U.S. ambient air monitoring network has inadequate coverage under new PM2.5 standard

The Clean Air Act (CAA) in the United States relies heavily on regulatory monitoring networks, yet monitoring sites are sparsely located, especially among historically disadvantaged communities. For ambient fine particulate matter (PM2.5), we compare the air quality monitoring data with spatially complete concentrations derived from empirical models to quantify the gaps of existing U.S. monitoring networks in capturing concentration hotspots and exposure disparities. Recently, the U.S. Environmental Protection Agency adopted a more stringent annual-average air quality standard for PM2.5 (9 µg/m3). Here, we demonstrate that 44% of urban areas exceeding this new standard – encompassing ~ 20 million people – would remain undetected because of gaps in the current PM2.5 monitoring network. Crucially, we find that “uncaptured” hotspots, which contain 2.8 million people in census tracts that are misclassified as in attainment of the new PM2.5 standard, have substantially higher percentages of minority populations (i.e., people of color, disadvantaged communities, and low-income populations) compared to the overall US population. To address these gaps, we highlight 10 priority locations that could reduce the population in the uncaptured hotspots by 67%. Overall, our findings highlight the urgent need to address gaps in the existing monitoring network.