Isotopes of atmospheric nitrate from the GEOTRACES Atlantic and Pacific Sections

Meredith Galanter Hastings, Brown University, Department of Earth, Environmental and Planetary Sciences, and Institute at Brown for Environment and Society, Providence, RI, United States, Therese Carter, Brown University, Department of Chemistry, Providence, RI, United States and Karen L Casciotti, Stanford University, Earth System Science, Stanford, United States
Abstract:
As part of the GEOTRACES North Atlantic and Pacific Peru-Tahiti Sections, the isotopic composition of aerosol nitrate was quantified. From an atmospheric perspective, the isotopic composition of nitrate offers potential to trace the sources and chemistry that contribute to its formation. The nitrogen isotopic composition (δ15N) of oceanic nitrate is a “core parameter” of each GEOTRACES cruise and can provide constraints on major processes such as N fixation, denitrification, and lateral transport. Constraint of atmospheric input is important as it can influence the interpretation of isotopic results. In both the North Atlantic and South Pacific, atmospheric nitrate is a source of low δ15N and very high δ18O. In the North Atlantic, δ15N of atmospheric nitrate varies from -10.0 to +1.1 ‰ vs. air-N2 (median -2.5‰), while in the Pacific δ15N varies from -13.1 to -3.2 ‰ (median -5.7‰). This source of δ15N is much lower than typically found in surface waters in either basin. The oxygen isotopic composition reflects oxidation processes in the atmosphere and because of the influence of ozone on the formation of nitrate, atmospheric nitrate contains uniquely high δ18O and Δ17O (Δ17O = δ17O – 0.52 δ18O) values compared to other sources of oceanic nitrate. In the North Atlantic, the δ18O ranges from 75.6 to 85.6‰ vs. VSMOW, and the Δ17O varies from 27.5 to 32.7‰. In the Pacific, δ18O varies from 68.5 to 79.3‰ and Δ17O from 23.5 to 28.4‰. The lower oxygen isotopic composition in the South Pacific likely reflects a combination of halogen chemistry, which can extract O atoms from ozone in the marine boundary layer, and hydroxyl radical chemistry that is more prominent in the tropical latitudes than the mid-latitudes of the North Atlantic cruise. Comparisons of the isotopic composition of bulk aerosols compared to size segregated aerosols shows similar results. Sampling of different parts of the filters used to the collect the aerosol samples also shows very consistent isotopic results, often much more consistent than concentration of nitrate.
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