A PULSED CONDENSATION PARTICLE COUNTER FOR LOW-COST MONITORING OF ULTRAFINE AIRBORNE PARTICLES ABSTRACT This project will design, develop and validate a new approach for monitoring the number concentration of fine and ultrafine airborne particles. Ultrafine particles are specifically implicated in health, and yet are not detected by currently-used low-cost sensors. Our approach is a Pulsed Condensation Particle Counter which uses adiabatic expansion combined with single particle counting. With modern optical sensors, single particle counting is quite feasible, and is more precise than the ensemble measurements of prior automated adiabatic counters. Our analysis shows that our new method should be much more energy-efficient than the laminar flow condensation methods now used, as no heating or cooling of the components is required. Our target is an affordable ($<3000), portable instrument that measures the particle number concentration with known accuracy and precision, and that bridges the gap in between the low-cost “citizen science” devices and research-grade instruments. This Phase I project will assess the feasibility of our concept as a low-cost sensor through modeling and experiment. It addresses the critical method components, namely system sizing, expansion rate, humidification and optical detection. Numerical modeling will examine the saturation ratio resulting from expansion in the presence of heat and water vapor transport from the walls, and how this varies with the aspect ratio of the expansion volume. This modeling will guide the design of critical components, which will then be built and tested with laboratory aerosols of known size and composition. The project will examine cost-efficient means of optical detection of the condensationally enlarged particles, including coincidence corrections. These components will be tested using existing electronics, and these experimental data will provide a basis for estimating the accuracy, precision, size, weight, power use and cost of a fully packaged system.