Pa^2/Hz to dB/Hz conversion
This depends a bit on what you are after. As Laurent Duval has pointed out, you need can only take the logarithm of a unitless quantity so you need to do some form of normalization


If you want Sound Pressure Level (dbSPL) you would normalize to the standard reference pressure of 20 uPa (20e-6 Pa)
You could also divide by the free field acoustic impedance of air, which would give you the intensity in W/m^2. 
The most common units for a PSD would be W/Hz. To calculate that you'd have to integrate over the entire volume flow and so you need to make an assumption for the shape of your sound wave. That's pretty easy for spherical and plane waves, but difficult for more complex shapes.