This photo of what looks like "nozzles" is currently being sharing on social media as supposed evidence of a secret spraying program using commercial jets (i.e. the "chemtrail" theory).
However, these are just static discharge wicks, short sections of wire bolted to the wing that discharge any build up of static electricity. Without these wicks, static electricity can build up on the wing, and uncontrolled sparks can interfere with communications and navigation equipment.
The image that is being passed around has a very forced perspective. It look like the wicks go along the entire wing, but it's actually just a few feet towards the tip of the wing. The wicks are barely visible in most photo of a 747-8F, because the plane is so big, and the wicks are tiny.
From inside the plane they are barely visible, way out on the end of the wing.
This video shows the wicks in context, and gives you a better idea of their size.
Here's a patent for a static discharge wick. It's basically a piece of multi-strand steel cable, covered in plastic, less than half an inch thick.
There is actually one "nozzle" in the photo though, the fuel dump vent. Due to the force perspective it looks like it's in the middle of the wing, but it's actually near the end (with the wicks)
Heres the vent in action. Jets sometimes have to dump fuel when they have to return after takeoff (or land midway to their destination) due to some emergency.
And here's a shot of a fuel dump with the static wicks visible, giving a good sense of scale. This is on a 777.