Fugitive dust emissions play an important role in urban air quality. Much research on fugitive dust's effects has focused on human health and societal impacts, with limited work investigating effects on other species. The endangered Apodemia mormo langei butterfly is endemic to the Antioch Dunes, a small area on the south bank of the San Joaquin River in northern California, largely protected as a National Wildlife Refuge. Between the two protected portions of the dunes is a gypsum processing facility. Deposition of gypsum dust may adversely affect endangered insects, especially in their vulnerable larval life stage. Persistent westerly winds blow from the western section of the refuge, across the industrial facility, to the eastern protected dune area. Ambient particulate matter (PM) was collected at 30 sites in both sections of the refuge using passive samplers deployed at times matching the butterfly life cycle. The prevailing wind maintained upwind-downwind sampling orientation throughout the study. PM samples were analyzed for total mass, and elemental composition via X-ray fluorescence. Downwind concentrations of gypsum-related elements were between 4 (strontium) and 12 (sulfur) times higher than upwind loadings, suggesting deposition of PM from the gypsum facility. The effect of fugitive emissions was strongest at the industrial facility's fenceline, closest to a conveyor belt that loads gypsum. Combined with documented reductions in insect larval longevity when exposed to gypsum dust, the results suggest that gypsum deposition may be affecting the ecosystem and endangered species in the downwind unit of the Antioch Dunes National Wildlife Refuge.Implications: Fugitive dust has impacts not only on humans, but on other organisms. The Antioch Dunes National Wildlife Refuge (ADNWR) in California, set aside to protect the endangered Apodemia mormo langei butterfly, consists of two land units separated by a gypsum processing facility in between them. In this study, we demonstrate fugitive gypsum dust deposition on the downwind unit of the ADNWR, which may impact the endangered butterfly and its ecosystem.