Practical Limitations of Aerosol Separation by a Tandem Differential Mobility Analyzer-Aerosol Particle Mass Analyzer.

A cavity ring-down spectrometer and condensation particle counter were used to investigate the limitations in the separation of singly and multiply charged aerosol particles by a tandem differential mobility analyzer (DMA) and aerosol particle mass analyzer (APM). The impact of particle polydispersity and morphology was investigated using three materials: nearly-monodisperse polystyrene latex nanospheres (PSL); polydisperse, nearly-spherical ammonium sulfate (AS) and polydisperse lacey fractal soot agglomerates. PSL and AS particles were easily resolved as a function of charge. For fresh soot, the presence of multiply charged particles severely affects the isolation of the singly charged particles. In cases where the DMA-APM was unable to fully resolve the singly charged particles of interest, the peak mass deviated by up to 13 % leading to errors in the mass specific extinction cross section of over 100 %. For measurements of non-spherical particles, non-symmetrical distributions of concentration as a function of mass were a sign of the presence of multiply charged particles. Under these conditions, the effects of multiply charged particles can be reduced by using a second charge neutralizer after the DMA and prior to the APM. Dilution of the aerosol stream serves to decrease the total number concentration of particles and does not remove the contributions of multiply charged particles.