National Assessment of Near-Road Air Quality in 2016: Multi-Year Pollutant Trends and Estimation of Near-Road PM 2.5 Increment

Continuous monitoring of PM 2.5 , NO 2 , and other pollutants occurs at near-road air quality monitoring locations throughout the United States, as required by Environmental Protection Agency (EPA) regulations. Near-road monitoring sites were set up from 2014 through 2016, with 68 NO 2 monitors in 2016, 31 of which also monitored PM 2.5 . Annual and multi-year statistics of NO 2 and PM 2.5 concentrations were compared with the National Ambient Air Quality Standards. A PM 2.5 “increment”—the difference between concentrations measured in the near-road environment and those concentrations measured at nearby sites—was estimated. Several methods were used to select nearby sites from which to calculate “background” concentrations: (1) sites within 25 km, 50 km, and 100 km of the near-road site, and sites whose 24 h PM 2.5 concentrations were well correlated with those at the near-road site; (2) sites within 40 km of the near-road site, where PM 2.5 concentrations were interpolated using an inverse distance weighting; and (3) a site in the “upwind” direction from the near-road site. The mean PM 2.5 increment across all near-road sites ranged from +0.6 to +1.1 μg/m 3 (or 6–10% of the average PM 2.5 ), depending on the method. Understanding the variation in the PM 2.5 increment can be useful to assess calculations of PM 2.5 background, which is required by EPA under various regulatory frameworks. The PM 2.5 increment generally decreased with increasing distance from monitoring site to roadway, and increased with increasing traffic volume.