On the electrical charge of motor vehicle exhaust particles

The electrical characteristics of particles emitted in motor vehicle exhaust are examined via single and tandem differential mobility analysis (DMA). The measurements reveal that large fractions of the particles, approximately 60–80% are electrically charged, but with nearly equal numbers of positively and negatively charged particles, which leaves the exhaust aerosol essentially electrically neutral. The tandem DMA approach provides a means to sort the particles according to both their size and charge level. It reveals exhaust particles with up to ±4±4 units of electrical charge. As the charge increases, the mean particle size increases, but the width of the distribution and particle concentration decrease. Plotted as a function of charge, at fixed size, the data follow a Boltzmann charge distribution with temperatures in the range of 800–1100 K. The experimental measurements are also examined in terms of a charged particle coagulation model. The model, based solely on the initial condition of a monodisperse aerosol with small equal fractions of ++ and -- particles, predicts size and charge distributions in semiquantitative agreement with experiment, and helps explain why the Boltzmann form is maintained after ionization ceases. Nucleation mode particles that form during dilution and cooling of the exhaust remain neutral.