You can do this with a timer, and a method to stop the emitter generating new particles...
At the end of your existing code add:
[newEmitter setName:@"newEmitter"];
[NSTimer scheduledTimerWithTimeInterval:2.0
target:self
selector:@selector(stopEmitter:)
userInfo:nil
repeats:NO];
Then make a stopEmitter: method…
- (void) stopEmitter:(NSTimer*)timer
{
[emitter setValue:@0
forKeyPath:@"emitterCells.newEmitter.birthRate"];
}