Influence of U.S. 95 Traffic on Outdoor Black Carbon Concentrations at 3 Near-road Schools in Las Vegas, NV

JENNIFER L. DEWINTER (1), Steven G. Brown (1), Paul T. Roberts (1), Michael C. McCarthy (1), David L. Vaughn (1)

(1) Sonoma Technology, Inc.

     Abstract Number: 305
     Last modified: May 5, 2010

Abstract
Due to the health risks associated with exposure to air pollution near heavily trafficked roadways, a monitoring campaign was conducted to characterize ambient concentrations of air pollutants surrounding the U.S. 95 freeway, in Las Vegas, Nevada, from June 2007 through September 2008. Aethalometer black carbon (BC), carbon monoxide (CO), and nitrogen oxide (NO$_X) were monitored at three near-roadway schools located between 20 m and 130 m from the U.S. 95 freeway, as well as at one urban background school more than 2 km from the freeway. Aethalometer BC, CO, and NO$_X measurements were examined for diurnal and weekday-weekend patterns. Weekday BC, CO, and NO$_X concentrations share a bimodal diurnal shape with morning and evening/overnight peaks, similar to the pattern routinely observed in urban environments. The influence of U.S. 95 traffic on ambient concentrations was assessed using downwind/upwind concentration ratios; downwind measurements at the near-road sites were compared to upwind measurements at the urban background site to estimate the downwind excess concentrations due to roadway emissions. The median ratio of the near-road/urban background concentration pairs varied from 1.4-2.2, suggesting that the freeway accounts for 27%-53% of the BC concentrations at the downwind sites. The concentration ratios were evaluated by time of day and by month. Median ratios were elevated during the middle of the day; however, this was not the result of increased emissions. Higher wind speeds in the middle of the day contributed to greater dispersion and therefore lower concentrations at the background site. A statistical analysis was conducted to assess the contribution of different parameters associated with high concentrations near the road, including wind speed, wind direction, traffic volume, speed, and vehicle size. Air pollutant concentrations associated with vehicle emissions are higher near the roadway; distance from the roadway, wind speed/wind direction, and changes in emissions were important drivers.