Design and Evaluation of a Thermal Precipitation Aerosol Electrometer (TPAE)

Abstract. Aerosol electrometers are widely applied to measure the number concentration and other physical parameters of particles. In this work, a new aerosol electrometer, i.e., Thermal Precipitation Aerosol Electrometer (TPAE), was designed and its performance was experimentally evaluated. The TPAE integrates a thermal precipitator with a micro- current measurement circuit board for measuring electrical charges carried by particles. The thermal precipitator was in the disk-to-disk configuration. Heating paster and air cooling were adopted to establish a temperature gradient in the precipitator. At a sample flow rate of 0.3 L/min and temperature gradient of 254 K/cm, the precipitation efficiency of particles reaches ~100 %. The measurement range of the designed aerosol electrometer was ±5×10⁵ fA, with the accuracy of ±2 fA. In the evaluation, the electrical performance of TPAE was evaluated using DMA (differential mobility analyzer)-classified sodium chloride and soot particles and compared it to that of the reference faraday-cage aerosol electrometer. The precipitation performance of TPAE was then studied as the functions of temperature gradient, sampling flowrate and particle size. It is shown that the particle collection efficiency of built-in thermal precipitator is inversely proportional to the sampling flow rate, and proportional to the temperature gradient. The effect of particle size on the above efficiency was minor for sodium chloride particles. Different from that observed for NaCl particles, the slightly positive correlation of the collection efficiency with the electrical mobility size was observed for soot particles (in the size range of 30 ~ 160 nm). It might be due to soot agglomerates. The designed aerosol electrometer with the soft particle collection could be used in, e.g., field studies requiring to both measuring the particle charges and collecting particles for offline morphology, chemistry, and other studies.