Nanoparticle emissions on Minnesota highways

The objective of this project was to characterize on-road aerosol on highways surrounding the Minneapolis area. Data were collected under varying on-road traffic conditions and in residential areas to determine the impact of highway traffic on air quality. The study was focused on determining on-road nanoparticle concentrations, and estimating fuel-specific and particle emissions km^-1. On-road aerosol number concentrations ranged from 10⁴ to 10⁶ particles cm^-3. The highest nanoparticle concentrations were associated with high-speed traffic. At high vehicular speeds engine load, exhaust temperature, and exhaust flow all increase resulting in higher emissions. Less variation was observed in particle volume, a surrogate measure of particle mass. Most of the particles added by the on-road fleet were below 50nm in diameter. Particles in this size range may dominate particle number, but contribute little to particle volume or mass. Furthermore, particle number is strongly influenced by nucleation and coagulation, which have little or no effect on particle volume. Measurements made in heavy traffic, speeds<32kmh^-1, produced lower number concentrations and larger particles. Number concentrations measured in residential areas, 10-20m from the highway, were considerably lower than on-road concentrations, but the size distributions were similar to on-road aerosol with high concentrations of very small (<20nm) particles. Much lower number concentrations and larger particles were observed in residential areas located 500-700m from the highway. Estimated emissions of total particle number larger than 3nm ranged from 1.9 to 9.9×10¹⁴ particles km ^-1 and 2.2-11×10¹⁵ particles (kg fuel)^-1 for a gasoline-dominated vehicle fleet.