Air pressure, a few details aside, must be equal to the weight of the air molecules sitting above the spot where you are measuring it. If we add more gas to the atmosphere, sea level air pressure must increase. One way to think about it: the weight of the additional gas smooches down on the existing atmosphere, compressing it and thus increasing the pressure.

Of course all of this is academic in any event; addressing climate change by diluting the CO2 is a nice attempt at out-of-the-box thinking but it's not actually plausible.