Quantifying the Contributions from Multiply Charged Particles to Improve Calibration Procedures for the ACSM

Ernie R. Lewis, Arthur J. Sedlacek, Ogochukwu Enekwizu, Maria Zawadowicz, AMIE DOBRACKI, Brookhaven National Laboratory

     Abstract Number: 390
     Working Group: Instrumentation and Methods

Abstract
The Aerosol Chemical Speciation Monitor (ACSM, Aerodyne Inc.) measures non-refractory submicron aerosol mass concentrations. Calibration of the ACSM typically involves aerosolizing an ammonium nitrate solution and selecting particles with a 300 nm electrical mobility diameter using a differential mobility analyzer (DMA). It is commonly reported that there is minimal contribution from multiply charged particles of the same diameter when an ammonium nitrate solution with a concentration between 0.005 and 0.018 M is used. However, at these conditions, our findings reveal that these multiply charged particles contribute between 20-40 % of the measured nitrate mass. To address the contribution from multiply charged particles, we employed a Centrifugal Particle Mass Analyzer (CPMA; Cambustion) in conjunction with the DMA to isolate singly charged particles based on both their electrical mobility diameter and mass. Our results reveal that the ACSM sampling efficiency at electrical mobility diameters of 100, 200, and 400 nm is substantially less than 100 % when both the DMA and CPMA are used, based on a sampling efficiency of 100 % for ammonium nitrate particles selected only by the DMA at 300 nm electrical mobility diameter. We propose a simple protocol for correcting the bias introduced by multiply charged particles during laboratory experiments and field measurements when a CPMA is not available.