Characterizing the near-road NOx 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 (NO 2 ) emissions. This study synthesizes literature regarding NO x and NO 2 gradients in the near-road environment to understand community exposures. We synthesize literature reporting measured NO x and NO 2 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 NO 2 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 NO 2 decrease with increasing distance from major roadways, following nonlinear decay patterns. Analyzing these data revealed a decreasing power decay trendline. NO x and NO 2 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 nitrogen oxides 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 nitrogen oxides. Various meteorological and traffic variables were found to impact near-road gradients, though the gradient was driven by distance from the road. • Synthesized recent studies characterizing NO 2 and NO x near-road concentration decay • Created a generalized near-road concentration gradient for NO 2 and NO x • Highlighted data gaps and variables that may impact near-road gradient of NO 2 and NO x