The Scanning DMA Transfer Function

Abstract

The scanning differential mobility analyzer (DMA) has been widely employed for measurement of rapidly evolving aerosol size distributions. Interpretation of data from scanning DMAs is greatly facilitated when an exponential voltage ramp is prescribed, since the shape of the instrumental transfer function remains constant throughout a scan. However, that transfer function may differ significantly from that expected for fixed voltage operation. Because no simple analytical description of the scanning DMA transfer function exists, it has been evaluated numerically by simulating particle trajectories within a TSI 3081 cylindrical DMA. These computations yield transfer functions for the DMA up scan that are roughly triangular but with widths significantly greater than those for fixed voltage operation, and transfer functions for the down scan that are highly asymmetric. The impact of these distortions is most obvious when the size distribution of the measured aerosol is narrow, but errors in recovered size and concentration can be significant even when the aerosol size distribution is much broader than the transfer function. The magnitude of these errors is dependent upon the ratio of the mean gas residence time to the exponential voltage time constant, the sheath-to-aerosol-flow ratio, and the technique used to determine the instrument plumbing time. Experimental results for scans across broad and narrow size distributions compare favorably with predictions based on the simulated transfer functions. Simplified corrections are provided that can be used to adjust the concentration and mobility of size distributions recovered using a fixed voltage transfer function.

Experimental Comparison of Two Portable and Real-Time Size Distribution Analyzers for Nano/Submicron Aerosol Measurements

“…4). This can be explained by the influence of the flow ratio on the DMA transfer function (Collins et al, 2004).…”

A New Approach to Calculate Diffusional Transfer Functions of Scanning DMAs

“…At the high-voltage limit, however, the scanning DMA resolutions do not reach the theoretical limit of Knutson and Whitby (1975), with the maximum resolution attained decreasing with scan time. The reasons for the reduced resolution in the nondiffusional limit under a fast scanning operation have been previously discussed (Collins et al 2004;Dubey and Dhaniyala 2008;Mamakos et al 2008).…”

Differential Mobility Analysis of Aerosols: A Tutorial

“…The transfer function of the DMA operated in scanning mode was shown to be identical to that of one operated at the average voltage during the particle transit time. An additional subtlety was identified by Collins et al (2004); the changing voltage during the particle transit time can alter how long a particle spends in different regions within the velocity profile of the classifier. This is reflected in a difference in the size distribution measured during a scan of increasing voltage (an up-scan) and that measured with decreasing voltage (a down-scan).…”