Characterizing Nitrogen Oxides and Fine Particulate Matter near Major Highways in the United States Using the National Near-Road Monitoring Network

As part of the United States Environmental Protection Agency’s 2010 Nitrogen Dioxide (NO(2)) National Ambient Air Quality Standards (NAAQS) review, a national network of near-road sites was established to characterize pollutant behavior, interaction, and dispersion in the ambient near-road environment. Using spatial interpolation to estimate the near-road concentration increments of NO(2) and particulate matter with an aerodynamic diameter of 2.5 micrometers and less (PM(2.5)) relative to nearby non-near-road monitors, we found that the 2013–2018 national average increment is 6.9 ppb and 1.0 μg m(−3) for NO(2) and PM(2.5), respectively. Analyses of the hourly near-road NO(2,) nitric oxide (NO), and PM(2.5) increments showed distinct diurnal cycles; the NO(2) increment peaks at ~9 ppb during the early afternoon (2–4pm local time) while the NO and PM(2.5) increments peak during the morning rush hour (5–8am local time) at 25 ppb and 1.8 μg m(−3) for NO and PM(2.5), respectively. Although long term trends are not yet available for this network of sites, a similar analysis of the NO(2) and PM(2.5) increment at a quasi-near-road site outside of the official network in Elizabeth, NJ showed gradual decreases in the increment over time since the mid-2000s.