Constraint Particle System to only x&y axis?

The title says it all. I'm working in a 2D game and the particle emitter shape is a Cone, Sphere, Hemisphere etc. There's a way to shape the emitter to a circle or a quad? So particles are only generated in a 2-Dimensional world.

Update 1

This script sets the z-velocity of each particle to 0, successfully keeping the particles in the x-y plane. The only drawback I see is that this can cause some serious performance problems.

 using UnityEngine;
 using System.Collections;
 
 public class ExplosionController : MonoBehaviour {
 
     public ParticleSystem particleSystem;
 
     void Update()
     {
         ParticleSystem.Particle[] particleArray = new ParticleSystem.Particle[particleSystem.maxParticles];
         int particles = particleSystem.GetParticles(particleArray);
         for (int i = 0; i < particles; i++)
         {
             Vector3 vel = particleArray[i].velocity;
             vel.z = 0;
             particleArray[i].velocity = vel;
         }
         particleSystem.SetParticles(particleArray, particles);
     }
 }

Update 2

As I only need to set the particles z-velocity to 0 only one time, because the explosion is a non-looping particle system, it's easy to disable the loop option from the inspector and trigger the emission from the script itself:

 using UnityEngine;
 using System.Collections;
 
 public class ExplosionController : MonoBehaviour {
 
     public ParticleSystem particleSystem;
 
     void Awake()
     {
         particleSystem = gameObject.GetComponent<ParticleSystem>();
     }
 
     void Start()
     {
         particleSystem.Emit(particleSystem.maxParticles);
         ParticleSystem.Particle[] particleArray = new ParticleSystem.Particle[particleSystem.maxParticles];
         int particles = particleSystem.GetParticles(particleArray);
         for (int i = 0; i < particles; i++)
         {
             Vector3 vel = particleArray[i].velocity;
             vel.z = 0;
             particleArray[i].velocity = vel;
         }
         particleSystem.SetParticles(particleArray, particles);
         
     }
 }

It is also easy to adapt this script to your own needs, creating functions to alter color and whatnot.

Best regards.