National Assessment of Near-Road Air Quality in 2016: Multi-Year Pollutant Trends and Estimation of Near-Road PM2.5 Increment

Continuous monitoring of PM2.5, NO2, 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 NO2 monitors in 2016,31 of which also monitored PM2.5. Annual and multi-year statistics of NO2 and PM2.5 concentrations were compared with the National Ambient Air Quality Standards. A PM2.5 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 PM2.5 concentrations were well correlated with those at the near-road site; (2) sites within 40 km of the near-road site, where PM2.5 concentrations were interpolated using an inverse distance weighting; and (3) a site in the upwind direction from the near-road site. The mean PM2.5 increment across all near-road sites ranged from +0.6 to +1.1 mu g/m(3) (or 6-10% of the average PM2.5), depending on the method. Understanding the variation in the PM2.5 increment can be useful to assess calculations of PM2.5 background, which is required by EPA under various regulatory frameworks. The PM2.5 increment generally decreased with increasing distance from monitoring site to roadway, and increased with increasing traffic volume.