Animating already emitted CAEmitterCell objects

Question

I have a CAEmitterCell that I am using to emit a continuous background for my game. When the player "dies", I want the background to slow to a stop, so I did this:

- (void)stopAllEmitters:(NSTimeInterval)duration
{
    for(CAEmitterLayer* layer in self.emitters)
    {
        CAEmitterCell* e = layer.emitterCells[0];

        CABasicAnimation* anim = [CABasicAnimation animationWithKeyPath:@"emitterCells.emitter.velocity"];
        anim.fromValue = @(e.velocity);
        anim.toValue = @(0.0);
        anim.duration = duration;
        anim.fillMode = kCAFillModeForwards;
        anim.removedOnCompletion = NO;
        [layer addAnimation:anim forKey:@"emitterAnim"];
    }
}

Note that all emitterCells are named @"emitter", and each layer only has one emitterCell.

This does work for all future-spawned emitter objects, but not for the ones that have already been spawned when I call this.

Is there any way to apply the velocity animation to emitter objects that are already on-screen?