Isotopic evidence for source changes of nitrate in rain at Bermuda

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Abstract
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 15N/14N and lower 18O/16O ratios are observed in the warm season (April–September) in comparison to the cool season (October–March): The mean δ15N of nitrate for the warm and cool seasons is −2.1‰ and −5.9‰ (versus air N2), respectively, while the mean δ18O is 68.6‰ and 76.9‰ (versus Vienna Standard Mean Ocean Water). The few cool season rain events that had high 15N/14N and low 18O/16O 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 15N/14N and 18O/16O of the nitrate. The source region provides precursor nitrogen oxides (NOx), influencing the 15N/14N of nitrate, and contributes to the chemistry that produces nitrate from NOx, which determines the 18O/16O of nitrate. While the range in nitrate 15N/14N observed during the cool season is consistent with anthropogenic emissions from North America, the higher warm season 15N/14N 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 18O/16O of nitrate in rain at Bermuda is high throughout the year (δ18O = 60.3 to 86.5‰) as a result of interactions of precursor NOx with ozone, which has a high 18O/16O ratio. The lower nitrate 18O/16O 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.