Air on earth is mostly nitrogen, about 78%, and 21 % oxygen. Plants can´t produce nitrogen out of the thin CO2 atmosphere that is martian air so there can be no earth air on Mars. Even if all the CO2 was converted into O2 it still wouldn´t be breathable because the pressure would still be about 1/100th of earth pressure. You die in such low pressure. While the nitrogen that was part of Mars primordial denser atmosphere has drifted off into space the oxygen might remain. It wouldn´t be stable though. It would react with the soil and with all the CO2 gone there would be no more source of oxygen and the martian atmosphere would become thinner and thinner as the oxygen is used up until eventually the atmosphere would be even more of a vacuum than it has ever been.