Estimation of pH value in the cloud water using CCTM or MCIP species


I’m trying to estimate pH value in the cloud water using CCTM results and MCIP results.

I know this is really basic problem, but I want to know whether my thinking is right or not and the most appropriate method.

First, I estimated pH value using OH concentration from ACONC file since pH can be calculated through the formula ‘pH = 14 - pOH’.

But this method is not suitable because the OH concentration from ACONC file does not mean OH concentration in the cloud water.

And then, I tried to find any species in MCIP results for estimating the pH value, but I couldn’t find suitable species.

So, I want to know how to estimate or calculate pH value in the cloud water using CMAQ CCTM or MCIP results.

And other ideas are also okay.

I hope any response for my question, please let me know any ideas from you.

Thank you.

HyeonYeong Park

Hi, HyeonYeong.

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-).