The invention relates to an ion source (50) for generating elemental ions and/or ionized metal oxides from aerosol particles, comprising: a reduced pressure chamber (61) having an inside; an inlet (56) and a flow restricting device (60) for inserting the aerosol particles in a dispersion comprising the aerosol particles dispersed in a gas, in particular in air, into the inside of the reduced pressure chamber (61), the inlet (60) fluidly coupling an outside of the reduced pressure chamber (61) via the flow restricting device (60) with the inside of the reduced pressure chamber (60); a laser (62) for inducing in a plasma region (63) in the inside of the reduced pressure chamber (61) a plasma in the dispersion for atomizing and ionizing the aerosol particles to elemental ions and/or ionized metal oxides; wherein the reduced pressure chamber (61) is adapted for achieving and maintaining in the inside of the reduced pressure chamber (61) a pressure in a range from 0.01 mbar to 100 mbar. The invention further relates to a method for generating elemental ions and/or ionized metal oxides from aerosol particles, comprising the steps of inserting aerosol particles in a dispersion comprising the aerosol particles dispersed in a gas, in particular in air, through an inlet (56) via a flow restricting device (60) into an inside of a reduced pressure chamber (61), while maintaining in the inside of the reduced pressure chamber (61) a pressure in a range from 0.01 mbar to 100 mbar, preferably from 0.1 mbar to 100 mbar or from 1 mbar to 100 mbar, particular preferably from 0.1 mbar to 50 mbar or from 1 mbar to 50 mbar, most preferably from 0.1 mbar to 40 mbar or from 1 mbar to 40 mbar; and inducing with a laser (62) in a plasma region (63) in the inside of the reduced pressure chamber (61) a plasma in the dispersion for atomizing and ionizing the aerosol particles to elemental ions and/or ionized metal oxides, wherein the laser (62) is adapted for inducing in the plasma region (63) in the inside of the reduced pressure chamber (61) the plasma in the gas of the dispersion for atomizing and ionizing the aerosol particles to elemental ions.