Thinking About Oil Rain

Recently, there has been discussion on the Internet of a video that purportedly shows evidence of it raining oil in Louisiana.



Is this evidence of oil rain? Is oil rain possible?

Evidence

Even if oil rain might be possible, certainly this video does not constitute evidence.  If such a phenomenon happened, someone ought to be able to collect rain falling from the sky and analyze the contents.  Such a phenomenon would be so odd that certainly many people would document it.

More Plausible Explanations

Before jumping to extraordinary claims, one ought to look for more plausible explanations.  Consider the following, in no particular order:

• There were already hydrocarbons on the street, perhaps from spilled gasoline or another source.
• The sewers backed up.
• Liquid oil was somehow blown in from a sea breeze, a strong wind gust, or a waterspout.
• An aerial release of dispersants or other hydrocarbons was somehow blown onto the ground.
• The video is a hoax

Until these explanations are ruled out, I consider the evidence extremely questionable at best.  Still, it invites an interesting question: is oil rain a physically reasonable phenomenon?  How could such a thing happen?

Physical Mechanism

An author from Jalopnik.com tried to imagine a mechanism:

under normal environmental temperatures, oil does not evaporate, however with the Deepwater Horizon disaster, the effects of seawater emulsification and the introduction of BP's dispersant of choice, Corexit 9500, may be allowing some degree of evaporation into the water cycle

This explanation completely misses the boat. Even under normal situations more volatile fractions of the oil will evaporate; evaporation is not the problem.  Condensation is the issue.  Volatile organic compounds evaporating from the oil spill is not enough to cause them to re-condense into rain.  The atmosphere is very big compared to the oil spill.  Volatile components will disperse in the atmosphere and be diluted.  The vapor will be most concentrated right near the spill and that concentration will taper off as one gets farther away.  What would cause the vapor to re-condense?  Consider the case with water.

Water Rain

Water vapor is everywhere.  For it to rain the vapor needs to condense into liquid water.  If the liquid water nucleates into big enough water droplets, it will rain.

This graph shows a curve of the equilibrium vapor pressure of water.  The curve represents 100% relative humidity, i.e., at a given temperature, air in equilibrium with water would hold the amount of vapor shown.  Air with 100% humidity is commonly referred to as "saturated."  That's not quite accurate, but it will do for my purposes.

Consider air at 80 degrees that is nearly saturated with water vapor and suppose it suddenly cools to 60 degrees.  There is now too much vapor in the air and the vapor condenses in order to approach equilibrium. The dew point is the point at which the air is cool enough to cause water to condense.

Oil Rain

The difference between the hydrocarbon vapors  from oil with the case of water is that it is extremely unlikely that the air could saturate with the vapor.  As one gets farther from the source of the vapor, the vapor is diluted by clean air.  In the case of water, any air with which the parcel of air mixes already has water vapor in it.  If the newly mixed air has too much water vapor, it can condense.  In the case of hydrocarbon vapor, the air is diluted with relatively clean air.  The concentration of hydrocarbon vapor will never be as large as it was near the source.

Evaporation is a molecular process.  The addition of dispersant may affect the phase diagram of hydrocarbons causing more or less to evaporate, but it does not change the fundamental fact that the vapor will be diluted.  The chances that the air can cool enough for these extremely dilute vapors to condense into liquid are extremely small.  If we lived on a planet on which the seas were composed of hexane, it would be a different story.

More exotic mechanisms

One can imagine more exotic mechanisms involving clusters or the absorption of vapor by water droplets.  The former case is contraindicated by the very small amount of clusters one would expect.  The latter case would require high concentrations of vapor in the proximity of where the rain droplets are formed.  Neither of these ideas is very plausible.

EPA

Jalopnik.com contacted EPA, which responded:

EPA has no data, information or scientific basis that suggests that oil mixed with dispersant could possibly evaporate from the Gulf into the water cycle.

Conclusion

Impossible is a strong word, and I will not use it in this context, but it seems very implausible that hydrocarbon vapors from the oil spill could condense into rain.  Certainly, the flimsy evidence provided gives no reason why one should take such a notion seriously.

Sources

• YouTube
• Huffington Post: Raining Oil in Louisiana?
• Jalopnik.Com: It's raining Oil in Louisiana
• Wikipedia: Vapour Pressure of Water

Acknowledgement

Thanks to John Merritt for pointing this story out to me.