How much CO2 is in a raindrop? the ocean? the air?

Published on June 1, 2021Written by budbromley.blog

Image: Union of Concerned Scientists
The atmospheric concentration of CO2 gas is continuously adjusting to maintain the concentration partition ratio K derived in Henry’s law. Henry’s law partition ratio is independent of the source of the CO2. The net average atmospheric concentration of CO2 (~400 ppmv) is independent of human CO2 emission.

Human CO2 is fully compensated in (and only a small part of) the natural global CO2 fluxes in the environment. Ocean has been estimated to make up 98 percent of the hydrosphere. (Mason, 1958) Rainwater is less than 2 percent of the hydrosphere.
Mason points out that ocean is 98 percent of the hydrosphere but he does not specify the rain portion. Mason states that no significant error will be made by assuming the average CO2 concentration in all water is the average of sea water.
According to Henry’s Gas Law, the giant mass of CO2 gas in ocean water, on the order of 40,000 gigatons of carbon (4 X 1013 metric tons) and temperature regulate the atmospheric CO2 gas concentration and the CO2 fluxes in the atmosphere, ocean, biosphere and even in rainwater droplets.  The timeframe for each of these fluxes is different.  Following Henry’s Law, the high solubility of CO2 gas in liquid water means the atmosphere is scrubbed of CO2 gas by the enormous volume of liquid water in the ocean, air and soil.
Flow and flux are not the same. Flux is a directional vector of an amount of material flowing per unit time through a unit area. In this case, the unit area is the surface area of water everywhere which is in contact with atmosphere. A CO2 flux is the amount of CO2 flowing per second per square meter of water surface. There are enormous, continuous fluxes of CO2 in two directions, into the atmosphere and into water, controlled by temperature and surface area, and both of these directional fluxes are more than 10 times larger than fossil fuel emissions.
“The total CO2 produced by the burning of the annual production of coal and oil is 6.2 X 1015 g or about 1/300th of the amount in the atmosphere today.  This might suggest that at the present rate of consumption of fossil fuels atmospheric carbon dioxide will be doubled in 300 years.  However, in this connection the importance of the hydrosphere as a reservoir of carbon dioxide should be emphasized; its significance has been discussed by Revelle and Suess (1957).
Sea water contains 20 g of CO2/cm2 of the earth’s surface, as against 0.4 g/cm2 in the atmosphere.  Oceanic and atmospheric carbon dioxide are interdependent, the former being a function of the partial pressure of CO2 in the atmosphere. Thus to double the partial pressure of carbon dioxide in the atmosphere would require the addition of much more than is now present therein, because most of that added would be absorbed by the ocean; similarly to decrease the carbon dioxide in the atmosphere by half would require removal of many times the present content.
It is apparent that the oceans, by controlling the amount of atmospheric CO2, play a vital part in maintaining stable condition suitable for organic life on the earth.” (Mason, Page 211-212.)
Note in the above quotation, the 50:1 ratio of grams of CO2 in sea water to grams of CO2 in atmosphere. The partition ratio of CO2 gas between water and air is governed by Henry’s Gas Law which results in about 50 times higher concentration of CO2 gas in water than in the air around or above the water. The absorption of CO2 gas into the surface of water is very fast (sub-second) and driven primarily by water temperature.
Colder water absorbs far more CO2 than warm water. Warm water emits CO2 gas into the air. The distribution of CO2 gas horizontally and vertically in atmosphere is not as fast, but I will not discuss these chaotic processes here.
The dissolution time of aqueous CO2 gas into its various dissolved carbonate forms is very fast (seconds.) The chemical reaction of carbonate ions with oceanic buffering systems is very fast (seconds.) The calcium buffering system of ocean will be discussed briefly as an example.
Absorption and emission of CO2 from the surface of water acts locally on every square centimeter of water surface every second.  Normalizing temperature and CO2 concentration by averaging removes information and adds no value to the analysis.  The temperature difference above and below about 26 C are the critical variables which define whether CO2 is being absorbed or emitted at a particular location and time.
The temperature controls the direction of the flux. The temperature difference and surface area at that temperature control the amount and the velocity of the flow. All of that information is missing when only a global average temperature is used. “The average temperature of the ocean surface water is about 17 °C (62.6 F).” (Temperature of Ocean Water, University of Michigan. August 31, 2001.)
The average temperature of ocean surface water is irrelevant to the Henry’s Law equilibrium and to the solubility chemistry of CO2 in water.  The average of very large chaotic fluxes is a meaningless number with no predictive value.
In the graphic below we can easily see where CO2 is emitting into air and where CO2 is absorbing into ocean and soil.  A global average temperature would tell us nothing. When temperature exceeds 26 C, CO2 will be emitted from water. When temperature is less than 26 C, CO2 will be absorbed into water. We can also infer from this graphic that there are enormous fluxes of atmospheric CO2 gas from the equator to higher latitudes near both poles. In fact there are many cells in the atmosphere and in the ocean each with its own CO2 flux.

At a few thousand meters altitude above sea level where water vapor and aerosols condense into liquid water droplets, and anywhere condensation occurs, the surface of water droplets will be either absorbing or emitting CO2 based dominantly on temperature. Henry’s law determines the solubility of CO2 gas in all water, not only in ocean water. The CO2 gas concentration in your beverage is changing in real time.
If the top of a can or bottle of a carbonated beverage is removed, or the beer is tapped from the keg into your glass, initially the CO2 gas concentration in the liquid beverage will immediately decline because the total pressure of the gases above the liquid is significantly less than the total pressure of the mixed gases above the liquid in the closed keg.
After that, the aqueous CO2 gas concentration in your beverage will continue to decline until the liquid and air above it reach the Henry’s Law equilibrium partition ratio K, which is based primarily on the temperature of your beverage.
Addition of salts or acids to the liquid increases the aqueous CO2 gas concentration. Carbonated beverages typically contain a small amount of acid, for example phosphoric acid, for retention of aqueous CO2 gas in the liquid. Rain scrubs chemicals such as sodium chloride from the air which become ionic in raindrops and that in turn changes the aqueous CO2 gas concentration in raindrops. That is all I will say about this subject here.
Henry’s Law only applies to the solubility of gases into liquids when the gas concentrations are low. When they are low, such as rare gas CO2 at 400 parts per million, then concentration of CO2 gas in the liquid and in the air above the liquid can be calculated and measured with very high accuracy and precision. Henry’s Law is the basis of the multi-billion dollar per year scientific instrumentation industry of gas chromatography. GC’s are used routinely in almost all industries involving chemistry from perfumes to paint to healthcare to refineries.
Henry’s Law partition only applies to the gas phase in the liquid, for example aqueous CO2 gas in ocean, and the gas above the liquid, for example CO2 in the air.  Aqueous CO2 gas reacts in seconds in water by disassociating into several forms of carbonate ions.  These carbonate ions then react with ionic forms of other molecules which are also dissolved in ocean water, for example calcium ions.  Calcium ions (Ca+2) react with a carbonate ions to form calcium carbonate (limestone, dolomite, CaCO3). This calcium carbonate precipitates as a solid and becomes slurry, sedimentation, then stone on the sea floor. This disassociation chemistry is not determined by Henry’s Law.
Ocean buffering systems such as this calcium chemistry are removing aqueous CO2 gas from the Henry’s Law equilibrium equation. This calcium buffering chemistry is very important to the concentration of CO2 in the ocean and atmosphere and is defined by other laws, but I will only briefly mention it in this article.
A rain droplet falls through air containing CO2 gas. The CO2 gas partition ratio between the air and the rain droplet is adjusting in real time (no significant lag, no equilibrium) to the temperature differential experienced in the rain droplet and the CO2 concentration in the surrounding air as the droplet falls.
As the rain droplets fall to earth, in tropical and temperate latitudes when the droplet temperature exceeds 26 C, the droplets begin emitting CO2 gas. In higher temperate and polar latitudes, when droplet temperatures are less than 26 C, the falling drops will be absorbing CO2 gas from the air as they fall.
Droplets of water nucleate (condense) on particles in the atmosphere. The types of particles vary widely based on geography. Salt and other minerals and gases are carried aloft by wind, currents, convection, storms over ocean. Oceans are ~70% of earth’s surface. Over land the chemical composition of raindrops is much more variable; no simple algorithm is possible. Rain droplet formation is discussed in detail in Professor Murry Salby’s text Physics of The Atmospheric and Climate, 2012.
The chemical composition of raindrops varies with the amount of rain falling during a given time period. Rain (and dew) scrub the air of particulates and gases, e.g., hydrocarbon gases. Hydrocarbon, sulfur and nitric gases are higher concentration in urban areas than over ocean, and these gases are found in raindrops in those areas, again obeying Henry’s Law K for each gas. The same is happening for CO2, methane, argon, and other gases found in air; each gas has its Henry’s Law solubility K for water. You have probably noticed that the air is cleaner after a good rain.
In general, wherever water temperature is below 26 C, that water is absorbing CO2 gas in real time, no delay, in proportion to the temperature difference above 26 C and in proportion to the area of water surface which is in contact with air at that temperature. Anywhere water temperature is above 26 C it will be emitting CO2 gas into air. Rain arriving at ocean surface changes the concentration of CO2 gas in ocean surface, which will then drive re-equilibration based on Henry’s Law partition ratio in that surface water.
Water droplets in clouds, falling from clouds, and condensing in air sum to a relatively high surface area compared to the flat 2 D surface area of the ocean.  Approximately 4πr2 verses r2.  Therefore, taken altogether, the additional sink and source due to raindrops would appear to be significant relative to other sinks and sources.  But, building an algorithm to calculate the size of this additional rain sink and source would be as uncertain as predicting the weather, primarily due to variances driven by water in all its phases and chaotic conditions.
For example, in Hawaii near the northern boundary between temperate zone and tropical zone, rain and clouds are cooler than ocean surface.  Raindrops have a larger ratio of surface area / volume ratio than ocean surface.  Cooler raindrops temporarily increase the aqueous CO2 gas concentration in ocean surface water in Hawaii and all of the tropics.
But since ocean water in the tropics is usually warmer than 26 degrees, that additional aqueous CO2 gas will be rapidly (seconds) emitted to atmosphere as the temperature of the cooler rainwater rapidly warms to the temperature of the ocean’s massive heat sink.  Thus, raindrops are another large, chaotic CO2 gas flux between sink and source.  It would be difficult or impossible to model with accuracy this chaotic bi-directional CO2 flux between sink and source.
To calculate how much CO2 is in rain, we would need to know the amount of precipitation that is liquid, the surface area of rain drops, the temperature gradients in the global atmosphere and ocean, of course Henry’s Law for myriad conditions, etc.  Some of the needed information is measured and estimated. Volume of global precipitation is calculated by taking the product of the Earth’s surface area and its average annual rainfall.
Total annual volume of precipitation of water in all phases is about 5.1 × 1014 m3.  In other words, fossil fuel CO2 gas emission on the order of 5.5 X 109 metric tons (see graphic) is being absorbed into a volume of rain that is on the order of 1014cubic meters.   Raindrops are a large sink and source for CO2 gas.  Rain is scrubbing the air of CO2 just as it scrubs air of other gases and particles.  Whether rain is a sink or source of CO2 depends on temperature in that location.
Read the rest here: budbromley.blog
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