For the systems with equilibrium exposed at air, we will impose hypothesis that the basic ions of alkaline metals will not precipitate. This is the case of mixed cooling systems, more accurate working conditions of cooling towers, where air contact surface is very high so the transfer between air and water of CO₂ have maximum speed. The Henry 's law governs this process. The quantity of gas dissolved on to known volume of solvent at constant temperature, is proportional with gas or liquid pressure at equilibrium.

• [15] C = Ks * Pco2; Ks = 0.03 at 25 ⁰ C.

For the system, we have now another unknown value and another equation, at last seven unknown variables and five equations. To solve the problem we must to know two of them in this case the known values are Pco2 and m.Solving the system in the same manner like for closed systems will give a solution
• [16] [H⁺]= K_w + K_b [H₂CO₃]+{SQRT{K_w + K_b [H₂CO₃]^2+8m K_b K₂ [H₂CO₃]} /{2m}

The previous computer program is changing, as entry data is partial pressure for CO₂ and m. From equation (2) I know [CO₂] and [H₂CO₃], from (16) will result [H⁺] and from this point will result all values like on to previous program. For practical use, we have here the diagram of dependency [H⁺] and m for usual conditions so you can to evaluate water pH after the cooling towers as a function of water alkalinity at the known temperature.