Isotopic evidence for source changes of nitrate in rain at Bermuda

Rainwater collected on the island of Bermuda between January 2000 and January 2001 shows pronounced seasonal variation in the nitrogen and oxygen isotopic composition of nitrate. Higher N-15/N-14 and lower O-18/O-16 ratios are observed in the warm season (April-September) in comparison to the cool season (October-March): The mean delta(15)N of nitrate for the warm and cool seasons is -2.1parts per thousand and -parts per thousand5.9% (versus air N-2), respectively, while the mean delta18O is 68.6% and 76.9% (versus Vienna Standard Mean Ocean Water). The few cool season rain events that had high N-15/N-14 and low O-18/O-16 exhibited trajectory paths originating from the south, similar to those of warm season samples. Accordingly, the region from which air is transported to the island determines the N-15/N-14 and O-18/O-16 of the nitrate. The source region provides precursor nitrogen oxides (NOx), influencing the N-15/N-14 of nitrate, and contributes to the chemistry that produces nitrate from NOx, which determines the O-18/O-16 of nitrate. While the range in nitrate N-15/N-14 observed during the cool season is consistent with anthropogenic emissions from North America, the higher warm season N-15/N-14 suggests that lightning is a significant source of nitrate to Bermuda. The isotopic evidence for a significant southern source of nitrate to Bermuda helps to explain the previous observation of unexpectedly high nitrate concentrations in warm season rain. The O-18/O-16 of nitrate in rain at Bermuda is high throughout the year (delta(18)O=60.3 to 86.5%) as a result of interactions of precursor NOx with ozone, which has a high O-18/O-16 ratio. The lower nitrate O-18/O-16 in the warm season and in cool season air masses from the south is consistent with elevated concentrations of hydroxyl radical (OH), which dilutes the isotopic signal of ozone. Our limited data set suggests that the relative importance of the OH sink for NOx during the cool season varies spatially over as large a range as is observed between the warm and cool seasons.