Multiple Charging Effects on the CCN Activity of Black Carbon Particles

OGOCHUKWU ENEKWIZU, Ernie R. Lewis, Arthur J. Sedlacek, Brookhaven National Laboratory

     Abstract Number: 227
     Working Group: Carbonaceous Aerosol

Abstract
The interactions of black carbon (BC)-containing particles with water vapor in the atmosphere are of interest because they can influence cloud formation in addition to influencing BC optical properties, and BC lifetimes through removal by scavenging and precipitation. Accurate measurements of hygroscopicity and cloud condensation nuclei (CCN) activity are needed given the positive forcing impacts of BC on climate. Measurements of hygroscopicity of BC-containing particles are typically conducted on particles selected only by their mobility with a differential mobility analyzer (DMA). However, a single DMA system also transmits larger particles, with multiple charges, that have the same mobility as the singly-charged particles. These larger particles will activate at lower supersaturations than the singly-charged particles, affecting measurements and resulting in overestimation of CCN activity. In this study, we combine a DMA with a centrifugal particle mass analyzer (CPMA) to select particles by both mobility and mass, thus resulting in only singly-charged particles. We demonstrate the use of the DMA-CPMA system in resolving biases caused by multiply charged particles in the CCN activation measurements of different BC surrogate particles with mobility diameters ranging from 100 to 300 nm. Additionally, we determine the hygroscopicity parameter (κ) of the BC surrogates. We find that multiply charged particles result in an overestimation of κ by 12 to 75 %, depending on the particle mobility diameter and composition of the BC surrogate. Our results also show that the effect of multiple charging on the CCN activity of the BC particles is strongly influenced by morphology dispersion, i.e., variability in morphology of particles with the same mobility and mass that results in variability in other properties such as CCN activity. Our findings demonstrate that use of particles selected by mobility alone will not lead to accurate results for the CCN activation and hygroscopicity of BC-containing particles.