Roadside ultrafine and nanoparticle number distributions in northern Central Valley, California and relationships to meteorology and traffic

Health studies indicate that ultrafine particles may be detrimental to human health and the assessment of health impacts as well as potentially controlling future ambient concentrations can effectively occur only if sources and ambient concentrations of ultrafine and nanoparticles are understood. This dissertation presents roadside ultrafine and nanoparticle number distributions and single-diameter particle number concentrations in order to characterize roadside ultrafine particle number concentrations and to determine the relationships between the particle number concentrations and meteorological and vehicular parameters. Specifically, this study focused on roadways in the suburban-agricultural interface found in the central valley of northern California. Roadside particle number distributions (6 nm < Dp < 237 nm) were simultaneously measured upwind and downwind of the freeway at two sample sites in a pilot study (November 11, 12 of 2000; July 9, 10, 11 of 2001) and full study (July 9, 10, 12, 15, 17, 19, 22, 24 of 2002). Daily average downwind particle number distributions had a single peak with a modal diameter between 10 nm and 30 nm, and the greatest proportion of particles was in the nanometer size range (Dp < 50 nm). Upwind modes were more variable and generally of larger diameter than downwind modes. Average downwind total particle number concentrations (TPN) ranged from 1.7 x 104 to 7.1 x 104 particles/cm3 while average upwind TPN were from 4.4 x 103 to 8.6 x 103 particles/cm3 . This study was the first to measure real-time size resolved particle number concentrations using second by second measurements over periods of hours. Relationships between integrated particle numbers (6 nm < Dp < 237 nm) and single diameter particle number concentrations and vehicle volumes and meteorological parameters are described. Overall, meteorological conditions were shown to be extremely important in determining particle number concentrations. In addition, the results indicate that for the relatively low volume roads measured in this study, heavy-duty vehicle volumes and meteorology may be more important than total vehicle volumes for predicting roadside particle number concentrations.