Isotopic composition of atmospheric nitrate in a tropical marine boundary layer

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 NO ₓ (= NO + NO ₂), particularly the importance of nighttime chemical NO ₓ 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 BrNO ₃ sink at a level of ca. 20 ± 10% of nitrate formation pathways. The results also suggest that the N ₂O ₅ pathway is a negligible NO ₓ sink in this environment. Observations further indicate a possible link between the NO ₂/NO ₓ ratio and the nitrogen isotopic content of nitrate in this low NO ₓ environment, possibly reflecting the seasonal change in the photochemical equilibrium among NO ₓ 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.