Creating a ring of fog in 2D with particle system

I’m currently working on a 2D top-down project.

I need to implement a fog ring effect with a safe (or void) area that shrinks over time. Player should take damage every second if they are outside this safe area.
I’ve tried using a Particle System to create a donut shape for the fog ring, but it didn’t look as good as I expected.

Is it a right approach and I’m doing something wrong or are there any other ways to implement this?