Diurnal variations in ambient fine and ultrafine particle concentrations near a major highway in El Paso, Texas

Particle number concentrations were measured near a major highway to study the diurnal variations of fine and ultrafine particle number concentrations. The monitoring site is located near the University of Texas at El Paso main campus and is near I-10 highway. The Aerodynamic Particle Sizer (APS) (measurement size range 0.5--20 mum) and the Scanning Mobility Particle Sizer (SMPS) (measurement size range 0.005--0.25 mum) were used to measure particle number concentrations and size distributions in ambient air in a pseudo-continuous manner from March 29 to April 15, 2007. Diurnal variations were observed for mass concentrations as well as for particle number concentrations. Both particle number and mass concentrations peak during the afternoon hours. However, during the morning hours, particle number concentrations show a more significant peak as compared to mass concentrations. Additionally, size distributions show that the particle number concentration morning peak is driven by the smaller particles that contribute minimum mass and thus have a minimal effect on mass concentration. The size distributions also show that the particle number concentration afternoon peak is driven by larger particles, which contribute the most mass and this have a meaningful effect on the mass concentration.