Pb-210 and Be-7 as Coupled Flux and Source Tracers for Aerosols in the Pacific Ocean

Deposition of aerosols to the surface ocean is an important factor affecting primary production in the surface ocean. However, the sources and fluxes of aerosols and associated trace elements remain poorly defined. Aerosol Pb-210, Po-210, and Be-7 data were collected on US GEOTRACES cruise GP15 (Pacific Meridional Transect, 152 degrees W; 2018). Pb-210 fluxes are low close to the Alaskan margin, increase to a maximum at similar to 43 degrees N, then decrease to lower values. There is good agreement between Pb-210 fluxes and long-term land-based fluxes during the SEAREX program (1970-1980s), as well as between GP15 and GP16 (East Pacific Zonal Transect, 12 degrees S; 2013) at adjacent stations. A normalized fraction f(Be-7, Pb-210) is used to discern aerosols with upper (high f) versus lower (low f) troposphere sources. Alaskan/North Pacific aerosols show significant continental influence while equatorial/South Pacific aerosols are supplied to the marine boundary layer from the upper troposphere. Lithogenic trace elements Al and Ti show inverse correlations with f(Be-7, Pb-210), supporting a continental boundary layer provenance while anthropogenic Pb shows no clear relationship with f(Be-7, Pb-210). All but four samples have Po-210/Pb-210 activity ratios <0.2 suggesting short aerosol residence time. Among the four samples (Po-210/Pb-210 = 0.42-0.88), two suggest an upper troposphere source and longer aerosol residence time while the remaining two cannot be explained by long aerosol residence time nor a significant component of dust. We hypothesize that enrichments of Po-210 in them are linked to Po enrichments in the sea surface microlayer, possibly through Po speciation as a dissolved organic or dimethyl polonide species.