A bipolar SMPS network for measuring atmospheric aerosols using natural air ions

Abstract

To study atmospheric aerosols, it is essential to build networks that are continuously monitoring particle number size distribution (PNSD). PNSDs are often measured using electrical mobility spectrometers such as scanning mobility particle sizer (SMPS) which includes a neutralizer that conditions the charge state of particles to the pre-assumed distributions. During long-term atmospheric measurement, the charge state can deviate from these assumed values. Measuring both positively and negatively charged particles using a bipolar SMPS helps to capture the variations in aerosol charge state. In this study, we set up a network of bipolar SMPSs at three urban sites and three suburban sites in six Chinese cities. Removing the conventional neutralizer, the bipolar SMPS utilizes the natural aerosol charging process by atmospheric ions. Newly developed aerosol classifiers and detectors were used. Harmonized quality assurance procedures were adopted to assure the performance and the data quality, including laboratory calibrations and intercomparisons, maintenance and monitoring during field measurements, and data inversion and validation. PNSDs are effectively measured by bipolar SMPSs using natural charging process at all six sites. During six months of field measurement in these sites, ion mobility ratios fluctuate significantly while the daily averaged values are stable around 1.0. Particle number concentrations integrated from PNSDs are consistent with those directly measured by the detectors (within a variation of 10%).