资源环境科技发展态势分析平台

Atmospheric nitrous oxide (N2O) levels have been continuously growing since preindustrial times. Mitigation requires information about sources and sinks on the regional and global scales. Isotopic composition of N2O in the atmosphere could contribute valuable constraints. However, isotopic records of N2O in the unpolluted atmosphere remain too scarce for large-scale N2O models. Here, we report the results of discrete air samples collected weekly to biweekly over a 5-year period at the high-altitude research station Jungfraujoch, located in central Switzerland. High-precision N2O isotopic measurements were made using a recently developed preconcentration and laser spectroscopy technique. The measurements of discrete samples were accompanied by in situ continuous measurements of N2O mixing ratios. Our results indicate a pronounced seasonal pattern with minimum N2O mixing ratios in late summer, associated with a maximum in delta N-15(bulk) and a minimum in intramolecular 15N site preference (delta N-15(SP)). This pattern is most likely due to stratosphere-troposphere exchange (S 1E), which delivers N2O-depleted but 15N-enriched air from the strato-sphere into the troposphere. Variability in delta(15)N(SP)induced by changes in STE may be masked by biogeochemical N2O production processes in late summer, which are possibly dominated by a low-delta N-15(SP) pathway of N2O production (denitrification), providing an explanation for the observed seasonality of delta N-15(SP). Footprint analyses and atmospheric transport simulations of N2O for Jungfraujoch suggest that regional emissions from the planetary boundary layer contribute to seasonal variations of atmospheric N2O isotopic composition at Jungfraujoch, albeit more clearly for delta N-15(SP) and delta O-18 than for delta N-15(bulk) With the time series of 5 years, we obtained a significant interannual trend for delta(15)N(bulk )after deseasonalization ( -0.052 +/- 0.012 parts per thousand a(-1)), indicating that the atmospheric N2O increase is due to isotopically depleted N2O sources. We estimated the average isotopic signature of anthropogenic N2O sources with a two-box model to be -8.6 +/- 0.6 parts per thousand for delta N-15(bulk) 34.8 +/- 3 parts per thousand for delta O-18 and 10.7 +/- 4 parts per thousand for delta N-15(SP). Our study demonstrates that seasonal variation of N2O isotopic composition in the background atmosphere is important when determining interannual trends. More frequent, highprecision and interlaboratory-compatible measurements of atmospheric N2O isotopocules, especially for delta N-15(SP), are needed to better constrain anthropogenic N2O sources and thus the contribution of biogeochemical processes to N2O growth on the global scale.