Destroy Particles if they are outside their emitter

you could also just use the trigger event in the particle system and have them killed if they leave a volume

I did something similar by destroying particles that are a certain distance from the emitter:

 using System.Linq;
 /// <summary>
 /// Destroys particles that are farther than DistanceToDestroy from the particlesystem emitter
 /// </summary>
 [RequireComponent(typeof(ParticleSystem))]
 public class ParticleMaxDistance : MonoBehaviour
 {
     public float DistanceToDestroy;

     private ParticleSystem cachedSystem;
     private Vector3 staticPosition;

     void Start()
     {
         cachedSystem = this.GetComponent<ParticleSystem>();
         staticPosition = this.transform.position;
     }

     void Update()
     {
         ParticleSystem.Particle[] ps = new ParticleSystem.Particle[cachedSystem.particleCount];
         cachedSystem.GetParticles(ps);

         // keep only particles that are within DistanceToDestroy
         var distanceParticles = ps.Where(p => Vector3.Distance(staticPosition, p.position) < DistanceToDestroy).ToArray();
         cachedSystem.SetParticles(distanceParticles, distanceParticles.Length);
     }
 }