Louisiana residents 45 miles off the Gulf of Mexico claim to have videotaped an oily substance raining down. Worst case scenario? It's petroleum mixed with Corexit, the cancer-causing dispersant BP's spraying on its oil slick. Best case scenario? Dirty roads.
Video is circulating which purports to show the effects of an oily rain coming down off the coast of Louisiana. It could be as simple as the sheen which develops on the road following rain on a dirty road or someone attempting a sensationalist hoax. At this point nobody knows. A smattering of reports from along the coast indicate similar occurrences accompanied by a strong odor of oil.
Our first thoughts were oil we're accustomed to dealing with generally doesn't evaporate, and that's true for engine oil sitting on a shelf, however crude oil at sea is an entirely different story. According to a 2003 study titled "Oil in the Sea III: Inputs, Fates, and Effects" put out by Ocean Studies Board, Marine Board, and Transportation Research Board, oil behaves very differently when on the open water. The study states:
Within a few days following a spill, light crude oils can lose up to 75 percent of their initial volume and medium crudes up to 40 percent. In contrast, heavy or residual oils will lose no more than 10 percent of their volume in the first few days following a spill. Most oil spill behavior models include evaporation as a process and as a factor in the output of the model.
The oil included in the Deepwater Horizon disaster is most certainly crude, and was at one point a heavy crude, which reduces the overall loss to evaporation, however it's been mixed up by the effects of the ocean and become an emulsification, which according to the study, enhances the likelihood of evaporation:
Emulsification, if it occurs, has a great effect on the behavior of oil spills at sea. As a result of emulsification, evaporation slows spreading by orders of magnitude, and the oil rides lower in the water column, showing different drag with respect to the wind.
So considering the effect of light and crude oil evaporation and seawater emulsification oil from the Deepwater Horizon may be having an effect on the water cycle. An unknown variable on the overall cycle is the introduction of BP's dispersant of choice, Corexit 9500, which may be either helping or hurting the degree of evaporation. Dispersants break up the natural surface tension in oil, sending small droplets into the water column and reducing the surface area which may evaporate, however it also changes the chemistry of the oil which remains on the surface. We have yet to find any science on the subject of the evaporation rates of these compounds or their likelihood to come back down as contaminated rain.