An integrated electrical condensation particle counter for compact and low-cost ultrafine particle measurement system

Abstract

In this study, we developed a compact and low-cost measurement system that can measure the real-time number concentration of airborne ultrafine particles (UFPs, particles smaller than 100 nm). Our system is based on the condensation nucleation method; however, it counts particles based on their electrostatic properties (i.e., dielectric constant) to overcome the limitations of complex optical systems that have traditionally been employed. The new system consists of two components: a microfluidic particle growth channel to grow UFPs into micron-sized water droplets and a capacitive particle counter to count the grown droplets. To effectively reduce the size and cost of the system, each system was fabricated into a single module using a semiconductor manufacturing process prior to integration. Super-hydrophilic micropillar wicks were monolithically integrated with Printed Circuit Board substrate channels to use water as the condensing working fluid. A glass substrate interdigitated electrode chip with a 3 μm gap was fabricated and placed on the impaction plate of an inertial impactor to be used as a sensor. Numerical calculations were performed to verify the dimensions of the channel required for efficient supersaturated vapor generation. Quantitative experiments using NaCl, (NH4)2SO4, and Ag2O UFPs showed that our system could grow particles larger than 50 nm into micron-sized droplets and count up to 10,300 N cm−3 particles with linear electrostatic properties.