| Summary: | The comprehensive isotopic composition of atmospheric nitrate (i.e., the simultaneous measurement of all its stable isotope ratios: N-15/N-14, O-17/O-16 and O-18/O-16) has been determined for aerosol samples collected in the marine boundary layer (MBL) over the Atlantic Ocean from 65 degrees S (Weddell Sea) to 79 degrees N (Svalbard), along a ship-borne latitudinal transect. In nonpolar areas, the delta N-15 of nitrate mostly deriving from anthropogenically emitted NOx is found to be significantly different (from 0 to 6%) from nitrate sampled in locations influenced by natural NOx sources (-4 +/- 2)%. The effects on delta N-15(NO3-) of different NOx sources and nitrate removal processes associated with its atmospheric transport are discussed. Measurements of the oxygen isotope anomaly (Delta O-17 = delta O-17 - 0.52 x delta O-18) of nitrate suggest that nocturnal processes involving the nitrate radical play a major role in terms of NOx sinks. Different Delta O-17 between aerosol size fractions indicate different proportions between nitrate formation pathways as a function of the size and composition of the particles. Extremely low delta N-15 values (down to -40%) are found in air masses exposed to snow-covered areas, showing that snowpack emissions of NOx from upwind regions can have a significant impact on the local surface budget of reactive nitrogen, in conjunction with interactions with active halogen chemistry. The implications of the results are discussed in light of the potential use of the stable isotopic composition of nitrate to infer atmospherically relevant information from nitrate preserved in ice cores.
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