Roadside nanoparticles corresponding to vehicle emissions during one signal cycle

Abstract

The behavior of nanoparticles (NPs) in the roadside atmosphere has not been clarified because it involves unstable volatile components. It was thought that the number concentration (NC) and NP size distribution change due to variations in traffic conditions (e.g., traffic volume [TV], velocity, acceleration, etc.) near the intersection, but the SMPS (Scanning Mobility Particle Sizer) lacks the temporal resolution required for rapid, transient measurements. Using a fast-response aerosol spectrometer capable of providing near-instantaneous particle NC measurements in real time, the behavior of NPs during one signal cycle became clear, and it was understood that the effect of condensation/evaporation processes is important, in addition to coagulation. As for the relation of the NC in proportion to the TV, this did not show a constant line but rather a hysteresis curve during the signal cycle, because the gas-particle equilibrium state at the roadside atmosphere was variable. Using two points of simultaneous measurement and on-board measurement, the behavior of NPs could be confirmed in response to the characteristics of automotive exhaust, which varied due to the on-road driving state, engine conditions, vehicle position, or traffic light timing, at the intersection. The on-board measurement of NP size distribution in the exhaust plume from a diesel vehicle was carried out as a reference for direct particle emissions, compared with the roadside NPs. The coagulation/deposition model simulation using the direct particle emissions underestimated the NCs compared with the observed values. The gas-particle equilibrium model could explain the underestimated portion caused by the condensation of ambient VOC (Volatile Organic Compounds) onto the particles. If this hypothesis is correct, the condensable VOC amount in the roadside atmosphere is suggested to be very large.