Characterization of Ultrafine Particles and Other Vehicular Pollutants Near an International Bridge in United States – Mexico Border Region

Abstract

An intensive continuous hotspot study was conducted in the City of Laredo to characterize levels of ultrafine particles (UFP), black carbon (BC), nitrogen oxides (NOx), ozone (O3) and carbon monoxide (CO) from December 2007 to January 2008. Data were categorized into three different time periods: (1) Before Christmas, (2) During Christmas, and (3) After Christmas. The mean UFP number concentration throughout the study was 1.87 × 104 per cm3. Average traffic density was 1,099 per hour. The average concentrations of BC, CO, CO2, NOx, and O3 were 1.825 μg/m3, 0.05 parts per million (ppm), 499 ppm, 28 parts per billion (ppb), and 21.6 ppb, respectively. There was a significant correlation between both NOx and BC with UFP particle count, with R 2 = 0.63 and 0.60, respectively. Wind direction was found to have a significant effect on UFP and other pollutants. An exponential decay in UFP (20–290 nm) concentrations with increases in wind speed was observed. Significant correlation (R 2 = 0.99) was observed between the vehicle speed and the particle number emission factors. About a 1.5-fold increase in concentration of particle number emission factors was observed when the vehicle speed increased from 16 to 50 km/h.