Diurnal and Seasonal Variations of Traffic-related PM Pollution at an International Border Crossing

Hector A. Olvera (1), Veronica Guerrero (1), Mario Lopez (1), Wen-Whai Li (1)

Department of Civil Engineering, The University of Texas at El Paso, El Paso, TX 79968

     Abstract Number: 258
     Last modified: November 9, 2009

Abstract
The international Ports Of Entry (POEs) between El Paso, Texas and Cd. Juarez, Chihuahua are significant emission sources of hazardous air pollutants that pose serious health threat to nearby residents, custom inspectors, and bridge users. Among the POEs between the U.S. and Mexico, the Bridge of the Americas (BOTA) in El Paso has the largest combined traffic of private and commercial vehicles. The pollution problem at the BOTA has become more pronounced in recent years due to the increased volume of traffic and prolonged wait time for the U.S. custom border inspection, resulting in large numbers of vehicles idling in queues that may last hours.

This poster presents temporal and seasonal variations of PM mass and number concentrations observed at the BOTA with possible correlations to traffic and meteorological conditions. Fourteen days of continuous PM measurements were conducted each season for 1 year (2008 - 2009). PM10 and PM2.5 mass concentrations were measured at 2 locations by the BOTA using Thermo TEOMs and particle number concentrations (from 5 nm to 0.02 mm) were monitored every ten-minute at one location using a pair of TSI particle sizers (SMPS and APS). Concurrent traffic information was recorded with a digital video recorder while meteorological information was recorded at a nearby weather station. Approximately 2,000 heavy-duty diesel trucks and 25,000 privately own vehicles crossed the BOTA every day during the course of the study. Temporal traffic characteristics showed distinct patterns between commercial and private traffic creating several different emission scenarios. Commercial trucks were abundant northbound during the morning and early afternoon hours (6:00 - 13:00 hrs) and southbound at noon and during the afternoon hours (16:00 - 19:00 hrs). Passenger vehicles were abundant northbound most of the day and southbound during the afternoon hours (16:00 - 19:00 hrs).

PM mass and number levels had different seasonal patterns. In general, PM mass (either PM10 or PM2.5) concentration was high in spring and low in fall whereas the total number concentration (TNC) was high in winter and low in summer. PM10 mass concentration had a positive correlation with wind speed whereas TNC showed a negative correlation with the same parameter. Daily variations of PM10 and PM2.5 at the BOTA were distinguishably different from the diurnal pattern typically observed at other regional air monitoring stations regardless of the season. TNC, however, had similar daily variations among all four seasons, elevated throughout the day and reached the peak in the afternoon. During the winter TNC averaged at approximately 100,000 particles per cubic centimeter between 17:00 and 18:00 hrs that coincided with the southbound diesel-traffic peak hours. During the entire study, ultrafine particles (PM0.1) accounted for approximately 95% of the TNC. Short-term TNC reached as high as 1,000,000 particles per cubic centimeter when the monitor was occasionally engulfed in truck exhausts. Overall, TNC was dominated by particles of sizes ranging from 20 and 40 nm. Associations among the PM mass and number concentrations, traffic counts, meteorology, time of the day, and day of the week are currently being analyzed.