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 (NO₂) 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 NO₂ and particulate matter with an aerodynamic diameter of 2.5 μm 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 NO₂ and PM_2.5, respectively. Analyses of the hourly near-road NO₂, nitric oxide (NO), and PM_2.5 increments showed distinct diurnal cycles; the NO₂ increment peaks at ∼9 ppb during the early afternoon (2–4 pm local time) while the NO and PM_2.5 increments peak during the morning rush hour (5–8 am 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 NO₂ 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.