The Effect of Charger Ion Properties on Particle Size Distributions using a Scanning Mobility Particle Sizer

The World Health Organization recently called for expanded monitoring of aerosol particle size distributions (PSDs), implemented across air quality monitoring stations worldwide. A mobility particle size spectrometer (MPSS), commonly implemented using a Scanning Mobility Particle Sizer (SMPS), is the standard method for measuring PSDs. It consists of a bipolar charge conditioner, a differential mobility analyzer (DMA) and a condensation particle counter. The bipolar charge conditioner ionizes gas molecules (or charger ions) in the carrier gas, which then charge or discharge the aerosol particles. The charging probabilities, or charge fractions, are used in the inversion to a PSD.

Charge fractions are dependent on the particle size and charger ion properties. These vary with temperature, pressure, and trace atmospheric gas composition. However, MPSS measurements are typically inverted under the assumption that the charger ion properties do not change with environmental conditions. When the actual charger ion properties differ from those assumed by the inversion, the charge fractions are incorrect and errors in the inversion occur. Full knowledge of the PSD using an MPSS is only possible when the charger ion properties are known.

In this study, we examine the error in the inverted PSD geometric mean diameter (GMD), geometric standard deviation (GSD) and number concentrations (N) due to differences in actual and inversion charger ion properties. We use the non-diffusive DMA transfer functions and Hoppel multiple charge correction algorithm to generate and invert a mock MPSS signal. The charge fractions are generated using established approximations of the Fuchs charging model. We find substantial error in the inverted GMD, GSD, and N occurs with a realistic difference between actual and inversion charger ion properties. As the error is greater for negatively charged particles, we recommend measuring the positively charged particles in standard SMPS scans.