Wavelength dependence of isotope fractionation in N<sub>2</sub>O photolysis

Abstract

Abstract. In previous reports on isotopic fractionation in the ultraviolet photolysis of nitrous oxide (N 2 O) only enrichments of heavy isotopes in the remaining N 2 O fraction have been found. However, most direct photolysis experiments have been performed at wavelengths far from the absorption maximum at 182 nm. Here we present high-precision measurements of the 15 N and 18 O fractionation constants (ε) in photolysis at 185 nm. Small, but statistically robust depletions of heavy isotopes for the terminal atoms in the linear N 2 O molecule are found. This means that the absorption cross sections σ ( 15 N 14 N 16 O) and σ ( 14 N 2 18 O) are larger than σ ( 14 N 2 16 O) at this specific wavelength. In contrast, the central N atom becomes enriched in 15 N. The corresponding fractionation constants (±1 standard deviation) areTo our knowledge, this is the first documented case of such a heavy isotope depletion in the photolysis of N 2 O which supports theoretical models and pioneering vacuum ultraviolet spectroscopic measurements of 15 N substituted N 2 O species that predict fluctuations of ε around zero in this spectral region (Selwyn and Johnston, 1981). Such a variability in isotopic fractionation could have consequences for atmospheric models of N 2 O isotopes since actinic flux varies also strongly over narrow wavelength regions between 175 and 200 nm due to the Schumann-Runge bands of oxygen. However, the spacing between maxima and minima of the fractionation constants and of the actinic flux differ by two orders of magnitude in the wavelength domain. The wavelength dependence of fractionation constants in N 2 O photolysis can thus be approximated by a linear fit with negligible consequences on the actual value of the spectrally averaged fractionation constant. In order to establish this linear fit,Correspondence to: J. Kaiser (kaiser@princeton.edu) additional measurements at wavelengths other than 185 nm were made using broadband light sources, namely D 2 , Hg/Xe and Sb lamps. The latter lamp was used in conjunction with various interference filters to shift the peak photolysis rate to longer wavelengths. From these experiments and existing data in the literature, a comprehensive picture of the wavelength dependence of N 2 O photolysis near room-temperature is created.

The isotopic composition of methane in the stratosphere: high-altitude balloon sample measurements

“…Due to slightly different reaction rate coefficients for the different isotopologues, chemical removal by these reactions leads to a shift in the isotope ratio of the residual material. Neglecting other effects, this can be described by the wellknown Rayleigh fractionation equation (Kaiser et al, 2002b).…”

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

“…Analytical advances in the late 1990s then allowed the position-dependent measurement of the nitrogen isotope distribution between the terminal and central nitrogen atoms in N 2 O (Brenninkmeijer and Röckmann, 1999;Esler et al, 2000;Toyoda and Yoshida, 1999). The new techniques were immediately adopted for extensive laboratory measurements of kinetic isotope effects during photolysis (summarized in Kaiser et al, 2003b andvon Hessberg et al, 2004) and the reaction of N 2 O with O( 1 D) (Kaiser et al, 2002a;Toyoda et al, 2004). Our present understanding of these isotope effects can be considered to be very good.…”

Ultra-violet absorption cross sections of isotopically substituted nitrous oxide species: <sup>14</sup>N<sup>14</sup>NO, <sup>15</sup>N<sup>14</sup>NO, <sup>14</sup>N<sup>15</sup>NO and <sup>15</sup>N<sup>15</sup>NO

“…As the wavelength region 200 to 210 nm is the most important in terms of stratospheric photolysis of nitrous oxide (Yung and Miller, 1997), the data from Selwyn and Johnston are insufficient. Since the red shoulder of the N 2 O absorption spectrum above about 188 nm has very little vibrational structure (Yoshino et al, 1997), broadband photolysis and subsequent isotopic analysis appears to be a reasonable way of characterizing the magnitude of isotopic fractionation (Kaiser et al, 2002b(Kaiser et al, , 2003cRöckmann et al, 2001). Kaiser et al (2003c) used the collective results from broadband photolysis and "single wavelength" photolysis experiments to derive a linear fit of fractionation constant vs. wavelength for the range 190 to 220 nm.…”

“…A flurry of field studies Rahn and Wahlen, 1997;Röckmann et al, 2001;Toyoda et al, 2001b;Yoshida and Toyoda, 2000) and laboratory experiments (Kaiser et al, 2002b(Kaiser et al, , 2003cRahn et al, 1998;Stanford Research Systems, PS 325 Lock-in amplifier Princeton Applied Research , 5209 (settings: 12 dB/ Stanford Research Systems, SR850 octave filters and a time constant of 300 ms; the full (employed settings: time constant: 300 ms, scale sensitivity used was in the range: 10-100 mV, 18 dB/octave filters, shunt resistor 1.5 M , the shunt resistor: 1 M ) sensitivity: 0.5-50 mV)…”