Open AccessIsotopic evidence for source changes of nitrate in rain at Bermuda
Author(s) -
Hastings Meredith Galanter,
Sigman Daniel M.,
Lipschultz Fred
Publication year - 2003
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2003jd003789
Subject(s) - nitrate , nitrogen , seasonality , environmental science , environmental chemistry , chemistry , ecology , biology , organic chemistry
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 15 N/ 14 N and lower 18 O/ 16 O ratios are observed in the warm season (April–September) in comparison to the cool season (October–March): The mean δ 15 N of nitrate for the warm and cool seasons is −2.1‰ and −5.9‰ (versus air N 2 ), respectively, while the mean δ 18 O is 68.6‰ and 76.9‰ (versus Vienna Standard Mean Ocean Water). The few cool season rain events that had high 15 N/ 14 N and low 18 O/ 16 O 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 15 N/ 14 N and 18 O/ 16 O of the nitrate. The source region provides precursor nitrogen oxides (NO x ), influencing the 15 N/ 14 N of nitrate, and contributes to the chemistry that produces nitrate from NO x , which determines the 18 O/ 16 O of nitrate. While the range in nitrate 15 N/ 14 N observed during the cool season is consistent with anthropogenic emissions from North America, the higher warm season 15 N/ 14 N 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 18 O/ 16 O of nitrate in rain at Bermuda is high throughout the year (δ 18 O = 60.3 to 86.5‰) as a result of interactions of precursor NO x with ozone, which has a high 18 O/ 16 O ratio. The lower nitrate 18 O/ 16 O 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 NO x during the cool season varies spatially over as large a range as is observed between the warm and cool seasons.