Dependence of Pulse Height Distribution on Particle Size, Composition, and Concentration for 1-7 nm Particles

DARREN CHENG, Chongai Kuang, Coty Jen, Carnegie Mellon University

     Abstract Number: 183
     Working Group: Instrumentation and Methods

Abstract
New particle formation (NPF) describes the process where gas-phase molecules in the atmosphere nucleate at ~1nm and grow into stable particles. In order to study NPF events and quantify particle nucleation and growth rates, particle size distributions down to 1 nm are required. The most common instrument to perform these measurements is with a scanning mobility particle sizer (SMPS). However, SMPSs scan through particle sizes and therefore do not provide continuous measurements of a single particle size. SMPSs are also reliant on charging particles which is challenging due to extremely low and highly uncertain charging efficiencies for sub-10 nm particles. Here we present pulse height analysis (PHA) of a water-based MAGIC condensation particle counter (CPC) coupled with a diethylene glycol growth tube to measure near-instantaneous size distributions of particles between 1 and 7 nm. The PHA technique provides information on the sampled particle size distribution by relying on the differential condensational growth of particles through a CPC growth tube. Our results show that the pulse height distributions are dependent on particle size, composition, and concentration. For example, sodium chloride particles display a small shift in distribution to larger pulse heights between 1 – 2 nm. In contrast, mode pulse heights from grown ammonium sulfate particles continuously shift monotonically throughout the 1 – 7 nm range. We present how each factor influences the predicted particle size distribution measured from PHA. Overall, our results will improve the accuracy of the PHA technique in measuring sub 10 nm size distributions.