Demonstration of a Novel Ultrafine Sensor based on Pulsed Bipolar Charging

There remains a critical gap in low-cost measurement technologies for ultrafine particles (UFPs), despite their prevalence across urban and indoor environments and increasing links to health effects. Recent advances in charge-based UFP sensors are filling that gap, particularly when combined with Faraday cup detection in a compact, deployable form. However, existing devices face key limitations, including signal variability associated with corona discharge onset and fouling, and sensitivity to pre-existing charge on sampled particles in field deployment.

To address these challenges, this work demonstrates a bipolar charging sensor for ultrafine particles, in which particle charging is periodically modulated and the resulting induced current is measured. While sensing based on pulsed unipolar charging is widely utilized, to the authors knowledge, this is the first sensing method to utilize pulsed bipolar charging. An analytical model is also developed that directly links particle charging to the electrometer response. The testing results from the sensor agree with model predictions across a range of sample flow rates and pulse frequencies. This model also highlights that alternating the charge polarity on the aerosol particles yields stronger, more stable signals while reducing sensitivity to pre-existing particle charge.

Together, these results demonstrate a novel UFP sensor.