The present invention proposes an aerosol-generating system comprising a liquid storage portion (5) for storing a liquid aerosol-forming substrate; a vaporizer (7) for volatilizing the liquid aerosol-forming substrate; and a pump (3) for pumping air into the liquid storage portion to push out a volume of the liquid aerosol-forming substrate from the liquid storage portion, and to supply the pushed out volume of liquid aerosol-forming substrate to the vaporizer (7). The present invention also proposes a method for generating an aerosol.