Long-term continuous measurement of near-road air pollution in Las Vegas: seasonal variability in traffic emissions impact on local air quality

Excess air pollution along roadways is an issue of public health concern to Federal, State, and local government environmental agencies and the public. This concern was the motivation for a long-term study to measure levels of air pollutants at various distances from a roadway in Las Vegas, Nevada. This study represents a joint effort between the US Environmental Protection Agency and the US Department of Transportation’s Federal Highway Administration. Measurements of air pollutants—including carbon monoxide (CO), oxides of nitrogen (NO, NO2, NOX), and black carbon (BC)—were conducted concurrently at four distances from a major interstate (206,000 vehicles per day) for an entire year. With prevailing winds from the west, concentrations of all measured species at 20 m from the highway were significantly higher (non-overlapping 95% confidence intervals) than levels 300 m from the road. In addition, CO, NOX, and BC measured at 100 m from the road on the prevailing downwind side of the road were significantly higher than 100 m on the opposite side of the road. The disproportionate impact of the roadway emissions on the eastern side of the highway points to the importance of local meteorology in determining the extent of near-road impact. When isolating only time periods with winds from due west (±60°), CO, NO2, NOX, and BC levels at 20 m east of the highway were 60%, 46%, 122%, and 127% higher, respectively, than the concurrent measurements at the upwind site. Monthly average traffic volume and frequency of downwind conditions are not enough to explain the trends in monthly average excess CO at 20 m east of the road; average wind speed appears to be an important explanatory factor. The year-long extensive dataset afforded some unique data mining analyses—the maximum near-road impact (top 10% of 20 m east site minus 300 m east site) is associated with winds from the southwest to northwest, higher traffic volumes, and low wind speeds; meanwhile, the apparent maximum spatial extent in near-road impact (top 10% of 300 m east site minus to 100 m west site) occurred during evening to presunrise periods in the winter under conditions of low speed winds from due west, with moderate to low traffic volumes. This research confirms that excess air pollution associated with proximity to roads is significant over a year-long time frame and that local meteorology is a critical factor determining the extent of near-road impact.