When the water evaporates away you are left with a mass of various solids. In the case of this crude the “solids” are food for bacteria that target these solids.
Unfortunately, WATER is not the only 'liquid' or 'gas' involved that 'evaporates'. WATER is the least of the liquids or gases that 'evaporate' as WATER and OIL don't even mix very well, or for very long.
There's also an extremely huge amount of water under the surface of the GOM, and the OIL that is 'suspended in solution' won't be losing that liquid to evaporation unless the SUN GOES NOVA.
Now the GAS, methane, rises right to the surface, dispersant or not. The OIL on the surface of the ocean contains aromatic hydrocarbons that DO EVAPORATE under the glare of the Sun. It this were not so, then the process of REFINING OIL using HEAT to get GASOLINE would be impossible.
So.... I will still defend my position that the OIL, and the GAS, and the DISPERSANT are releasing certain chemicals into the atmosphere that could (and may already have ) cause(d) considerable damage, and that long term exposure to them (like over 4 hours, per the EPA and BP) is harmful to humans, plants, and animals.
These chemicals MAY or MAY NOT come back down with rain, MAY or MAY NOT have caused the crop damage.
The only TRUTH is that we don't know yet.
No one has found any other explanation for the crop damage yet, so.... what is causing it?
Remember, the oil will break down on its own absent the application of any dispersant. The dispersant just hastens the natural process. With or without it, all the volatile chemicals contained in the oil are going to escape into the atmosphere, if they themselves aren't degraded before then.
It is crude oil, light sweet crude oil. The extended aromatic hydrocarbons are released in the air if they attain enough heat, as you say. The purpose of the Corexit is to break the mass up into smaller globules with greater surface area for bacteria to attack. If, as you apparently believe with some conviction, there are other chemicals coming up with the crude (aside from methane, which was the cause of the head blowing and blowing up the rig- a considerable “slug” of methane came up the 5K plus feet of pipe to explode on the surface- recall that the psi at 5k feet of water is very great,and the gas expands as it comes to the surface losing this outside pressure-boom!)then those chemicals are currently well known to our EPA, to oil drillers in general and the industry worldwide. If it is a particularly fine light sweet crude then some other hydrocarbons might be there, but they don’t just pffft evaporate into water vapor to make a foul rain. Doesn’t happen.