Volume rendering - Density vs Alpha

[malkuth]

Hi,

I want to render a custom pass for clouds in mantra. The reference jpg shows a custom pass from Maya (density / ambient) and the alpha for the same clouds. In Maya a gradient ramp was used on the incandescence channel. Basically where the clouds are denser should be black and the opposite, for lower density I should have white. Can I replicate that with mantra? I'm using the sky rig tool so everything is calculated at render time. Thank you!