Acid rain pH calculation

Hi all,
I would like to know how to calculate the pH for investigating acid rain using WRF-CMAQ. Could you please guide me a bit to the equation and variable that should be used?
@hogrefe.christian @cgnolte @Ben_Murphy @lizadams

@Ryan perhaps this question is already answered here: Estimation of pH value in the cloud water using CCTM or MCIP species