| A comprehensive set of stratospheric balloon and aircraft samples was analyzed
for the position-dependent isotopic composition of nitrous oxide (N2O).
Results for a total of 220 samples from between 1987 and 2003 are presented,
nearly tripling the number of mass-spectrometric N2O isotope measurements
in the stratosphere published to date. Cryogenic balloon samples were obtained
at polar (Kiruna/Sweden, 68° N), mid-latitude (southern France,
44° N) and tropical sites (Hyderabad/India, 18° N). Aircraft
samples were collected with a newly-developed whole air sampler on board of the
high-altitude aircraft M55 Geophysica during the EUPLEX 2003 campaign. For
mixing ratios above 200 nmol mol−1, relative isotope enrichments
(δ values) and mixing ratios display a compact relationship,
which is nearly independent of latitude and season and which can be explained
equally well by Rayleigh fractionation or mixing. However, for mixing ratios
below 200 nmol mol−1 this compact relationship gives way to meridional,
seasonal and interannual variations. A comparison to a previously published
mid-latitude balloon profile even shows large zonal variations, justifying the
use of three-dimensional (3-D) models for further data interpretation.
In general, the magnitude of the apparent fractionation constants (i.e.,
apparent isotope effects) increases continuously with altitude and decreases
from the equator to the North Pole. Only the latter observation can be
understood qualitatively by the interplay between the time-scales of N2O
photochemistry and transport in a Rayleigh fractionation framework. Deviations
from Rayleigh fractionation behavior also occur where polar vortex air mixes
with nearly N2O-free upper stratospheric/mesospheric air (e.g., during the
boreal winters of 2003 and possibly 1992). Aircraft observations in the polar
vortex at mixing ratios below 200 nmol mol−1 deviate from isotope
variations expected for both Rayleigh fractionation and two-end-member mixing, but
could be explained by continuous weak mixing between intravortex and extravortex
air (Plumb et al., 2000). However, it appears that none of the simple approaches
described here can capture all features of the stratospheric N2O isotope
distribution, again justifying the use of 3-D models. Finally, correlations
between 18O/16O and average 15N/14N isotope ratios or
between the position-dependent 15N/14N isotope ratios show that
photo-oxidation makes a large contribution to the total N2O sink in the
lower stratosphere (possibly up to 100% for N2O mixing ratios above
300 nmol mol−1). Towards higher altitudes, the temperature dependence of
these isotope correlations becomes visible in the stratospheric observations. |