AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Quantification of Ultrafine Particles with Electric Charges in On- and Near-freeway Environments
EON LEE, Bin Xu, Yifang Zhu, University of California, Los Angeles
Abstract Number: 89 Working Group: Urban Aerosols
Abstract Previous studies have reported the presence of ions and charged particles in motor vehicle exhaust. However studies characterizing electric charges on particles on and near traffic emissions are limited. This study presents the fraction of charged particles measured on and near two major freeways in Los Angeles. A tandem Differential Mobility Analyzer (DMA) system was used to evaluate the fraction of ultrafine particles (UFPs) carrying different number of charges across a broad range of electrical mobility diameters (30, 50, 80, and 100 nm). We first attempted to characterize the fraction of charged particles on two distinctively different freeways: a gasoline-vehicle dominated freeway (I-405) and a diesel traffic dominated freeway (I-710 i.e., up to 20 % diesel traffic). We found that the fraction of charged particles was higher on I-710 than on I-405, and the fractions on both freeways were significantly higher than the background. We also found that the background UFPs carried only up to two charges but traffic-induced particles could carry up to three charges across the tested electrical mobility diameters. Downwind from the I-405, we found the decay rates of charged particles were generally slower than that of total cluster ions but faster than that of particle number concentration. Particles with higher charges had the faster decay; that is, the fastest decay for triply charged particles and the slowest decay for singly charged particles. Further investigation on bipolar charged particle distributions showed strong net positive charges on nucleation mode particles. In comparison with Boltzmann steady-state charge equilibrium theory, we found that charged particles (especially, those in nucleation mode) might not necessarily reach the charge equilibrium distributions in the near-freeway environments.