Development of a Fast-Response Electrical Mobility Spectrometer for Ambient Ultrafine Particle Monitoring: Design and Numerical Optimization

Accurate monitoring of ultrafine particles (UFPs) in ambient air is critical for environmental health assessment, yet it is often limited by the low time resolution and low charging efficiency of conventional bipolar scanning systems. This study presents the design and numerical optimization of a wide-range aerosol spectrometer (10–1000 nm) specifically engineered for high-temporal-resolution ambient monitoring. The instrument integrates a PM1.0 aerodynamic impactor, a high-intensity unipolar corona charger, and a fast-response electrometer. Unlike traditional designs, this system utilizes unipolar charging to achieve near-unity charging efficiency for accumulation mode particles, enabling a 1-minute scan time suitable for capturing dynamic environmental events. A comprehensive numerical model validating impactor aerodynamics, corona discharge physics, and transfer functions demonstrates that the system achieves a stable Townsend discharge with an N _i t product exceeding 10 ¹³ s.ions/m ³ . A Non-Negative Least Squares (NNLS) inversion algorithm was successfully implemented to recover size distributions with high fidelity. This optimized design provides a robust, cost-effective solution for expanding UFP monitoring capabilities in air quality networks.