterly negligible as a contribution to the weight of the air, which can still be treated as a ‘perfect gas’ in most meteorological calculations.
The focus of this essay is:
The natural atmosphere, however clean it may appear to be, is always supplied with a sufficient number of minute particles of salts, acids, or other substances which serve just as well as liquid water in capturing water molecules from the vapour. These are the ‘nuclei of condensation’.
Another well-known fact is that the natural atmosphere is always supplied with the ‘trace’ gas—carbon dioxide. What I have never read is about, in the context of the natural atmosphere, is the natural relationship between carbon dioxide and water. This is not to imply that no one has ever written about this relationship.
When I look in the Handbook of Chemistry and Physics published by Chemical Rubber Publishing Co. I can read about Carbonic Acid (H2CO3) and that it only exists in solution. I can read that Carbon Dioxide (CO2) has a solubility of 171.3 cm3/100ml of cold water at 0oC. For comparison with other gas solubilities I consider Argon (Ar) 5.9 cm3/100ml of cold water at 0oC and Krypton (Kr) 11.0 cm3/100ml of cold water at 0oC. Based upon this information alone, I must consider that the significant difference between the solubility of carbon dioxide and of these two other gases is that a carbon dioxide molecule (CO2) reacts with a water molecule (H2O) to form the carbonic acid molecule (H2CO3) and these other two atoms do not react with water to form molecules.
That the compound carbon dioxide reacts with the compound water (dihydrogen oxide) to form the compound carbonic acid in water solution is common knowledge in chemistry but evidently not well known in meteorology. So probably the cause of the unique physical properties of the compound water are also not well known in meteorology. This cause is what chemist term—hydrogen bonding. Hydrogen bonds are strongest type of possible intermolecular bonds between small molecules like water molecules and they are highly directional (one molecule must be precisely positioned relative to the position of another molecule). Hence, ice (solid water), because of its precise structure, is significantly less dense than liquid water as evidenced by the fact that ice floats on liquid water.
When meteorologists consider the composition of the necessary condensation nuclei, it seems the general (common) choice of the possibilities—salts, acids, or other substances—is salt. For the reasoning is that the majority of the earth surface is covered by water (oceans and seas) and most are familiar with sea foam generated by turbulent wave action and surf. So the mechanism by which bits of salt are injected into the atmosphere is the bursting bubbles of sea foam which quickly evaporate leaving tiny particles of salt. And it has been observed that salt, primarily common sodium chloride, are sometimes trace impurities of rain water.
However, Sutcliffe and most other authors never review what the importance of salts, acids, or other substances are to the necessary condensation nuclei. The only hint of a potential problem given by Sutcliffe is: that is relative to a flat surface of pure water. Most people are familiar with the observation that water droplets (and other droplets not contacting liquid or solid surfaces) appear to be spheres. I have been a science teacher and I liked to tell my students that there are two different types of water molecules in a pure water droplet and then ask: What are they? I asked this question because the answer should be obvious; but only the best science students were likely to see it. And as a former chemistry student, I question if I could have seen the answer at that early time. And the fact I have never read anyone writing about the relationship between carbon dioxide and condensation nuclei is my evidence that the obvious can be most difficult to see.
I read that Einstein stated: The true sign of intelligence is not knowledge but imagination. And, Imagination is more important than knowledge.
To begin to see (understand) the importance of carbon dioxide to condensation nuclei requires imagination. In a droplet of water, which drops from an ordinary eyedropper, are billions of trillions etc. of tiny, tiny, water molecules which are in constant, vigorous motion held together with the possible very weak (relative to the strong intramolecular forces holding the three individual atoms of the water molecule in a precise alignment) intermolecular forces (attractions): hydrogen bonding, electrical dipole-dipole forces, or most weak (because of the small size of water molecules) van der Walls forces.
Have you seen the two types of water molecules yet? They are the molecules on the surface and the molecules beneath the surface. Why is the shape of a water droplet spherical? The simple answer is the sphere is the shape whose volume relative to surface area is the greatest. Hence, these billions of trillions etc. tiny, tiny molecules, which are in constant, vigorous motion, arrange themselves in the shape of a sphere to minimize the number of water molecules on the surface; which cannot have as many interactions with other water molecules as those molecules beneath the surface can have with each other. If this observed natural fact (the spherical droplet) does not stretch your imagination, it would seem nothing might.
In my physical chemistry textbook written by Farrington Daniels and Robert A. Alberty I read:
The properties of the surface layer of a substance are different from those of the bulk phase. … Surface Tension. A liquid surface tends to contract to the minimum area as a result of unbalanced forces of molecular attraction at the surface. The Dependence of Vapor Pressure on the Radius of Curvature. The vapor pressure over a meniscus [curved surface] which is concave toward the vapor phase if smaller than that over a flat surface of the liquid, and that over a meniscus which is convex toward the vapor phase is greater. … As a result of this effect very small droplets with their convex surfaces evaporate in a closed system and condensation occurs on larger droplets.
As I write (copy quotes) I am learning something very important which I did not know (or have recently considered if I once knew it). Like the previous quoted sentence. But I continue.
Equation 11 {This equation gives the increased vapor pressure of a convex liquid surface with radius of curvature r.} is of interest in the consideration of the initial formation of liquid droplets from a supersaturated vapor. If a number of molecules come together to form a small drop, the vapor pressure of the liquid in this drop will be greater than that of the bulk [flat] liquid. Therefore, a very small droplet will evaporate rather than grow. … Apparently in order for droplets to be formed at low degrees of supersaturation dust particles [other] or ions must be present to give a larger mass of material and lesser convex curvature. … Surface Tension of Solutions. The addition of a third component may lower the surface tension considerably; but if the third component causes an increase in surface tension, the effect is small. Solutes are classified as “capillary active” or “capillary inactive” on the basis on the surface tension. In the case of the aqueous solution-air interface, inorganic electrolytes, salts of organic acids, bases of low molecular weight, and certain nonvolatile nonelectrolytes such as sugar and glycerin are capillary inactive. Solutes which are capillary active are organic acids, alcohols, esters, ethers, amines, ketones, etc. The effect of capillary-active substances on the surface tension of water may be very great.
Given this which chemists pretend to know, sea salt (inorganic electrolyte) should not be greatly effective as a solute of a condensation nuclei which reduces a small droplets surface tension. At the same I have long known there is a colligative property of solutes which should reduce the vapor pressure of a ‘flat’ liquid. However, carbonic acid is an organic acid which should significantly reduce the surface tension of a small droplet which would seemingly cause the spherical droplet to possibly become a blob. Because I am just learning this as I copy (quote), I have a hard time imagining how a tiny spherical condensation nuclei might become a blob.
What I can imagine is that a large, relative to a water molecule, carbonic acid molecule which would be slower moving in its constant, less violent, motion than that of the several water molecules it could replace on the droplet’s surface. But possibly the most important feature of the carbonic acid molecule is it can form hydrogen bonds with both other acid molecules and water molecules. Hence, I can easily imagine that these acid molecules would have a tendency to migrate to the surface so more surface water molecules could join the others in the bulk liquid. Thus forming a ‘surface skin’ which has a lower vapor pressure than that of only a water molecule surface given the same radius of curvature.
And I know a very important attribute of the relationship between the condensation nuclei and carbon dioxide is with all other solutes the concentration of the solute would probably decrease as more water vapor condensed on the droplet. But since carbon dioxide is always naturally present in the natural atmosphere, more carbon dioxide can naturally dissolve as the droplet increases in size and thus maintain a nearly constant concentration of solute.
So, what do you imagine and conclude?
Author’s Note: While I have had this idea for several weeks, if not months, it was Robert ‘Bob’ Beatty’s comment to Hans Schreuder’s PSI article (https://principia-scientific.
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