Krakatoa uses the particle system’s “opacityPP” channel directly as its “Density”. So the values in “opacityPP” should be reflected in the renders.
Does the particle system have an “opacityPP” attribute that you want to use directly? Or are you wanting to compute a density from opacityPP and lifespanPP?
I’m not entirely sure what your setup is and what you are seeing. Can you attach a sample scene that shows what you are seeing?
Note that our “Density” is not directly analogous to opacity. It defines how much light gets attenuated when passing through the particles. A 0.5 Density does not mean the particle is half opaque. Maybe I’m misunderstanding what is happening though. A sample scene would be very helpful to try to diagnose things.