Improving Accuracy in Mobility-Based Aerosol Sizing by Measuring Bipolar-Charged Particles
JINGKUN JIANG, Jin Wu, Tsinghua University, China
Abstract Number: 42
Working Group: Instrumentation and Methods
Abstract
Scanning Mobility Particle Sizers (SMPS) are widely used to measure particle number size distribution (PNSD) of nanoparticles. A key component of SMPS is the neutralizer, which generate ions and bring aerosols to steady-state charge fractions. These charge fractions are important parameters when performing data inversion. Conventionally, the charge fractions are calculated using bipolar diffusion theory; for instance, the most widely used approximation formulas are derived from fixed ion properties of radioactive source (Wiedensohler, 1988) and soft X-ray source (Tigges et al., 2015).
However, the real-world charge fractions can differ significantly from those assumed in approximations or models, which could be affected by gas composition, ion source activity and neutralizer design. Our previous studies demonstrated that the concentrations of positively and negatively charged particles provide information about both charge fractions and ion properties. We have also designed and applied bipolar SMPS that can rapidly obtain the PNSD of positively and negatively charged aerosols.
In this research, we analyze the concentrations of positively and negatively charged aerosols conditioned by different neutralizers to evaluate the fluctuations of charge fractions in long-term field measurement. We find the actual charge fractions differ from the previously used approximations. As the neutralizer ages, the deviation becomes more pronounced and harder to predict. The number concentration of aerosols conditioned by neutralizers exhibit size dependencies that differ from the predictions of approximation formulas. Furthermore, by analyzing the data from 14 sites across China, we identified the characteristics of naturally bipolar charged aerosols. We also investigated whether using the summed concentrations of positively and negatively charged particles during inversion provides better results than using unipolar data alone. We suggest that employing bipolar high voltage and modifying the SMPS appropriately are beneficial for reducing bias in results and obtaining more comprehensive information in ambient measurements.