Location-specific strategies for eliminating US national racial-ethnic [Formula: see text] exposure inequality

Abstract

Air pollution levels in the United States have decreased dramatically over the past decades, yet national racial-ethnic exposure disparities persist. For ambient fine particulate matter ([Formula: see text]), we investigate three emission-reduction approaches and compare their optimal ability to address two goals: 1) reduce the overall population average exposure ("overall average") and 2) reduce the difference in the average exposure for the most exposed racial-ethnic group versus for the overall population ("national inequalities"). We show that national inequalities in exposure can be eliminated with minor emission reductions (optimal: ~1% of total emissions) if they target specific locations. In contrast, achieving that outcome using existing regulatory strategies would require eliminating essentially all emissions (if targeting specific economic sectors) or is not possible (if requiring urban regions to meet concentration standards). Lastly, we do not find a trade-off between the two goals (i.e., reducing overall average and reducing national inequalities); rather, the approach that does the best for reducing national inequalities (i.e., location-specific strategies) also does as well as or better than the other two approaches (i.e., sector-specific and meeting concentration standards) for reducing overall averages. Overall, our findings suggest that incorporating location-specific emissions reductions into the US air quality regulatory framework 1) is crucial for eliminating long-standing national average exposure disparities by race-ethnicity and 2) can benefit overall average exposures as much as or more than the sector-specific and concentration-standards approaches.

Keywords: air pollution; air quality regulatory; environmental justice; fine particulate matter.

Fig. 1.

${\text{PM}}_{2.5}$ exposure-disparity and concentration-reduction curves. (A) Concentration disparity between the most exposed racial-ethnic group (Asian, Black, Hispanic, White, or Other) and the population average (y axis) versus cumulative emission reduction (x axis). (B) Population average concentration versus cumulative emission reduction. (C) Concentration disparity versus population averaged concentration (i.e., the y axis values from A and B). For each panel, current conditions are the left side (i.e., “do nothing” at x = 0), and a complete (100%) emission reduction is the right side (i.e., achieving zero emissions: lower right, at x = 30.4 MT/y in A and B, at x $=$ 7.0 µg/m³ in C). Each panel compares three approaches to emission reduction: location (green line), sector (blue line), and NAAQS-like (i.e., employing a concentration standard; here, 6 µ${\text{g/m}}^3$; orange line). An “equal reduction” approach, where all emissions are reduced proportionately, would be a straight line (black dotted line). The location approach (green line) can eliminate national disparities with modest total emission reductions, whereas with the other two approaches, national disparities remain, even after substantial emission reductions (A and C). The location approach also does as well as or better than the other two approaches, for population average concentration (B and C).

Fig. 2.

Emission reductions for the three approaches: by location (i.e., corresponding to the green lines, Fig. 1) (A), by sector (corresponding to blue lines, Fig. 1) (B), and NAAQS-like (orange lines, Fig. 1) (C). A displays national results (Left) and zoomed-in results for 10 large areas (Right). Spatial units displayed in C are CBSAs, the geographic unit for NAAQS evaluation. The three approaches offer fundamentally different ways of formulating and prioritizing emission reductions. Ag., agriculture; Const., construction; Elec., electricity; HD, heavy duty; LD, light duty; Misc., miscellaneous; Res., residential; Veh., vehicle.