A novel electrical-mobility-based instrument for total number concentration measurements of ultrafine particles

A novel electrical-mobility-based technique to measure total particle number concentration over a selected size range is presented. Charged particles are condensed out onto an electrode that is shaped such that the product of its transfer function and the particle charging efficiency is a constant, independent of particle size. The resulting total current is then proportional to the number concentration of the sampled particles over the collected size range. The theoretical approach for the calculation of the electrode shape function is described. The extension of this technique for measurement of higher moments of the particle size distributions over a desired size range is briefly discussed.This concept is used to design a new instrument, called the tailored electrode concentration sensor (TECS). For validation of the theoretical concept, the collection electrode in the TECS instrument is designed for concentration measurements over a size range of 30–90 nm. In the TECS, the collection section is located downstream of an electrostatic precipitator section, where the sampled flow is split into aerosol and sheath flows, similar to the design of the MEAS [Ranjan, M., & Dhaniyala, S., (2007), Theory and design of a new miniature electrical-mobility aerosol spectrometer, Journal of Aerosol Science, 38(9), 950–963]. This results in a compact, low pressure drop instrument. Experimental results confirm that the response of the optimally-shaped electrode in the TECS system is only proportional to total number concentration over the selected size range.