Decadal time series of tropospheric abundance of N_2O isotopomers and isotopologues in the Northern Hemisphere obtained by the long-term observation at Hateruma Island, Japan

摘要

Decadal time series and short-term temporal variations in mixing ratio of atmospheric nitrous oxide (N_2O) and abundance of its isotopomers (~(14)N~(15)N~(16)O and ~(15)N ~(14)N~(16)O) and isotopologue (~(14)N ~(14)N~(18)O) relative to 14N14N 16O have been observed for the first time in the Northern Hemisphere at Hateruma Island (HAT), Japan during 1999-2010 by monthly air sampling. Results show that the bulk nitrogen isotope ratio δ~(15)N ~(bulk) decreased at the rate of -0.023 ± 0.006‰ yr ~(-1), although the N_2O mixing ratio increased at the rate of about 0.7 nmol mol~(-1) yr~(-1) (ppb yr~(-1)) during the period. Isotope budget calculation with the δ~(15)N ~(bulk) trend supports the earlier estimates showing that the isotopically light sources such as agriculture and industry contribute to the increase of atmospheric N_2O. However, the rate of decrease of δ~(15)N~(bulk) is slightly smaller in magnitude than the rates obtained virtually for the 20th century from firn air in polar regions and surface air in the Southern Hemisphere (Tasmania and Antarctica), which suggests greater contribution of ~(15) N-enriched N_2O sources in recent years or in the extra-polar Northern Hemisphere. In contrast, the oxygen isotope ratio (δ~(18)O) and intramolecular ~(15)N site preference (SP, difference between isotope ratios at central and terminal nitrogen atoms) of N_2O showed no significant trends, contrary to previous reports. Results show that no significant seasonal variation exists in δ~(15)N~(bulk), δ~(18)O, and SP of N _2O at HAT in the past decade within the limits of our sampling frequency and analytical precision.