Comprehensive isotopic composition of atmospheric nitrate in the Atlantic Ocean boundary layer from 65°S to 79°N

Abstract

The comprehensive isotopic composition of atmospheric nitrate (i.e., the simultaneous measurement of all its stable isotope ratios: 15N/14N, 17O/16O and 18O/16O) has been determined for aerosol samples collected in the marine boundary layer (MBL) over the Atlantic Ocean from 65°S (Weddell Sea) to 79°N (Svalbard), along a ship‐borne latitudinal transect. In nonpolar areas, the δ15N 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 δ15N(NO3−) of different NOx sources and nitrate removal processes associated with its atmospheric transport are discussed. Measurements of the oxygen isotope anomaly (Δ17O = δ17O − 0.52 × δ18O) of nitrate suggest that nocturnal processes involving the nitrate radical play a major role in terms of NOx sinks. Different Δ17O between aerosol size fractions indicate different proportions between nitrate formation pathways as a function of the size and composition of the particles. Extremely low δ15N 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.