Effects of Gas-Phase Components on Measurement of Secondary Organic Aerosol Size Distribution

YUANLONG HUANG, John Seinfeld, Richard Flagan, California Institute of Technology

     Abstract Number: 207
     Working Group: Instrumentation and Methods

Abstract
The measurement of airborne particle size can be divided into three steps: charging the particles, separating charged particles in an electrical field, and counting the separated particles, typically with a condensation particle counter. Each step should be well understood to carry out a precise size inversion. Here we focus on the charging process in a soft X-ray ambipolar diffusion charge conditioner to establish a fundamental understanding of ion generation and the interaction between particles and the ions used to charge them. The particle charge state depends on the residence time distribution inside the charge conditioner, the gas composition, and the ion concentrations and properties. It is typically parameterized for steady-state operation in dry ambient air. However, a non-equilibrium or biased charge state can exist due to the presence of unanticipated gas-phase species, particle shape, or downstream wall losses of ions of different mobilities, or electric fields due to charge build-up on insulating surfaces within the charge conditioner. We have developed a dual-polarity differential mobility analyzer that measures both positively and negatively charged particles. Using this instrument, we measure the particle charge distribution resulting from a custom soft X-ray charge conditioner in the presence of various gas-phase species in typical chamber and flow tube experiments. We investigate the effect of the presence of the most common oxidants that are usually present in excess during chamber experiments (e.g., O₃ and H₂O₂) and the humidity on the ion generation and the particle charge distribution in a soft X-ray diffusion charge conditioner. These effects must be considered when deriving the SOA yields from the experimental data.