There must be something else here, because reducing ambient light normally takes away the grain. But as Gary said, if you used volumetric lighting or atmosphere, you need to bump up the quality  slider up to 2 minimum.

What are your render settings? Final is not enough, but ultra is too much. Go for user settings, with 46% advanced effects quality boost, and min9 and max 12 for AA settings, with 60% quality.