How can I apply gravity to each particle within a 'Gravity Zone' region?

I have an object with a box trigger collider denoting a region of unique gravity, which I call a Gravity Zone. The idea behind a Gravity Zone is to add an additional effect of gravity in that region, in addition to whatever global gravity setting the scene has.

Gravity Zones have a force vector setting, which can be for example (0, -4, 0). Gravity Zones do two things...

First, they apply the specified force vector to each Rigidbody within the zone, at every FixedUpdate. This means that any object – with a Rigidbody – within the region gets tracked using OnTriggerEnter and OnTriggerExit events to know when it has entered or left, and then each FixedUpdate I am applying the force to all tracked rigidbodies.

Second, they need to apply the specified force vector to each particle within the zone, because I realized simply affecting Rigidbodies would be incomplete, and particles still act as if they weren't in the unique zone of gravity. However, I am unsure of how to get each particle within the zone. If I had them, I could update each of their velocity values by the force amount. Furthermore, I want to modify the force amount by the 'Gravity Modifier' setting of the particle's source Particle System, so I would need a way to get the Particle System that the particle originated from to read that particle's relevant Gravity Modifier setting value.

So to recap:
1. How can I get all of the particles inside my Gravity Zone?
2. How can I get the 'Gravity Modifier' value of the respective Particle System for each of those particles (which may be from multiple different Particle Systems)?