Bulk Aerosol Trace Element Concentrations and Deposition Fluxes During the U.S. GEOTRACES GP15 Pacific Meridional Transect

Atmospheric deposition of aerosols transported from the continents is an important source of nutrient and pollutant trace elements (TEs) to the surface ocean. During the U.S. GEOTRACES GP15 Pacific Meridional Transect between Alaska and Tahiti (September–November 2018), aerosol samples were collected over the North Pacific and equatorial Pacific and analyzed for a suite of TEs, including Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Pb. Sampling coincided with the annual minimum in dust transport from Asia, providing an opportunity to quantify aerosol TE concentrations and deposition during the low dust season. Nevertheless, peak concentrations of “crustal” TEs measured at ∼40–50°N (∼145 pmol/m3 Fe) were associated with transport from northern Asia, with lower concentrations (36 ± 14 pmol/m3 Fe) over the equatorial Pacific. Relative to crustal abundances, equatorial Pacific aerosols typically had higher TE enrichment factors than North Pacific aerosols. In contrast, aerosol V was more enriched over the North Pacific, presumably due to greater supply to this region from oil combustion products. Bulk deposition velocity (Vbulk) was calculated along the transect using the surface ocean decay inventory of the naturally occurring radionuclide, 7Be, and aerosol 7Be activity. Deposition velocities were significantly higher (4,570 ± 1,146 m/d) within the Intertropical Convergence Zone than elsewhere (1,764 ± 261 m/d) due to aerosol scavenging by intense rainfall. Daily deposition fluxes to the central Pacific during the low dust season were calculated using Vbulk and aerosol TE concentration data, with Fe fluxes ranging from 19 to 258 nmol/m2/d. Plain Language Summary Both natural material such as soil dust and industrial emissions such as soot can be transported thousands of miles in the atmosphere as small particles before gradually settling out of the atmosphere or being stripped out by rain. This process can be an important mechanism for delivering material from the continents to surface waters of the open ocean and also introduces elements that are essential for algal growth, but present in the ocean in very low concentrations (known as trace elements). In this study, we measured the concentrations of several TEs on airborne particles during fieldwork in the Pacific Ocean, between Alaska and Tahiti. The observed amounts of TEs were low because the timing of the work coincided with the annual minimum in atmospheric transport of dust from Asia. The deposition rate