If you have the option, you may just want to output the cloud water pH based on “HPLUS” calculated in aqchem.F. pH = -LOG10(HPLUS)
If not, to get an estimate of (resolved) cloud water pH, I think you’d have to take all species in the ACONC file (gas and aerosol + CO2) that dissociate in cloud water and solve the system of equations that comes from Henry’s Law relationships, ionic dissociation equations, and electroneutrality. One can use aqchem.F as an example. You can use the following variables from your MCIP files to calculate liquid water content to perform necessary unit conversions: WCBAR (kg/m3) = ( QC( COL, ROW, LAY ) + QR( COL, ROW, LAY ) + QG( COL, ROW, LAY ) ) * DENS( COL, ROW, LAY )
One word of caution: OH in the ACONC file represents hydroxyl, not hydroxide (OH-).
And I have one more question about estimating atmospheric pH not cloud water.
I know CMAQ v5.3.1 is using ISORROPIA II for equilibrium of aerosol.
Then, can I get estimated pH value directly from ISORROPIA II ??
If it is not possible, should I use some species (e.g. Na+, K+, NH4+, TSO4, NO3-, Cl-, Ca2+, Mg2+) from ACONC file of CMAQ to estimate atmospheric pH value??