Assessment of the Near-road (Monitoring) Network including comparison with nearby monitors within U.S. cities

  • March 2020
  • Peer-Reviewed Articles
  • Multiple

Lal, R.M., Ramaswami, A., & Russell, A. (2020). Assessment of the Near-road (Monitoring) Network including comparison with nearby monitors. Environmental Research Letters (accepted manuscript), doi: 10.1088/1748-9326/ab8156.

ABSTRACT: Emissions from mobile sources have historically been an important anthropogenic contributor to ambient air pollution leading to high levels of air pollution near major roadways. The US EPA recently implemented the Near-Road (monitoring) Network to measure NO2 concentrations by high-traffic roadways in urban centers throughout the US, as these locations were believed to characterize worst-case human exposures to traffic-related pollutants. Many near-road sites also include PM2.5 and CO measurements, which along with the NO2 observations, were compared in a pairwise manner against non-near-road monitors located within the city-scale boundary. After controlling for primary emissions from the target highways, we found no statistical difference (α = 0.05) in PM2.5 concentrations between the near-road and non-near-road urban sites (δ = 0.42 (-0.08-0.90) µg m-3, n=35 comparisons). NO2 and CO levels, on average were significantly higher at the near-road sites compared to the non-near-road urban sites by 5.0 (3.4-6.5) ppb (n=44 comparisons) and 9.2×10-2 (0.04-0.14) ppm (n=42 comparisons), respectively. The average PM2.5 difference found here is 5%, and at 14 of the 35 (~40%) urban monitor comparisons and 28 of the 72 (~39%) overall comparisons, PM2.5 is actually higher at the non-near-road site relative to its near-road pair. Cleaner vehicle fleets, formation of secondary PM from on-road emissions occurring downwind (i.e., away from the road), decreased SOA formation rates in the near-road environment, the prevalence of other low-volume vehicular and local, non-vehicular sources of emissions at the non-near-road sites (e.g., railyards, truck yards, ports, biomass-fueled heating, backyard barbecuing, and commercial cooking, etc.) and local meteorology (e.g. wind speed and wind direction) explain this finding. The same observational data was used to assess mobile source emission estimates from the EPA National Emission Inventory, and analysis of the observations are in rough agreement with the current ratio of NOx to CO from on-road mobile sources.

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