Roadside Particle Number Distributions and Relationships between Number Concentrations, Meteorology, and Traffic along a Northern California Freeway: Fid-Bau Portal

Roadside Particle Number Distributions and Relationships between Number Concentrations, Meteorology, and Traffic along a Northern California Freeway

Nanzetta, M. Katherine / Holmén, Britt A.

Abstract

Particle number distributions were bimodal, with a primary within 10 m of the traveled lanes. mode at ∼10–25 nm, indicating that newly formed particles were sampled. Total downwind 6–237 nm particle number concentrations (N _tot) ranged from 9.3 × 10³ to 2.5 × 10⁵ cm⁻³, with higher daily average concentrations in November compared with July. N_tot correlated with wind speed, temperature, and relative humidity. Upwind photochemically initiated nucleation likely led to elevated background nanoparticle concentrations in July, as evidenced by increasing upwind distribution modal diameter with increasing temperature and a strong correlation between upwind N _tot and solar radiation. Also in summer, N _tot showed stronger correlation with Particle number distributions were measured simultaneously upwind and downwind of a suburban-agricultural freeway to determine relationships with traffic and meteorological parameters. Average traffic volumes were 6330 vehicles/hr with 10% heavy-duty vehicles, and volumes were higher in July than November. Most downwind par heavy-duty vehicle volumes than wind speed, temperature, and relative humidity. These results indicate the importance of measuring background particle size distributions simultaneously with roadside distributions. There may be a minimum vehicle volume from which useful real-world vehicle particle number distributions can be measured at roadside, even when collecting samples within 10 m of the traveled lanes.