Probing stratospheric transport and chemistry with new balloon and aircraft observations of the meridional and vertical N<sub>2</sub>O isotope distribution

Abstract

Abstract.A comprehensive set of stratospheric balloon and aircraft samples was analyzed for the position-dependent isotopic composition of nitrous oxide (N 2 O). Results for a total of 220 samples from between 1987 and 2003 are presented, nearly tripling the number of mass-spectrometric N 2 O 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 N 2 O photochemistry and transport in a Rayleigh fractionation framework. Deviations from Rayleigh fractionation behavior also occur where polar vortex air mixes with nearly N 2 O-free upper stratospheric/mesospheric air (e.g., during the boreal winters of 2003 and possibly 1992). Aircraft observations Correspondence to: J. Kaiser (J. Kaiser@uea.ac.uk) 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 N 2 O isotope distribution, again justifying the use of 3-D models. Finally, correlations between 18 O/ 16 O and average 15 N/ 14 N isotope ratios or between the position-dependent 15 N/ 14 N isotope ratios show that photo-oxidation makes a large contribution to the total N 2 O sink in the lower stratosphere (possibly up to 100% for N 2 O mixing ratios above 300 nmol mol −1 ). Towards higher altitudes, the temperature dependence of these isotope correlations becomes visible in the stratospheric observations.

Stable carbon isotope fractionation in the UV photolysis of CFC-11 and CFC-12

“…On the other hand, laboratory fractionations obtained with this same lamp are considered to be in good agreement with stratospherically observed fractionations, excepting the impact of dynamic processes, which reduce the magnitude of the fractionation by about half (Röckmann et al, 2001;Kaiser et al, 2006;Laube et al, 2010a). Ascertaining the scale of wavelength dependence is beyond the scope of this study and is a viable point of examination for future experiments.…”

“…However, these are likely to be less due to the effect of atmospheric mixing, which causes apparent (observed) fractionation in the atmosphere ε app to be approximately half of the intrinsic (laboratory measured) fractionation. Unphotolyzed CFC is continuously brought into the stratosphere and mixed with CFC that has been photolyzed previously, changing the isotope ratio of the whole and suppressing the inherent fractionation (Röckmann et al, 2001;Kaiser et al, 2006;Laube et al, 2010a).…”

“…In the case of N 2 O, laboratory experiments, field measurements and modelling studies have shown strong heavy isotope enrichments in the stratosphere, which have an important effect on the isotopic composition in the troposphere (e.g. Rahn and Wahlen, 1997;Röckmann et al, 2001;Kaiser et al, 2003 and2006;Blake et al, 2003;von Hessberg et al, 2004;Morgan et al, 2004). For CFC-12, recent measurements of the stable chlorine isotope ratio ( 37 Cl/ 35 Cl) of stratospheric CFC-12 have shown strong 37 Cl enrichments.…”

Constraining N<sub>2</sub>O emissions since 1940 using firn air isotope measurements in both hemispheres

“…In this study, the δ anth values are the averaged values over the whole investigated period. ε L values used in this study are averaged values from the lower stratosphere from Kaiser et al (2006) and ε L values from Park et al (2012) were used from Park et al (2004). and the remainder returns from the stratosphere to the troposphere (F exch ).…”

“…Here, we used the average ε app of all lowermost stratospheric measurements from Kaiser et al (2006) (Table 3). Note that slightly different fractionations ε app have been used in previous studies by Rahn and Wahlen (2000), Röckmann et al (2001), and Park et al (2012Park et al ( , 2004.…”