Extending the Spider-MAGIC Sizing Range for the Detection of Nucleation Mode Particles

STAVROS AMANATIDIS, Gregory S. Lewis, Steven Spielman, Susanne Hering, Aerosol Dynamics Inc.

     Abstract Number: 546
     Working Group: Instrumentation and Methods

Abstract
The Spider-MAGIC is a compact, dual-polarity scanning mobility spectrometer that consists of the radial-flow “Spider” Differential Mobility Analyzer (DMA) and the “MAGIC” water-based Condensation Particle Counter (CPC). The Spider DMA was originally designed to classify 9 – 420 nm particles with 30 s scans, using 0.90 L/min sheath and 0.30 L/min aerosol flow rates. These operating parameters were considered a good trade-off between required instrument size and attainable measurement capabilities. While the instrument covers an important fraction of the complete size distribution for atmospheric particle measurements, it can only partially detect nucleation mode particles in the <10 nm range. In this work we discuss the challenges and design modifications for extending the instrument measurement range down to 4 nm, while maintaining its original physical size.

To attain mobility separation at 4 nm without changing the size of the DMA requires nearly a three-fold increase in the sheath flow rate. Yet, initial experiments at higher flowrates showed evidence of performance degradation. Finite element modeling of the flow in the Spider DMA revealed the development of flow mixing in the DMA classification region. This originated from the formation of high-velocity jets as the sheath flow passes through an array of small flow-distribution holes. No mixing was observed at the original flow conditions.

To minimize the flow effects, we considered a new design approach, wherein a “flow impingement surface” is placed immediately below the sheath flow distribution holes. We show through modeling that this relatively simple modification dampens the jet formation and eliminates the flow mixing observed in the original design. Both designs were evaluated through tandem-DMA experiments which corroborate the modeling results and validate the instrument performance.