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) 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 and particulate matter with an aerodynamic diameter of 2.5 μm and less (PM) 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 for NO and PM, respectively. Analyses of the hourly near-road NO, nitric oxide (NO), and PM increments showed distinct diurnal cycles; the NO increment peaks at ∼9 ppb during the early afternoon (2-4 pm local time) while the NO and PM increments peak during the morning rush hour (5-8 am local time) at 25 ppb and 1.8 μg m for NO and PM, respectively. Although long-term trends are not yet available for this network of sites, a similar analysis of the NO and PM 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.