Dry deposition is a significant input of nutrients and pollutants to ecosystems, especially in arid and semi-arid regions like the Colorado Front Range. As dust activity increases across the southwestern United States due to land use change and increasing aridity, the contribution of dry deposition to total atmospheric deposition will likely increase as well. Although the importance of dry deposition has been well established for the Rocky Mountains, the spatial and temporal variability in dry deposition fluxes is poorly understood, especially at lower elevations more proximal to anthropogenic dust sources. We collected monthly bulk deposition samples from nine sites across an elevation gradient in the Colorado Front Range from November 2017 to November 2018. We analyzed the particulate and liquid fractions for a suite of 27 elements and major ions, respectively. To relate the chemistry results to dust source locations, we calculated air parcel back trajectories for each of our collection intervals using the Stochastic Time-Inverted Lagrangian Transport (STILT) model and High Resolution Rapid Refresh reanalysis data (HRRR). Every 3 hours during the duration of the collection period, 200 air parcels were initiated from 2 m above the ground surface and tracked back for 24 hours. We assessed locations where air parcels passed within a half meter of the ground surface, assuming that those could be dust source locations.

Our results reveal that lower elevations received more particulate deposition than higher elevations, with all elevations experiencing a springtime peak. Across the elevation transect, particulate deposition was enriched in P, S, Cu, Zn, Pb, Li, Mo, and Cd relative to the composition of the upper continental crust. All of the major ions measured in the liquid fraction were higher than National Atmospheric Deposition Program (NADP) averages for the same sites, pointing toward the importance of soluble dry deposition. For K, Mg, and P, the particulate fractions accounted for more than half of the total elemental fluxes, suggesting that dry deposition is important for these elements, but that it falls in a relatively insoluble form. Seasonal and elevational differences in the composition of the particulate fraction corresponded to different back-trajectory footprints, with potential spring and summer dust source locations more localized than during fall and winter. Our results point toward the importance of dry deposition in contributing to total atmospheric deposition and highlight the need for monitoring over multiple spatial and temporal scales.