Disentangling Natural and Anthropogenic Sources of Dust Deposition to a Montane Ecosystem at San Jacinto Peak, Southern California

Abstract The composition and flux of mineral dust are largely driven by the entrainment and transport of sediment from both natural and human sources, resulting in varying ecological impacts at the deposition site. To investigate the influence of natural and human sources of dust in montane environments, we measured dust composition and deposition rate on San Jacinto Peak in Southern California from 2019 to 2022 at six sites spanning 2,462 m in elevation and 20 km in distance. We find unique interannual variations in fine dust (0.2–30 μm) flux and chemical composition between sites. The greatest average dust flux occurs during July–November (0.14–3.50 g m −2 y −1 ), followed by March–July (0.24–4.07 g m −2 y −1 ) and is lowest during November–March (0.29–2.76 g m −2 y −1 ). Wildfires led to significant increases in dust flux, with the highest dust flux occurring at the lowest elevation site following the 2020 Snow Creek fire. Greater enrichment of metals and depletion of rare earth elements at higher relative to lower elevations indicate spatial and temporal variability in dust sources, consistent with variations in natural and anthropogenic inputs. Positive matrix factorization (PMF) indicates that high elevation sites on average receive a higher proportion of anthropogenic dust input (64%–75%), whereas low‐elevation sites receive a higher proportion of alluvium and local rock inputs (35%–63%), particularly on the north side of the mountain. This study highlights the complexity of interannual dust deposition in mountain environments and the modulation of dust flux and composition by anthropogenic activity and wildfire.