What light model are you using? I would recommend using GI.
What quality setting are you using in the Light tab of the atmosphere settings? I would recommend boosting this up. What level of ambient light are you using? The more ambient light you have, the more you have to boost this up.
What anti-aliasing settings are you using? For this scene, I would recommend not using any of the standard render settings but instead use "user settings" and manually increase AA to min=10, max=25, quality>85. I would recommend not using texture AA.
Are you using volumetric lights? Volumetric lights tend to produce more grain.
In the render settings, do not check "optimize volumetric lights" and do not check "optimize last render pass". In Vue language, "optimize" means less quality, generally speaking.