The Expanded Regulatory Significance of Saharan Dust Plumes in the United States

Given the recent reduction in the U.S. National Ambient Air Quality Standard (NAAQS) for annual PM_2.5 from 12 to 9 μg m^-3, the contribution of exceptional, though natural, particulate transport events has assumed greater regulatory relevance. This study examines the evolution of a June 2022 trans-Atlantic Saharan dust outbreak and contextualizes its impact on the 2023 PM_2.5 design values (DVs) for continental U.S. air quality monitors. The results demonstrate that the intrusion of Saharan dust yielded mean 24 h PM_2.5 values > 35 μg m^-3 for multiple days across the Central U.S. and single-handedly elevated the 2022 mean annual PM_2.5 by > 0.5 μg m^-3 for some Gulf Coast communities. Moreover, the Saharan dust outbreak led to ∼0.1 μg m^-3 increases in the 2023 PM_2.5 DVs for approximately 20 monitoring sites in Texas and Louisiana, and in the case of Port Allen, La., and at least two other monitors, the Saharan dust pushed the DV above the 9 μg m^-3 NAAQS. As this June 2022 episode illustrates, seemingly rare atmospheric phenomena are positioned to play disproportionately large regulatory roles, given the tighter air policy landscape.