Beschreibung:
<jats:p>Abstract. 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
δ15Nbulk and a minimum in intramolecular 15N site
preference (δ15NSP). This pattern is most likely due to
stratosphere–troposphere exchange (STE), which delivers N2O-depleted
but 15N-enriched air from the stratosphere into the troposphere.
Variability in δ15NSP induced by changes in STE may be
masked by biogeochemical N2O production processes in late summer, which
are possibly dominated by a low-δ15NSP pathway of N2O
production (denitrification), providing an explanation for the observed
seasonality of δ15NSP. 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 δ15NSP and δ18O than
for δ15Nbulk. With the time series of 5 years, we
obtained a significant interannual trend for δ15Nbulk
after deseasonalization (-0.052±0.012 ‰ 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 ‰ for δ15Nbulk, 34.8±3 ‰ for δ18O and 10.7±4 ‰ for δ15NSP. Our study demonstrates
that seasonal variation of N2O isotopic composition in the background
atmosphere is important when determining interannual trends. More frequent,
high-precision and interlaboratory-compatible measurements of atmospheric
N2O isotopocules, especially for δ15NSP, are needed
to better constrain anthropogenic N2O sources and thus the
contribution of biogeochemical processes to N2O growth on the global
scale.
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