American Association for Aerosol Research - Abstract Submission

AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA

Abstract View


Control of Condensation onto 1-2 nm Particles in Laminar Growth Tubes via Lewis Number Modulation in He-CO2 Gas Mixtures

JIKKU THOMAS, Anne Maisser, Christopher Hogan Jr., University of Minnesota

     Abstract Number: 84
     Working Group: Aerosol Physics

Abstract
Condensation particle counters (CPCs) can detect particles down to sizes of a few nanometers by exposing them to a supersaturated vapor and growing these particles to macroscopic sizes, followed by optical detection. The supersaturation achieved determines the lowest particle size that can be activated and thus detected. In many laminar flow CPCs, a flow with saturated vapor is introduced into a condenser tube maintained at a cooler temperature, and variable amounts of radial transport of heat and vapor leads to supersaturation in the growth tube. The saturation profile thus achieved strongly depends on the Lewis number (Le), i.e. the ratio between thermal diffusivity of the gas and mass diffusivity of the vapor molecules. When Le > 1, heat transfer occurs faster, leaving supersaturated vapor in the center of the tube. Increasing Le, therefore, directly increases the saturation ratio, enabling condensation onto smaller particles. In this study, we show it is possible to control the Lewis number in a commercially available CPC (TSI 3025 butanol based) by operating the CPC in a gas mixture of CO2 and helium. The Lewis number in He exceeds that in air, while the Le in CO2 is lower; hence, a large range of Le can be achieved by employing He-CO2 mixtures. We show computationally and experimentally (via experiments with tetra-alkylammonium ions), by operating at different gas ratios, the saturation profile inside the growth tube can be changed while all other operation parameters can be kept constant. This shifts the activation efficiency curves of the instrument and thus, is a promising method to use for particle sizing employing only a CPC without requiring any pre-classification.