A system and method comprising a charger for ionizing aerosols; a spectrometer coupled to the charger and operable to select for a predetermined particle size; a porous charge collector coupled to the spectrometer, and a MEMS electrometer. In some embodiments the charge collector may be a metal frit electrically coupled to the electrometer. The electrometer may include a comb drive actuator coupled to a moving shuttle supported on flexures.