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Software:
3ds max 2019 | Arnold 5

Adding a ‘Volume Light’ effect in Arnold for 3ds max is fairly simple:

1. In the Render Setup windows > Arnold Renderer tab, under Environment, Background & Atmosphere:
2. Click the Scene Atmosphere material slot, add an Arnold Atmosphere Volume material to it,
   And drag it as an instance to the Material Editor to edit it’s parameters.
   
3. Set the Density to a higher than 0.0 value, so the material will have an effect.
4. You’ll probably need to significantly raise the number of samples in the Atmosphere Volume material, and also the number of Volume samples in the light settings in order to get a clean render.

Related:
Arnold for 3ds max Volume Shading