Particle lifespan

programming flash
1

This is driving me nuts:
(im new to actionscript btw)
I have created the particle emitter below, but the particles last forever.
How can I remove each particle after it has existed a certain amount of time? (eg. 3secs)


var particleArray:Array = new Array();
    var maxParticles:Number = 10;
    
    for (var i:int = 0;i < 20;i++) {
    addParticle();
    }
    
    function addParticle() {
     var dot:Particle = new Particle();
     dot.x = hero.mc.x;
     dot.y = hero.mc.y-15;
     dot.alpha = Math.random() * .8 + .2;
     dot.scaleX = dot.scaleY = Math.random() * .8 + .2;
     var xMovement:Number = Math.random() * 8 - 4;
     var yMovement:Number = Math.random() * 8 - 4;
     particleArray.push(dot);
     gamelevel.addChild(dot);
     
     if (particleArray.length >= maxParticles) {
      gamelevel.removeChild(particleArray.shift());
     }
     
     dot.addEventListener(Event.ENTER_FRAME,moveParticle);
     
     function moveParticle()
     {    
      dot.x += xMovement;
      dot.y += yMovement;
     }
     
     
     
    }

Cheers