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Abstract
Nitrous oxide (N2O) is an important trace gas in the atmosphere. It is an active greenhouse
gas in the troposphere and it also controls ozone concentration in the stratosphere
through nitric oxide production. One way to trace the geochemical cycle of N2O is
by measuring the natural abundance of stable isotopes, namely 15N and 18O (refs 2-15).
Here we report the intramolecular distribution of 15N within the linear NNO molecule,
determined by measuring molecular and fragment ions of N2O on a modified mass spectrometer.
This revealed a preference for 15N at the central N position, or alpha-site, within
N2O isotopomers (isotope-containing molecules). Moreover, this preference varied significantly
throughout the atmosphere. In the troposphere, low alpha-site preference indicates
local emission of N2O from soils and fossil-fuel combustion, each with distinct isotopomer
signatures, which then mixes with background N2O. In the stratosphere, on the other
hand, loss of N2O is observed as enhanced alpha-site preference for 15N, due to fractionation
during ultraviolet photolysis of N2O. We have constructed an atmospheric mass balance
of N2O, incorporating isotopomer abundance, which shows that the intramolecular distribution
of 15N is a parameter that has the potential to increase significantly the resolution
with which sources and sinks of N2O can be identified and quantified in the atmosphere.