Aerosol quantification with the Aerodyne Aerosol Mass Spectrometer: detection limits and ionizer background effects

Abstract. Systematic laboratory experiments were performed to investigate quantification of various species with two versions of the Aerodyne Aerosol Mass Spectrometer, a Quadrupole Aerosol Mass Spectrometer (Q-AMS) and a compact Time-of-Flight Aerosol Mass Spectrometer (c-ToF-AMS). Here we present a new method to continuously determine the detection limits of the AMS analyzers during regular measurements, yielding detection limit (DL) information under various measurement conditions. Minimum detection limits range from 0.03 μg m⁻³ (nitrate, sulfate, and chloride) up to 0.5 μg m⁻³ (organics) for the Q-AMS. Those of the c-ToF-AMS are found between 0.003 μg m⁻³ (nitrate, sulfate) and 0.03 μg m⁻³ (ammonium, organics). The DL values found for the c-ToF-AMS were ~10 times lower than those of the Q-AMS, mainly due to differences in ion duty cycle. Effects causing an increase of the detection limits include long-term instrument contamination, measurement of high aerosol mass concentrations and short-term instrument history. The self-cleaning processes which reduce the instrument background after measurement of large aerosol concentrations as well as the influences of increased instrument background on mass concentration measurements are discussed. Finally, improvement of detection limits by extension of averaging time intervals, selected or reduced ion monitoring, and variation of particle-to-background measurement ratio are investigated.