Figure 2. Average ${\mathrm{P}\mathrm{M}}_{2.5}$ concentration in 2000 and 2016 across low- and high-income ZCTAs:

We assign all ZCTAs percentile ranks from 1 to 100 based on median household income and categorize them into ten income groups. We designate the lowest and highest three income groups as low-income and high-income respectively. The maps in panel (a) show ${\mathrm{P}\mathrm{M}}_{2.5}$ levels for the year 2000 in low-income (left) and high-income (right) ZCTAs. Panel (b) shows the same information for the year 2016. Disparities in exposure to ${\mathrm{P}\mathrm{M}}_{2.5}$ among the two groups are apparent and it can be visually seen that in both 2000 and 2016, low-income ZCTAs are exposed to higher ${\mathrm{P}\mathrm{M}}_{2.5}$ concentrations as compared to high-income ZCTAs (video 4 in supplementary material).