Although Dissolved inorganic carbon, total alkalinity and pH data have been previously indirectly used to derived pCo2, is there a more direct approach in its determination?
The question is a bit confusing to me. Partial pressure of a species usually refers to a pressure in a gaseous phase. I think you mean the pressure in the vicinity of the gas/water boundary. If so, it depends on the thickness of the layer above the surface. It might be determined at different levels and interpolated to zero distance.