Isotope Effects and the Atmosphere.

Chemical physics plays a large role in determining the isotopic compositions of gases in Earth's atmosphere, which in turn provide fundamental insights into the sources, sinks, and transformations of atmospheric gases and particulates and their influence on climate. This review focuses on the kinetic and photolysis isotope effects relevant to understanding the isotope compositions of atmospheric ozone, carbon dioxide, methane, nitrous oxide, and other gases and their historical context. The discussion includes non-mass-dependent isotope compositions of oxygen-containing species and a brief overview of the recent growth of clumped isotope measurements at natural isotopic abundances, that is, of molecules containing more than one rare isotope. The intention is to introduce chemistry researchers to the field of using isotope compositions as tracers of atmospheric chemistry and climate both today and back in time through ice and rock records and to highlight the outstanding research questions to which experimental and theoretical physical chemists can contribute.

Unimolecular Decomposition Rate of the Criegee Intermediate (CH3)2COO Measured Directly with UV Absorption Spectroscopy.

The unimolecular decomposition of (CH3)2COO and (CD3)2COO was measured by direct detection of the Criegee intermediate at temperatures from 283 to 323 K using time-resolved UV absorption spectroscopy. The unimolecular rate coefficient kd for (CH3)2COO shows a strong temperature dependence, increasing from 269 ± 82 s(-1) at 283 K to 916 ± 56 s(-1) at 323 K with an Arrhenius activation energy of ∼6 kcal mol(-1). The bimolecular rate coefficient for the reaction of (CH3)2COO with SO2, kSO2, was also determined in the temperature range 283 to 303 K. Our temperature-dependent values for kd and kSO2 are consistent with previously reported relative rate coefficients kd/kSO2 of (CH3)2COO formed from ozonolysis of tetramethyl ethylene. Quantum chemical calculations of kd for (CH3)2COO are consistent with the experiment, and the combination of experiment and theory for (CD3)2COO indicates that tunneling plays a significant role in (CH3)2COO unimolecular decomposition. The fast rates of unimolecular decomposition for (CH3)2COO measured here, in light of the relatively slow rate for the reaction of (CH3)2COO with water previously reported, suggest that thermal decomposition may compete with the reactions with water and with SO2 for atmospheric removal of the dimethyl-substituted Criegee intermediate.

Unexpected variations in the triple oxygen isotope composition of stratospheric carbon dioxide.

We report observations of stratospheric CO2 that reveal surprisingly large anomalous enrichments in (17)O that vary systematically with latitude, altitude, and season. The triple isotope slopes reached 1.95 ± 0.05(1σ) in the middle stratosphere and 2.22 ± 0.07 in the Arctic vortex versus 1.71 ± 0.03 from previous observations and a remarkable factor of 4 larger than the mass-dependent value of 0.52. Kinetics modeling of laboratory measurements of photochemical ozone-CO2 isotope exchange demonstrates that non-mass-dependent isotope effects in ozone formation alone quantitatively account for the (17)O anomaly in CO2 in the laboratory, resolving long-standing discrepancies between models and laboratory measurements. Model sensitivities to hypothetical mass-dependent isotope effects in reactions involving O3, O((1)D), or CO2 and to an empirically derived temperature dependence of the anomalous kinetic isotope effects in ozone formation then provide a conceptual framework for understanding the differences in the isotopic composition and the triple isotope slopes between the laboratory and the stratosphere and between different regions of the stratosphere. This understanding in turn provides a firmer foundation for the diverse biogeochemical and paleoclimate applications of (17)O anomalies in tropospheric CO2, O2, mineral sulfates, and fossil bones and teeth, which all derive from stratospheric CO2.

Electronic quenching of O((1)D) by Xe: Oscillations in the product angular distribution and their dependence on collision energy.

The dynamics of the O((1)D) + Xe electronic quenching reaction was investigated in a crossed beam experiment at four collision energies. Marked large-scale oscillations in the differential cross sections were observed for the inelastic scattering products, O((3)P) and Xe. The shape and relative phases of the oscillatory structure depend strongly on collision energy. Comparison of the experimental results with time-independent scattering calculations shows qualitatively that this behavior is caused by Stueckelberg interferences, for which the quantum phases of the multiple reaction pathways accessible during electronic quenching constructively and destructively interfere.

Isotope effects and spectroscopic assignments in the non-dissociative photoionization spectrum of N2.

Photoionization efficiency spectra of (14)N2, (15)N(14)N, and (15)N2 from 15.5 to 18.9 eV were measured using synchrotron radiation at the Advanced Light Source at Lawrence Berkeley National Laboratory with a resolution of 6 meV, and significant changes in peak energies and intensities upon isotopic substitution were observed. Previously, we reported the isotope shifts and their applications to Titan's atmosphere. Here, we report more extensive experimental details and tabulate the isotope shifts of many transitions in the N2 spectrum, including those for (15)N(14)N, which have not been previously reported. The isotope shifts are used to address several long-standing ambiguities in spectral peak assignments just above the ionization threshold of N2. The feature at 15.677 eV (the so-called second "cathedral" peak) is of particular interest in this respect. The measured isotope shifts for this peak relative to (14)N2 are 0.015 ± 0.001 eV for (15)N2 and 0.008 ± 0.001 eV for (15)N(14)N, which match most closely with the isotope shifts predicted for transitions to the (A (2)Πu v' = 2)4sσ(g) (1)Π(u) state using Herzberg equations for the isotopic differences in harmonic oscillator energy levels plus the first anharmonic correction of 0.0143 eV for (15)N2 and 0.0071 eV for (15)N(14)N. More generally, the isotope shifts measured for both (15)N2 and (15)N(14)N relative to (14)N2 provide new benchmarks for theoretical calculations of interferences between direct and indirect autoionization states which can interact to produce intricate resonant structures in molecular photoionization spectra in regions near ionization thresholds.

UV absorption spectrum of the C2 Criegee intermediate CH3CHOO.

The UV spectrum of CH3CHOO was measured by transient absorption in a flow cell at 295 K. The absolute absorption cross sections of CH3CHOO were measured by laser depletion in a molecular beam to be (1.06 ± 0.09) × 10(-17) cm(2) molecule(-1) at 308 nm and (9.7 ± 0.6) × 10(-18) cm(2) molecule(-1) at 352 nm. After scaling the UV spectrum of CH3CHOO to the absolute cross section at 308 nm, the peak UV cross section is (1.27 ± 0.11) × 10(-17) cm(2) molecule(-1) at 328 nm. Compared to the simplest Criegee intermediate CH2OO, the UV absorption band of CH3CHOO is similar in intensity but blue shifted by 14 nm, resulting in a 20% slower photolysis rate estimated for CH3CHOO in the atmosphere.

The non-statistical dynamics of the ¹8O + ³²O2 isotope exchange reaction at two energies.

The dynamics of the (18)O((3)P) + (32)O2 isotope exchange reaction were studied using crossed atomic and molecular beams at collision energies (E(coll)) of 5.7 and 7.3 kcal/mol, and experimental results were compared with quantum statistical (QS) and quasi-classical trajectory (QCT) calculations on the O3(X(1)A') potential energy surface (PES) of Babikov et al. [D. Babikov, B. K. Kendrick, R. B. Walker, R. T. Pack, P. Fleurat-Lesard, and R. Schinke, J. Chem. Phys. 118, 6298 (2003)]. In both QS and QCT calculations, agreement with experiment was markedly improved by performing calculations with the experimental distribution of collision energies instead of fixed at the average collision energy. At both collision energies, the scattering displayed a forward bias, with a smaller bias at the lower E(coll). Comparisons with the QS calculations suggest that (34)O2 is produced with a non-statistical rovibrational distribution that is hotter than predicted, and the discrepancy is larger at the lower E(coll). If this underprediction of rovibrational excitation by the QS method is not due to PES errors and/or to non-adiabatic effects not included in the calculations, then this collision energy dependence is opposite to what might be expected based on collision complex lifetime arguments and opposite to that measured for the forward bias. While the QCT calculations captured the experimental product vibrational energy distribution better than the QS method, the QCT results underpredicted rotationally excited products, overpredicted forward-bias and predicted a trend in the strength of forward-bias with collision energy opposite to that measured, indicating that it does not completely capture the dynamic behavior measured in the experiment. Thus, these results further underscore the need for improvement in theoretical treatments of dynamics on the O3(X(1)A') PES and perhaps of the PES itself in order to better understand and predict non-statistical effects in this reaction and in the formation of ozone (in which the intermediate O3* complex is collisionally stabilized by a third body). The scattering data presented here at two different collision energies provide important benchmarks to guide these improvements.

A crossed beam study of 18O(3P)+NO2 and 18O(1D)+NO2: isotope exchange and O2+NO formation channels.

The products and dynamics of the reactions (18)O((3)P)+NO(2) and (18)O((1)D)+NO(2) have been investigated using crossed beams and provide new constraints on the structures and lifetimes of the reactive nitrogen trioxide intermediates formed in collisions of O((3)P) and O((1)D) with NO(2). For each reaction, two product channels are observed - isotope exchange and O(2)+NO formation. From the measured product signal intensities at collision energies of ∼6 to 9.5 kcal/mol, the branching ratio for O(2)+NO formation vs. isotope exchange for the O((3)P)+NO(2) reaction is 52(+6/-2)% to 48(+2/-6)%, while that for O((1)D)+NO(2) is 97(+2/-12)% to 3(+12/-2)%. The branching ratio for the O((3)P)+NO(2) reaction derived here is similar to the ratio measured in previous kinetics studies, while this is the first study in which the products of the O((1)D)+NO(2) reaction have been determined experimentally. Product energy and angular distributions are derived for the O((3)P)+NO(2) isotope exchange and the O((1)D)+NO(2)→O(2)+NO reactions. The results demonstrate that the O((3)P)+NO(2) isotope exchange reaction proceeds by an NO(3)∗ complex that is long-lived with respect to its rotational period and suggest that statistical incorporation of the reactant (18)O into the product NO(2) (apart from zero point energy isotope effects) likely occurs. In contrast, the (18)O((1)D)+NO(2)→O(2)+NO reaction proceeds by a direct "stripping" mechanism via a short-lived (18)O-O-NO∗ complex that results in the occurrence of (18)O in the product O(2) but not in the product NO. Similarly, (18)O is detected in O(2) but not NO for the O((3)P)+NO(2)→O(2)+NO reaction. Thus, even though the product energy and angular distributions for O((3)P)+NO(2)→O(2)+NO derived from the experimental data are uncertain, these results for isotope labeling under single collision conditions support previous kinetics studies that concluded that this reaction proceeds by an asymmetric (18)O-O-NO∗ intermediate and not by a long-lived symmetric NO(3)∗ complex, as earlier bulk isotope labeling experiments had concluded. Applicability of these results to atmospheric chemistry is also discussed.

Large and unexpected enrichment in stratospheric 16O13C18O and its meridional variation.

The stratospheric CO(2) oxygen isotope budget is thought to be governed primarily by the O((1)D)+CO(2) isotope exchange reaction. However, there is increasing evidence that other important physical processes may be occurring that standard isotopic tools have been unable to identify. Measuring the distribution of the exceedingly rare CO(2) isotopologue (16)O(13)C(18)O, in concert with (18)O and (17)O abundances, provides sensitivities to these additional processes and, thus, is a valuable test of current models. We identify a large and unexpected meridional variation in stratospheric (16)O(13)C(18)O, observed as proportions in the polar vortex that are higher than in any naturally derived CO(2) sample to date. We show, through photochemical experiments, that lower (16)O(13)C(18)O proportions observed in the midlatitudes are determined primarily by the O((1)D)+CO(2) isotope exchange reaction, which promotes a stochastic isotopologue distribution. In contrast, higher (16)O(13)C(18)O proportions in the polar vortex show correlations with long-lived stratospheric tracer and bulk isotope abundances opposite to those observed at midlatitudes and, thus, opposite to those easily explained by O((1)D)+CO(2). We believe the most plausible explanation for this meridional variation is either an unrecognized isotopic fractionation associated with the mesospheric photochemistry of CO(2) or temperature-dependent isotopic exchange on polar stratospheric clouds. Unraveling the ultimate source of stratospheric (16)O(13)C(18)O enrichments may impose additional isotopic constraints on biosphere-atmosphere carbon exchange, biosphere productivity, and their respective responses to climate change.

Extreme deuterium enrichment in stratospheric hydrogen and the global atmospheric budget of H2.

Molecular hydrogen (H2) is the second most abundant trace gas in the atmosphere after methane (CH4). In the troposphere, the D/H ratio of H2 is enriched by 120 per thousand relative to the world's oceans. This cannot be explained by the sources of H2 for which the D/H ratio has been measured to date (for example, fossil fuels and biomass burning). But the isotopic composition of H2 from its single largest source--the photochemical oxidation of methane--has yet to be determined. Here we show that the D/H ratio of stratospheric H2 develops enrichments greater than 440 per thousand, the most extreme D/H enrichment observed in a terrestrial material. We estimate the D/H ratio of H2 produced from CH4 in the stratosphere, where production is isolated from the influences of non-photochemical sources and sinks, showing that the chain of reactions producing H2 from CH4 concentrates D in the product H2. This enrichment, which we estimate is similar on a global average in the troposphere, contributes substantially to the D/H ratio of tropospheric H2.

Strong Negative Temperature Dependence of the Simplest Criegee Intermediate CH2OO Reaction with Water Dimer.

The kinetics of the reaction of CH2OO with water vapor was measured directly with UV absorption at temperatures from 283 to 324 K. The observed CH2OO decay rate is second order with respect to the H2O concentration, indicating water dimer participates in the reaction. The rate coefficient of the CH2OO reaction with water dimer can be described by an Arrhenius expression k(T) = A exp(-Ea/RT) with an activation energy of -8.1 ± 0.6 kcal mol(-1) and k(298 K) = (7.4 ± 0.6) × 10(-12) cm(3) s(-1). Theoretical calculations yield a large negative temperature dependence consistent with the experimental results. The temperature dependence increases the effective loss rate for CH2OO by a factor of ~2.5 at 278 K and decreases by a factor of ~2 at 313 K relative to 298 K, suggesting that temperature is important for determining the impact of Criegee intermediate reactions with water in the atmosphere.

Nonstatistical behavior of reactive scattering in the (18)O+(32)O(2) isotope exchange reaction.

The recombination of oxygen atoms with oxygen molecules to form ozone exhibits several strange chemical characteristics, including unusually large differences in formation rate coefficients when different isotopes of oxygen participate. Purely statistical chemical reaction rate theories cannot describe these isotope effects, suggesting that reaction dynamics must play an important role. We investigated the dynamics of the 18O + 32O2 --> O3(*) --> 16O + 34O2 isotope exchange reaction (which proceeds on the same potential energy surface as ozone formation) using crossed atomic and molecular beams at a collision energy of 7.3 kcal mol(-1), providing the first direct experimental evidence that the dissociation of excited ozone exhibits significant nonstatistical behavior. These results are compared with quantum statistical and quasi-classical trajectory calculations in order to gain insight into the potential energy surface and the dynamics of ozone formation.

Mass-dependent and non-mass-dependent isotope effects in ozone photolysis: resolving theory and experiments.

In addition to the anomalous (17)O and (18)O isotope effects in the three-body ozone formation reaction O+O(2)+M, isotope effects in the destruction of ozone by photolysis may also play a role in determining the isotopic composition of ozone and other trace gases in the atmosphere. While previous experiments on ozone photolysis at 254 nm were interpreted as evidence for preferential loss of light ozone that is anomalous (or "non-mass-dependent"), recent semiempirical theoretical calculations predicted a preferential loss of heavy ozone at that wavelength that is mass dependent. Through photochemical modeling results presented here, we resolve this apparent contradiction between experiment and theory. Specifically, we show that the formation of ozone during the UV photolysis experiments is not negligible, as had been assumed, and that the well-known non-mass-dependent isotope effects in ozone formation can account for the non-mass-dependent enrichment of the heavy isotopologs of ozone observed in the experiment. Thus, no unusual non-mass-dependent fractionation in ozone photolysis must be invoked to explain the experimental results. Furthermore, we show that theoretical predictions of a mass-dependent preferential loss of the heavy isotopologs of ozone during UV photolysis are not inconsistent with the experimental data, particularly if mass-dependent isotope effects in the chemical loss reactions of ozone during the photolysis experiments or experimental artifacts enrich the remaining ozone in (17)O and (18)O. Before the calculated fractionation factors can be quantitatively evaluated, however, further investigation of possible mass-dependent isotope effects in the reactions of ozone with O((1)D), O((3)P), O(2)((1)Delta), and O(2)((1)Sigma) is needed through experiments we suggest here.

Mass spectrometric method for the absolute calibration of the intramolecular nitrogen isotope distribution in nitrous oxide.

A mass spectrometric method to determine the absolute intramolecular (position-dependent) nitrogen isotope ratios of nitrous oxide (N2O) has been developed. It is based on the addition of different amounts of doubly labeled 15N2O to an N2O sample of the isotope ratio mass spectrometer reference gas, and subsequent measurement of the relative ion current ratios of species with mass 30, 31, 44, 45, and 46. All relevant quantities are measured by isotope ratio mass spectrometers, which means that the machines' inherent high precision of the order of 10(-5) can be fully exploited. External determination of dilution factors with generally lower precision is avoided. The method itself can be implemented within a day, but a calibration of the oxygen and average nitrogen isotope ratios relative to a primary isotopic reference material of known absolute isotopic composition has to be performed separately. The underlying theoretical framework is explored in depth. The effect of interferences due to 14N15N16O and 15N14N16O in the 15N2O sample and due to 15N2+ formation are fully accounted for in the calculation of the final position-dependent nitrogen isotope ratios. Considering all known statistical uncertainties of measured quantities and absolute isotope ratios of primary isotopic reference materials, we achieve an overall uncertainty of 0.9 per thousand (1 sigma). Using tropospheric N2O as common reference point for intercomparison purposes, we find a substantially higher relative enrichment of 15N at the central nitrogen atom over 15N at the terminal nitrogen atom than measured previously for tropospheric N2O based on a chemical conversion method: 46.3 +/- 1.4 per thousand as opposed to 18.7 +/- 2.2 per thousand. However, our method depends critically on the absolute isotope ratios of the primary isotopic reference materials air-N2 and VSMOW. If they are systematically wrong, our estimates will also necessarily be incorrect.