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-).


Hi, Dr. Fahey!

Thank you for your response :slight_smile:

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??

I hope I can get your response to this question.

HyeonYeong Park

Hi, HyeonYeong-

You can estimate fine aerosol pH in 5.3.1 using ACONC output species for fine mode H3O+ (AH3OP) and aerosol water (AH2O):

H+ (M) = ( AH3OPI + AH3OPJ ) / 19.0 / ( AH2OI +AH2OJ ) *1000.0

FINEPHF (when AH2O > 0.01 ug/m3) = -1*LOG10(H+)

Hope that helps,

Thank you for your response :slight_smile:

I think your response will really helpful for me!

Thank you for your answer again!! :slight_smile:

HyeonYeong Park


Starting in CMAQv5.3.2, fine aerosol acidity is also output by CMAQ. See CMAQ/ at main · USEPA/CMAQ · GitHub for notes (you will need to filter for when pHF is undefined due to a lack of aqueous phase).