A New Approach to Calculate Diffusional Transfer Functions of Scanning DMAs

A fast approach to calculate scanning differential mobility analyzer (DMA) transfer functions is introduced. The trajectory-based approach of Dubey and Dhaniyala (2008, Analysis of Scanning DMA Transfer Function. Aerosol Sci. Technol., 42: 544-555) is extended to determine mobility-based transfer functions of diffusional particles in scanning DMAs. The mobility-based transfer functions are obtained from the transformation of calculated arrival-times of the particles of a selected mobility, i.e., the arrival-time transfer function (ATF). The ATFs are calculated from Monte Carlo simulations of the motion of particles of different diameters, under a range of DMA operating conditions. The results suggest that the nondiffusional ATFs for up-scan operation can be approximated as triangular in shape and the diffusional ATFs as Gaussian. The simulation results show that for high-voltage operation, the effect of fast scanning determines the measurement resolution, while for low-voltage operation, the effect of diffusion is most important. An approach to calculate the instrument transfer function based on the ATF area and resolution is described. The transfer functions calculated by the new fast approach match well with those obtained from Monte Carlo simulations.