title: Isotopic composition of atmospheric nitrate in a tropical marine boundary layer.
abstract: Long-term observations of the reactive chemical composition of the tropical marine boundary layer (MBL) are rare, despite its crucial role for the chemical stability of the atmosphere. Recent observations of reactive bromine species in the tropical MBL showed unexpectedly high levels that could potentially have an impact on the ozone budget. Uncertainties in the ozone budget are amplified by our poor understanding of the fate of NOx (= NO + NO2), particularly the importance of nighttime chemical NOx sinks. Here, we present year-round observations of the multiisotopic composition of atmospheric nitrate in the tropical MBL at the Cape Verde Atmospheric Observatory. We show that the observed oxygen isotope ratios of nitrate are compatible with nitrate formation chemistry, which includes the BrNO3 sink at a level of ca. 20 ± 10% of nitrate formation pathways. The results also suggest that the N2O5 pathway is a negligible NOx sink in this environment. Observations further indicate a possible link between the NO2/NOx ratio and the nitrogen isotopic content of nitrate in this low NOx environment, possibly reflecting the seasonal change in the photochemical equilibrium among NOx species. This study demonstrates the relevance of using the stable isotopes of oxygen and nitrogen of atmospheric nitrate in association with concentration measurements to identify and constrain chemical processes occurring in the MBL.
authors: Centre National de la Recherche Scientifique/ Université Joseph Fourier Grenoble, Laboratoire de Glaciologie et Géophysique de l'Environnement, 38402 St. Martin d'Hères, France.
authors: Joel
authors: J
authors: Savarino
authors: Samuel
authors: S
authors: Morin
authors: Joseph
authors: J
authors: Erbland
authors: Francis
authors: F
authors: Grannec
authors: Matthew D
authors: MD
authors: Patey
authors: William
authors: W
authors: Vicars
authors: Becky
authors: B
authors: Alexander
authors: Eric P
authors: EP
authors: Achterberg
mesh_terms: D001272:Atmosphere / Q000032:analysis*; D055726:Cabo Verde; D002852:Chromatography, Ion Exchange; D008956:Models, Chemical; D009566:Nitrates / Q000032:analysis*; D012621:Seasons; D014329:Tropical Climate
chemical_list: D009566:Nitrates
keywords: aerosols; bromine chemistry; monitoring; oxidation; tropic