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Impactor Collection Efficiencies Can Modify the Uncertainty of Multiply Charged Particles in Optical Extinction Measurements



Chunning Mao, Akua Asa-Awuku, Christopher D. Zangmeister, Jamie Weaver, James Radney


The complex distribution of particle charge states resulting from neutralization processes by radioactive or soft X-ray charge neutralizers is a well-documented problem in aerosol science. Here, we demonstrate that non-idealities in the collection efficiency of an impactor allows for the transmission of an unexpected population of multiply charged particles by a differential mobility analyzer that can bias optical measurements. The extinction cross sections (Cext) of ammonium sulfate particles were quantified using cavity ring-down spectroscopy and particle counting. Particles were selected by electrical mobility (i.e., a metric of particle size) using a differential mobility analyzer (DMA) or electrical mobility and mass selected by a tandem DMA and aerosol particle mass analyzer (APM), respectively, to elucidate multiple charging artifacts. Measured Cext exhibited statistically significant differences at particle sizes near the impactor cut-point implying the presence of multiply charged particles that should not be present and could not be confirmed by parallel size distribution measurements. Additionally, comparison of Cext with Mie theory demonstrates that misclassification of the multiply charged particles can still give rise to numerically accurate results. To understand these observations, the collection efficiency (CE) of four impactors from similar electrostatic classifiers was investigated. From these measurements, it was determined that the nominal and actual diameters of the impactors differed by -0.5 % (457 μm versus (455 ± 1) μm, respectively), but the average Stk50 (the Stokes number at the cut-point, D50) values differed (0.23 versus 0.18 ± 0.01, respectively) by ≈ 23 %. The measured CE as a function of √Stk (a metric of particle aerodynamic size) exhibits a long tail towards higher √Stk, allowing for transmission of the larger and multiply charged particles observed in the optical measurements. These measurements highlight the utility of using orthogonal, spectroscopic methods to quantify the presence of multiply charged particles.
Aerosol Science and Technology


Aerosol, Impactor, Multiple charging, Cavity ring-down spectroscopy


Mao, C. , Asa-Awuku, A. , Zangmeister, C. , Weaver, J. and Radney, J. (2023), Impactor Collection Efficiencies Can Modify the Uncertainty of Multiply Charged Particles in Optical Extinction Measurements, Aerosol Science and Technology, [online],, (Accessed June 16, 2024)


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Created April 10, 2023, Updated April 11, 2023