Particle Detection Efficiencies of Aerosol Time of Flight Mass Spectrometers under Ambient Sampling Conditions

Abstract

Aerosol time-of-flight mass spectrometers (ATOFMS) measure the size and chemical composition of single aerosol particles. To date, these instruments have provided qualitative descriptions of aerosols, in part because the fraction of particles actually present in the atmosphere that is detected by these instruments has not been known. In this work, the particle detection efficiencies of three ATOFMS instruments are determined under ambient sampling conditions from the results of colocated sampling with more conventional reference samplers at three locations in southern California. ATOFMS particle detection efficiencies display a power law dependence on particle aerodynamic diameter (D_a) over a calibration range of 0.32 < D_a < 1.8 microns. Detection efficiencies are determined by comparison of ATOFMS data with inertial impactor data and are compared to detection efficiencies determined independently by the use of laser optical particle counters. Detection efficiencies are highest for the largest particles and decline by approximately 2 orders of magnitude for the smallest particles, depending on the ATOFMS design. Calibration functions are developed here and applied to scale ATOFMS data to yield continuous aerosol mass concentrations as a function of particle size over an extended period of time.