CAEmitterLayer emit continuously in a line

I am creating a very simple particle system with CoreAnimation.

This is my cell:

UIImage *image = [UIImage imageNamed:@"spark"];
CAEmitterCell *cell = [CAEmitterCell emitterCell];
[cell setContents:(id)image.CGImage];
[cell setBirthRate:250.f];
[cell setScale:.25f];
[cell setColor:[UIColor colorWithRed:1.0 green:0.2 blue:0.1 alpha:0.5].CGColor];
[cell setLifetime:5.0f];

and layer:

CAEmitterLayer *emitterLayer = [CAEmitterLayer layer];
[emitterLayer setEmitterCells:@[cell]];
[emitterLayer setFrame:bounds];
[emitterLayer setRenderMode:kCAEmitterLayerAdditive];
[self.view.layer addSublayer:emitterLayer];

I add a very simple animation to the layer:

CGPoint startPos = CGPointZero;
CGPoint endPos = CGPointMake(bounds.size.width, bounds.size.height);

CABasicAnimation* ba = [CABasicAnimation animationWithKeyPath:@"emitterPosition"];
ba.fromValue = [NSValue valueWithCGPoint:startPos];
ba.toValue = [NSValue valueWithCGPoint:endPos];
ba.duration = .5f;
[emitterLayer addAnimation:ba forKey:nil];

What I expect is a line be be formed with particles emitted continuously. However, what I see particles being emitted at intervals, like this:

How can I fix this?