Disparities in ambient nitrogen dioxide pollution in the United States.

Average ambient concentrations of nitrogen dioxide (NO 2 ), an important air pollutant, have declined in the United States since the enactment of the Clean Air Act. Despite evidence that NO 2 disproportionately affects racial/ethnic minority groups, it remains unclear what drives the exposure disparities and how they have changed over time. Here, we provide evidence by integrating high-resolution (1 km × 1 km) ground-level NO 2 estimates, sociodemographic information, and source-specific emission intensity and location for 217,740 block groups across the contiguous United States from 2000 to 2016. We show that racial/ethnic minorities are disproportionately exposed to higher levels of NO 2 pollution compared with Whites across the United States and within major metropolitan areas. These inequities persisted over time and have worsened in many cases, despite a significant decrease in the national average NO 2 concentration over the 17-y study period. Overall, traffic contributes the largest fraction of NO 2 disparity. Contributions of other emission sources to exposure disparities vary by location. Our analyses offer insights into policies aimed at reducing air pollution exposure disparities among races/ethnicities and locations.