"Roadside particle number distributions and relationships between number concentrations, meteorology and traffic". This project is based on experimental data gathered in Northern California beside a major interstate. On-site measurements of number distributions, traffic volumes, and meteorological data were made. Since the roadside environment is a complex one, the correlations between variables were examined to assess the importance of each variable on measured concentrations and distributions. Current regulations are based on particle mass and the majority of particulate matter studies to date are mass-based. This research is important because current health studies show that high particle numbers, not necessarily high total mass, may be detrimental to human health. Vehicles are known to produce large numbers of ultrafine and nanoparticles, however their fate once emitted on the roadway is not well characterized. Developing an understanding of how vehicle volumes translate to roadside particle numbers will require an understanding of how all of the roadside parameters influence particle numbers.