Evidence of Source Changes for Nitrate in Precipitation at Bermuda

Nitric acid (HNO₃), a significant contributor to acid rain, is an important species in marine and atmospheric chemistry and the major sink for reactive atmospheric nitrogen oxides (NO_x = NO + NO₂). HNO₃ deposition at Bermuda in the winter and spring is primarily impacted by anthropogenic emissions of NO_x transported from North America. In the summer and fall, "clean" air masses are transported to the island primarily from south and southeast of the island. A time series of the nitrogen and oxygen isotopic composition of nitrate in Bermuda rain from January to November 2000 (n=63), shows distinct seasonal variations. The volume-weighted average of δ¹⁵N-NO₃^- for winter and spring samples is -5.8 ‰ (vs. air), while for summer and fall samples it is -1.3 ‰ . The volume-weighted average results for δ¹⁸O-NO₃^- are 77.4 ‰ (vs. VSMOW) for winter and spring and 67.7 ‰ for summer and fall. It has been shown that the volume-weighted average NO₃^- concentration does not vary significantly between the transport seasons at Bermuda [e.g. Moody and Galloway, Tellus, 1986]. This result is surprising since air masses primarily of marine origin offer no major anthropogenic sources of NO_x in comparison to transport from the East Coast of the United States. Analysis of simulations with a global chemical transport model (Geophysical Fluid Dynamics Laboratory GCTM) show fossil fuel combustion as the main source of nitrate deposition at Bermuda, except during the summer months when production of NO_x from lightning increases. The more enriched average value for δ¹⁵N-NO₃^- during summer and fall may be indicative of this contribution from lightning NO_x emissions to nitrate deposition. The δ¹⁸O for all NO₃^- samples is highly enriched relative to, for instance, water and atmospheric oxygen. These high values result from the interaction of NO_x and O₃ in the atmosphere [δ¹⁸O-O₃ = 85 to 120 ‰ (vs. VSMOW) based on Johnston et al., JGR, 1997] prior to formation and deposition of NO₃^-, a conclusion supported by the observation of mass independent behavior of δ¹⁷O in atmospheric nitrate [Savarino et al., Eos Trans. AGU, Spring 2002; Galanter et al., Eos Trans. AGU, 81(48), F191, 2000; Michalski and Thiemens, Eos Trans. AGU, 81 (48), F120, 2000]. The δ¹⁸O of deposited NO₃^- is sensitive to the pathway of production. The lower δ¹⁸O for all NO₃^- values during the summer and fall are consistent with production of HNO₃ via the NO₂+OH reaction, while the higher winter and spring values may reflect an increase in deposition of NO₃^- via the N₂O₅ pathway.