Where'd Those Particles Come From? Impactor Non-idealities, Multiple Charging, and Optical Measurement Biases

JAMES RADNEY, Chun-Ning Mao, Akua Asa-Awuku, Christopher Zangmeister, National Institute of Standards and Technology

     Abstract Number: 118
     Working Group: Instrumentation and Methods

Abstract
Multiple charging is a problem that has received significant attention in aerosol science that can be especially problematic for optical measurements as particle scattering and absorption cross sections are a strong and non-linear function of size; aerosol extinction cross sections (C_ext, the sum of absorption and scattering) can exhibit even stronger dependencies. Thus, C_ext measurements utilizing only size selection by a differential mobility analyzer (DMA) require the use of charge correction algorithms to account for the presence of larger particles bearing multiple charges. One fundamental assumption in these algorithms is that above some critical size the population of particles is zero thus imposing an upper size limit that is often enforced with an impactor. Ideally, impactor performance is described by a Dirac delta function, with a well-defined cut-point diameter above (below) which particle transmission is 0 % (100 %). In reality, particle transmission efficiency as a function of size has a finite slope and must instead described by the cut-point diameter, D₅₀, where particle transmission is 50 %. Here, we demonstrate that impactor non-idealities (or damage) can allow for an unexpected population of multiply charged particles larger than D₅₀ to be present. Harnessing the sensitivity of a cavity ring-down spectrometer (CRD), which measures aerosol extinction, we demonstrate that measurements of C_ext utilizing size selection only versus size and mass selection – which removes the contribution of multiply charged particles – may only converge at sizes larger than D₅₀ demonstrating the presence of this unexpected population of multiply charged particles. Failure to account for these particles can lead to significant deviation in inferred particle optical properties when performing refractive index retrievals or radiative forcing calculations.