Laboratory study of nitrate photolysis in Antarctic snow. II. Isotopic effects and wavelength dependence.

Atmospheric nitrate is preserved in Antarctic snow firn and ice. However, at low snow accumulation sites, post-depositional processes induced by sunlight obscure its interpretation. The goal of these studies (see also Paper I by Meusinger et al. ["Laboratory study of nitrate photolysis in Antarctic snow. I. Observed quantum yield, domain of photolysis, and secondary chemistry," J. Chem. Phys. 140, 244305 (2014)]) is to characterize nitrate photochemistry and improve the interpretation of the nitrate ice core record. Naturally occurring stable isotopes in nitrate ((15)N, (17)O, and (18)O) provide additional information concerning post-depositional processes. Here, we present results from studies of the wavelength-dependent isotope effects from photolysis of nitrate in a matrix of natural snow. Snow from Dome C, Antarctica was irradiated in selected wavelength regions using a Xe UV lamp and filters. The irradiated snow was sampled and analyzed for nitrate concentration and isotopic composition (δ(15)N, δ(18)O, and Δ(17)O). From these measurements an average photolytic isotopic fractionation of (15)ɛ = (-15 ± 1.2)‰ was found for broadband Xe lamp photolysis. These results are due in part to excitation of the intense absorption band of nitrate around 200 nm in addition to the weaker band centered at 305 nm followed by photodissociation. An experiment with a filter blocking wavelengths shorter than 320 nm, approximating the actinic flux spectrum at Dome C, yielded a photolytic isotopic fractionation of (15)ɛ = (-47.9 ± 6.8)‰, in good agreement with fractionations determined by previous studies for the East Antarctic Plateau which range from -40 to -74.3‰. We describe a new semi-empirical zero point energy shift model used to derive the absorption cross sections of (14)NO3 (-) and (15)NO3 (-) in snow at a chosen temperature. The nitrogen isotopic fractionations obtained by applying this model under the experimental temperature as well as considering the shift in width and center well reproduced the values obtained in the laboratory study. These cross sections can be used in isotopic models to reproduce the stable isotopic composition of nitrate found in Antarctic snow profiles.

Comparison of the Huggins band for six ozone isotopologues: vibrational levels and absorption cross section.

By use of the 3(1)A' ab initio potential energy surface (PES) of ozone and the multi-configuration time-dependent Hartree program for wavepacket propagation, we have determined numerous eigenstates of this state for six ozone isotopologues. These bound vibrational levels are the upper levels of the Huggins band, which covers the range from 27,000 to ~33,000 cm(-1). This study extends our previous work on the Hartley band, which was limited to the range ~32,000-50,000 cm(-1). Four isotopologues, (16)O(3), (16)O(17)O(16)O, (16)O(18)O(16)O, and (18)O(3) (noted hereafter 666, 676, 686, and 888), are symmetric, and two are asymmetric, (17)O(16)O(2) and (18)O(16)O(2) (noted hereafter 667 and 668). The PES of the 3(1)A' state has two equivalent minima of C(s) symmetry located at ~27,000 cm(-1) above the X(1)A(1) ground state. The equilibrium geometry of these two minima is r(e(1)) = 2.28 a(0), r(e(2)) = 3.2 a(0), and θ(e) = 107°. The dissociation limit of this PES, which correlates to the O((1)D) + O(2) ((1)Δ) "singlet" channel, is about 4300 cm(-1) above the two minima. For the (16)O(3) isotopologue, the 120 lowest bound eigenstates have been calculated and partially assigned up to 800 cm(-1) below the dissociation limit. The 60 lower eigenstates are easily assignable in term of three normal modes, the "long" bond (ν(1)), the bending (ν(2)), and the "short" bond (ν(3)). A new family of wave functions, aligned along the dissociation channels, appears at 3782 cm(-1) above the 3(1)A' (0,0,0) level. The 3(1)A' vibrational levels and the corresponding intensity factors from the (000), (010), (100), and (001) levels of the X(1)A(1) ground state have been calculated for the six isotopologues. The Huggins absorption cross sections of the six isotopologues have been calculated from the 3(1)A' vibrational energy levels and the corresponding intensity factors. The rotational envelope of each vibronic band has been empirically described by an ad hoc function. The ratio of the Huggins cross section of each ozone isotopologue with one of (16)O(3) provides the fractionation factor of each ozone isotopologue as a function of the photon energy. These various fractionation factors will allow predicting enrichments due to photolysis by various light sources like the actinic flux.

Ozone photodissociation: isotopic and electronic branching ratios for symmetric and asymmetric isotopologues.

We present new calculations of the branching ratios between the various electronic and isotopic photodissociation channels of ozone. Special emphasis is placed on the isotopic/isotopologue differences because the contribution of the ozone photodissociation to the oxygen isotope and ozone isotopologue enrichments or fractionations is important for atmospheric applications. These branching ratios, which depend on photon energy, have been calculated with a full quantum mechanical wavepacket propagation approach: the multiconfiguration time-dependent Hartree (MCTDH) method. Five ozone isotopologues are considered: three symmetric, (16)O(3) (noted 666), (16)O(17)O(16)O (676), and (16)O(18)O(16)O (686); two asymmetric, (16)O(2)(17)O (noted 667) and (16)O(2)(18)O (668). The 668 and 667 asymmetric isotopologues can dissociate into either 66 + 8 or 68 + 6 for 668 and into 66 + 7 or 67 + 6 for 667. In the ranges of the Chappuis and Hartley bands, the dissociation is very fast and electronic and isotopic branching ratios are obtained from the wavepacket fluxes through complex absorbing potentials (CAPs) located perpendicular to the dissociation channels of the potential energy surfaces (PESs) of the A (1)B(1) (Chappuis) and B 3(1)A' (Hartley/Huggins) electronic states. In the range of the Huggins band the dissociation is much slower and the isotopic branching ratios of 667 and 668 asymmetric isotopologues, (e.g; 668 → 66 + 8 or 86 + 6) are obtained from the ratios of two partial absorption cross sections corresponding to the selective excitation of one or the other of the two isomers of C(s) symmetry, which dissociate respectively into 66 + 8 and 86 + 6. We find that the photodissociation of the 668 asymmetric isotopologue favors the 68 + 6 channel with a propensity varying between 52% (Hartley) and 54% (Huggins) as a function of the photon energy. The electronic branching ratios to the singlet channel (O(3) + hυ → O((1)D) + O(2)((1)Δ)) are all close to 90% above ≈32,000 cm(-1). Below this energy, the singlet channel is energetically closed and only the triplet channel (O(3) + hυ → O((3)P) + O(2)((3)Σ)) is open. These branching ratios are required to calculate the photolysis rates of each ozone isotopologue, which in turn contribute to the atomic oxygen and the ozone isotopic enrichments in the atmosphere.

Absorption cross section of ozone isotopologues calculated with the multiconfiguration time-dependent hartree (MCTDH) method: I. The Hartley and Huggins bands.

The absorption cross sections of 18 isotopologues of the ozone molecule have been calculated in the range of the Hartley-Huggins bands (27000-55000 cm(-1)). All 18 possible ozone isotopologues made with (16)O, (17)O, and (18)O have been considered, with emphasis on those of geophysics interest like (16)O(3) (17)O(16)O(2), (16)O(17)O(16)O, (18)O(16)O(2), and (16)O(18)O(16)O. We have used the MCTDH algorithm to propagate wavepackets. As an initial wavepacket, we took the vibrational ground state multiplied by the transition dipole moment surface. The cross sections have been obtained from the autocorrelation function of this wavepacket. Only two potential energy surfaces (PESs) and the corresponding transition dipole moment are involved in the calculation. The dissociating R state has been omitted. The calculations have been performed only for J = 0. The comparison with the experimental absorption cross sections of (16)O(3) and (18)O(3) has been performed after an empirical smoothing which mimics the rotational envelop. The isotopologue dependence of the cross sections of 18 isotopologues can be split into two energy ranges, (a) from 27000 to 32000 cm(-1), the Huggins band, which is highly structured, and (b) from 32000 to 55000 cm(-1), the main part of the cross section which has a bell shape, the Hartley band. This bell-shaped envelop has been characterized by a new analytic model depending on only four parameters, amplitude, center, width, and asymmetry. The isotopologue dependence of these parameters reveals the tiny differences between the absorption cross sections of the various isotopologues. In contrast to the smooth shape of the Hartley band, the Huggins band exhibits pronounced vibrational structures and therefore shows large isotopologue differences which may induce a significant isotopologue dependence of the ozone photodissociation rates under actinic flux.