Characterizing the near-road NO_x gradient for exposure assessment purposes

Nationwide, in 2020, 24 % of the U.S. population lived near high volume roadways, a source of nitrogen oxides (NO_x) and nitrogen dioxide (NO₂) emissions. This study synthesizes literature regarding NO_x and NO₂ gradients in the near-road environment to understand community exposures. We synthesize literature reporting measured NO_x and NO₂ concentrations at increasingly large distances from a single roadway and variables impacting those concentrations.

Using the systematic literature search devised by U.S. EPA to develop Integrated Science Assessments, we identified publications that focused on near-road gradients of NO₂ and NO_x. The literature around near-road decay gradients were synthesized and summarized. Concentrations, distance to roadway, and other relevant information were extracted and analyzed using visual and regression methods to better understand the near-road decay gradient.

Concentrations of NO_x and NO₂ decrease with increasing distance from major roadways, following nonlinear decay patterns. Analyzing these data revealed a decreasing power decay trendline. NO_x and NO₂ concentrations typically reduced approximately to background levels by 500 m, with the steepest decline occurring within 50 m from roadways. Concentrations increased with cooler temperatures and higher traffic.

Gradients characterized by this review described how near-road populations are exposed to NO_x and the variables that may better describe community exposure. This review provides a quantitative research synthesis of variables most impactful on near-road gradient exposures to NO_x. Various meteorological and traffic variables were found to impact near-road gradients, though the gradient was driven by distance from the road.