Characterizing the near-road NOx gradient for exposure assessment purposes

Background: Nationwide, in 2020, 24 % of the U.S. population lived near high volume roadways, a source ofnitrogen oxides (NOx) and nitrogen dioxide (NO2) emissions. This study synthesizes literature regarding NOx andNO2 gradients in the near-road environment to understand community exposures. We synthesize literature reporting measured NOx and NO2 concentrations at increasingly large distances from a single roadway and variables impacting those concentrations.  Methods: 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 NOx. 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.Results: Concentrations of NOx and NO2 decrease with increasing distance from major roadways, following nonlinear decay patterns. Analyzing these data revealed a decreasing power decay trendline. NOx 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. Conclusion: Gradients characterized by this review described how near-road populations are exposed to NOx 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 NOx. Various meteorological and traffic variables were found to impact near-road gradients, though the gradient was driven by distance from the road.

Impact/Purpose

This work represents exposure implications for NOx species in the ISA, and fills data gaps on how near-road exposures can disproportionately affect near-road communities. The objective of this work is to collate and systematically synthesize recent existing studies that characterize NO2 and NOx gradient decay with distance from roadways. Using published concentration measurements, we compare different measured oxides of nitrogen concentrations at multiple distances from roadways. This includes an investigation of the distance at which NO2 or NOx reach background levels, sources of oxides of nitrogen and factors that affect this gradient, and a summary of the general spatial gradient moving away from roadways to better estimate human exposure to oxides of nitrogen.

Citation

Levasseur, J., Q. Meng, K. Rappazzo, AND P. Byrley. Characterizing the near-road NOx gradient for exposure assessment purposes. KeAi Communications Co., Ltd., Beijing, CHINA, 3:128-137, (2026). [DOI: 10.1016/j.epm.2026.01.002]

DOI: Characterizing the near-road NOx gradient for exposure assessment purposes