Design and evaluation of a thermal precipitation aerosol electrometer (TPAE)

Abstract

A new aerosol electrometer (AE), the thermal precipitation aerosol electrometer (TPAE), was designed for use with particles of sizes less than 300 nm, and its performance was experimentally evaluated. The TPAE combines the thermal precipitator with a microcurrent measurement circuit board (i.e., pre-amplifier) for measuring the current carried by collected particles. The thermal precipitator is in the disk-to-disk configuration. Heating paste and air cooling were adopted to establish the desired temperature gradient in the precipitation chamber. At a sample flow rate of 0.3 L min−1 and a temperature gradient of 264 K cm−1, the precipitation efficiency of 70 nm particles reaches ∼100 %. The measurement range of the designed aerosol electrometer is ±5×105 fA, and the accuracy is ±2 fA (2500 to 6.25×107 cm−3 using a flow rate of 0.3 L min−1 and assuming that only singly charged particles exist in the sample). During the evaluation process, the electrical performance of the TPAE was first tested using sodium chloride (NaCl) and soot particles previously classified by a differential mobility analyzer (DMA) and compared to the reference. The precipitation performance of the TPAE was then characterized as functions of the temperature gradient, sampling flow rate and particle size. It was shown that the particle collection efficiency of the 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 particle collection efficiency was minor for NaCl particles of sizes between 23 and 200 nm. Unlike that which was observed for the NaCl particles, a slightly positive correlation between the collection efficiency and the mobility size for soot particles (in the size range of 30–160 nm) was observed. This observation might be due to the existence of soot agglomerates. Compared to existing electrometers, the TPAE does not require the use of high-efficiency filters and includes the additional feature of the “soft” collection of particles for offline particle characterization as well as aerosol current measurement.