Impactor collection efficiencies can modify the uncertainty of multiply charged particles in optical extinction measurements

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 (C-ext) 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 C-ext exhibited statistically significant differences at particle sizes near the impactor cut point implying that these multiply charged particles should not be present and could not be confirmed by parallel size distribution measurements. Additionally, comparison of C-ext with Mie theory demonstrates that misclassification of the multiply charged particles can 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 mu m vs. (455 +/- 1) mu m, respectively (uncertainty is 1s standard deviation)) but the average Stk(50) (the Stokes number at the cut-point, D-50) values differed by approximate to 23% (0.23 vs. 0.18 +/- 0.01, respectively). The measured CE as a function of vStk (a metric of particle aerodynamic size) exhibits a long tail toward higher vStk values, 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. [GRAPHICS] .

Automated summary

The complex distribution of particle charge states caused by charge neutralization processes is a known issue in aerosol science. Here, we show that non-idealities in an impactor's collection efficiency can lead to the transmission of multiply charged particles, affecting optical measurements. By using cavity ring-down spectroscopy and particle counting, we quantified the extinction cross sections of ammonium sulfate particles and investigated the underlying reasons for the presence of multiply charged particles. The results highlight the importance of orthogonal spectroscopic methods in detecting multiply charged particles.