Ocean acidification is generally thought of in terms of a chemical equilibrium process so you don't take the gas exchange rate into account. In physical terms, this is equivalent to saying the ocean surface mixed layer equilibrates instantaneously with the increasing CO2 in the atmosphere. This is more or less true, the true equilibration time is on order of a year globally, but CO2 doesn't change all that much over a year.
So, what you need to do to calculate the effect of increasing atm. CO2 on ocean pH is get the manual for CO2 calculations in seawater published by CDIAC as part of the old DOE-Oceans CO2 program (google "Dickson Goyet Scripps CO2 manual" or similar terms and you should find it). Then you need to find maps of ocean surface total alkalinity and one of the parameters of the carbon system, either DIC or pCO2. Using the equations in the CO2 manual linked above, you can work out the CO2 chemistry regionally in the ocean, and from that figure out how the surface pH changes due to a doubling of atmospheric CO2.
Setting up the equations is relatively straightforward if you are used to dealing with acid-base chemical calculations. If you're not it can be a little confusing. There are canned programs for doing this (e.g., here, the Lewis and Wallace reference) and you could use one of them instead.
