Variation of number and mass concentration of particulate matter at the International Bridge of the Americas in El Paso, Texas

Abstract

Particle mass and number concentration measurements during different seasonal periods were conducted to study the effects caused by both privately own vehicles (POV) and commercial operated vehicles at the International Bridge of the Americas (BOTA), El Paso, Texas. There were two monitoring sites: UTEP air quality mobile monitoring site at the El Paso Water Utility (EPWU) located at the Chamizal National Memorial Park (CNMP) near the bridge. At this location measurements of traffic counts, size distribution, particle number and mass concentration PM₁₀ (EP₁₀; El Paso PM ₁₀) and PM_2.5 (EP_2.5; El Paso PM_2.5) were conducted. The second monitoring site was at the Mexican customs in Ciudad Juarez, Chihuahua where particle PM_2.5 (CJ _2.5; Ciudad Juarez PM_2.5) mass concentration measurements were conducted. Number concentration and size distribution measurements were conducted using a Scanning Mobility Particle Sizer (SMPS: measuring range of 5.94nm to 225nm) and the Aerodynamic Particle Sizer (APS: measuring range of 0.5 to 20 μm). The mass concentration measurements were obtained using the Tapered Element Oscillating Microbalance (TEOM), with different cyclones for PM₁₀ (EP₁₀) and two for PM_2.5 (EP _2.5 and CJ_2.5). Traffic counts were done manually at the site and also through video recordings from the Texas Department of Transportation (TXDOT) and the University of Texas at El Paso (UTEP).

Seasonal variations were observed for both particle number and mass concentrations at both sites. Winter and spring seasons had higher average total particle number concentrations than summer and fall seasons. In addition, diurnal variations were also observed during the morning and evening hours when traffic would elevate number and mass concentration. However, mass concentration was not very apparent as the number concentration peaked during the morning and evening hours. The particle number concentrations for the morning peaks were shown to be driven by smaller particle size distribution which contributed to a lesser mass concentration effects. In contrast, the particle number concentration for the evening peaks hours was shown to be driven by larger particle size distribution which contributed to the mass concentration effects. Furthermore, the average weekday and weekend demonstrated high ratios of number and mass concentrations for weekdays than weekends due to the closed commercial lanes on Sunday.