I don’t know whether the ac was pressurised or not but I believe it was equipped with sampling devices, so probably not.
The cockpit is pressurised by air from the compressor stage of the engines. That air comes from the front of the engine so whatever is outside is going to come in.
You can close the pressurisation valve before take off and fly unpressurised on 100% oxygen. This will MAY ensure that the same air is in the cockpit as when you left.