Trace elements in the North Atlantic troposphere: Shipboard results of precipitation and aerosols

Authors

  • T. M. Church,

  • A. Véron,

  • C. C. Patterson,

  • D. Settle,

  • Y. Erel,

  • H. R. Maring,

  • A. R. Flegal


Abstract

Samples for trace metals in precipitation (rain, snow, and fog) and total aerosols were collected at sea as part of the GCE/CASE/WATOX expedition during August and September 1988 in the greater North Atlantic between Bermuda, Iceland, the Azores, and Barbados. The samples were collected using ultra-clean equipment and methods for the analysis of the trace elements Al, Fe, Mn, Zn, Cu, Ni, Cd, and Pb (including stable isotopes). The samples were analyzed by graphite furnace atomic absorption spectrometry and thermal ionization mass spectrometry. Based on stable lead isotopes and retrospective isentropic air mass analyses, four air mass sources were evidenced including (1) temperate U.S. and (2) boreal Canadian westerlies, and (3) boreal and northern Europe easterlies associated with (4) Mediterranean and Saharan easterlies. Lead isotopic signatures clearly distinguished emission source regions over the North Atlantic with 206Pb/207Pb ratios of 1.19–1.22 for North American westerlies, 1.10–1.12 for European boreal easterlies, and 1.15–1.16 for Saharan trade easterlies. Also, the contrasting ratios between precipitation and aerosol demonstrate a vertical mixing of chemical sources in precipitation from contrasting air masses of different origin. The air mass trajectories were independently calculated and are in general agreement with the lead isotopic tracers. The isotopic tracers allowed more confident conclusions on air mass sources, particularly after a few days of oceanic transport. Also, the trajectories appeared to be very useful in explaining the strong variability of the trace metal concentrations during restricted periods of sampling time aboard a moving ship. To a lesser extent, the atmospheric emission sources were characterized by trace element concentrations. The occurrence of large dust loadings in the Saharan trade easterlies contributed abundant crustal elements and were also associated with enriched anthropogenic elements from southern Europe. Scavenging ratios were calculated for lead and cadmium during select periods when aerosol and precipitation were collected under the same meteorological regimes. The cadmium ratio is much greater than that of lead, indicating that cadmium and lead are each transported or scavenged on distinct populations of aerosols. For lead, the ratio is less in the North Atlantic than in the Pacific, suggesting an aerosol concentration dependence. Like the Pacific, the ratios increase for the lead, dust, and sea salt. One may thus conclude that marine scavenging ratios cannot yet be used effectively until factors such as concentration dependency and local conditions of tropospheric processing are better defined.

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