Krisltie A. Boering

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.

Tropospheric ¹⁸O¹⁸O is an emerging proxy for past tropospheric ozone and free‐tropospheric temperatures. The basis of these applications is the idea that isotope‐exchange reactions in the atmosphere drive ¹⁸O¹⁸O abundances toward isotopic equilibrium. However, previous work used an offline box‐model framework to explain the ¹⁸O¹⁸O budget, approximating the interplay of atmospheric chemistry and transport. This approach, while convenient, has poorly characterized uncertainties. To investigate these uncertainties, and to broaden the applicability of the ¹⁸O¹⁸O proxy, we developed a scheme to simulate atmospheric ¹⁸O¹⁸O abundances (quantified as ∆₃₆ values) online within the GEOS‐Chem chemical transport model. These results are compared to both new and previously published atmospheric observations from the surface to 33 km. Simulations using a simplified O₂ isotopic equilibration scheme within GEOS‐Chem show quantitative agreement with measurements only in the middle stratosphere; modeled ∆₃₆ values are too high elsewhere. Investigations using a comprehensive model of the O‐O₂‐O₃ isotopic photochemical system and proof‐of‐principle experiments suggest that the simple equilibration scheme omits an important pressure dependence to ∆₃₆ values: the anomalously efficient titration of ¹⁸O¹⁸O to form ozone. Incorporating these effects into the online ∆₃₆ calculation scheme in GEOS‐Chem yields quantitative agreement for all available observations. While this previously unidentified bias affects the atmospheric budget of ¹⁸O¹⁸O in O₂, the modeled change in the mean tropospheric ∆₃₆ value since 1850 CE is only slightly altered; it is still quantitatively consistent with the ice‐core ∆₃₆ record, implying that the tropospheric ozone burden increased less than 40% over the twentieth century.

Tropospheric 18O18O is an emerging proxy for past tropospheric ozone and free-tropospheric temperatures. The basis of these applications is the idea that isotope-exchange reactions in the atmosphere drive 18O18O abundances toward isotopic equilibrium. However, previous work used an offline box-model framework to explain the 18O18O budget, approximating the interplay of atmospheric chemistry and transport. This approach, while convenient, has poorly characterized uncertainties. To investigate these uncertainties, and to broaden the applicability of the 18O18O proxy, we developed a scheme to simulate atmospheric 18O18O abundances (quantified as ∆36 values) online within the GEOS-Chem chemical transport model. These results are compared to both new and previously published atmospheric observations from the surface to 33 km. Simulations using a simplified O2 isotopic equilibration scheme within GEOS-Chem show quantitative agreement with measurements only in the middle stratosphere; modeled ∆36 values are too high elsewhere. Investigations using a comprehensive model of the O-O2-O3 isotopic photochemical system and proof-of-principle experiments suggest that the simple equilibration scheme omits an important pressure dependence to ∆36 values: the anomalously efficient titration of 18O18O to form ozone. Incorporating these effects into the online ∆36 calculation scheme in GEOS-Chem yields quantitative agreement for all available observations. While this previously unidentified bias affects the atmospheric budget of 18O18O in O2, the modeled change in the mean tropospheric ∆36 value since 1850 C.E. is only slightly altered; it is still quantitatively consistent with the ice-core ∆36 record, implying that the tropospheric ozone burden increased less than ~40% over the twentieth century.

The expanding use of nitrogen‐containing fertilizers in agriculture has led to an increase in N₂O, an important greenhouse gas and ozone‐depleting substance, since preindustrial times. Isotopic measurements are a valuable tool to distinguish the contribution of different sources of N₂O, but the isotopic composition of N₂O formed in the low temperature plasma regimes in lightning has not previously been measured. Here, a corona discharge was generated in flowing or static zero air, and the N₂O formed at discharge cell pressures from ∼0.1 to 10 Torr (∼13–1,300 Pa) at a discharge voltage of 5 kV and discharge current of 1,500 μA was collected and measured by isotope ratio mass spectrometry. Experiments were also conducted by varying the discharge current from 75 to 1,500 μA at ∼0.5 Torr (∼67 Pa) and 5 kV. The results show enrichments in ¹⁵N in product N₂O of up to 32‰ relative to the reactant N₂ and even larger enrichments in ¹⁵N of up to 77‰ at the central nitrogen atom. Depletions in ¹⁸O as large as −71‰ relative to reactant O₂ were also measured. The isotope‐isotope relationships of the N₂O produced in the corona discharge are distinct from those of other sources of N₂O, suggesting that isotope measurements can be used to determine whether local or regional variations in the atmospheric concentration of N₂O – such as the enhanced N₂O levels measured in the upper tropical and subtropical troposphere during the HIPPO mission – can be attributed to lightning activity, soil emissions, or biomass burning.

Tropospheric ¹⁸ O ¹⁸ O is an emerging proxy for past tropospheric ozone and free-tropospheric temperatures. The basis of these applications is the idea that isotope-exchange reactions in the atmosphere drive ¹⁸ O ¹⁸ O abundances toward isotopic equilibrium. However, previous work used an offline box-model framework to explain the ¹⁸ O ¹⁸ O budget, approximating the interplay of atmospheric chemistry and transport. This approach, while convenient, has poorly characterized uncertainties. To investigate these uncertainties, and to broaden the applicability of the ¹⁸ O ¹⁸ O proxy, we developed a scheme to simulate atmospheric ¹⁸ O ¹⁸ O abundances (quantified as ∆ ₃₆ values) online within the GEOS-Chem chemical transport model. These results are compared to both new and previously published atmospheric observations from the surface to 33 km. Simulations using a simplified O ₂ isotopic equilibration scheme within GEOS-Chem show quantitative agreement with measurements only in the middle stratosphere; modeled ∆ ₃₆ values are too high elsewhere. Investigations using a comprehensive model of the O-O ₂ -O ₃ isotopic photochemical system and proof-of-principle experiments suggest that the simple equilibration scheme omits an important pressure dependence to ∆ ₃₆ values: the anomalously efficient titration of ¹⁸ O ¹⁸ O to form ozone. Incorporating these effects into the online ∆ ₃₆ calculation scheme in GEOS-Chem yields quantitative agreement for all available observations. While this previously unidentified bias affects the atmospheric budget of ¹⁸ O ¹⁸ O in O ₂ , the modeled change in the mean tropospheric ∆ ₃₆ value since 1850 C.E. is only slightly altered; it is still quantitatively consistent with the ice-core ∆ ₃₆ record, implying that the tropospheric ozone burden likely increased less than ~40% over the twentieth century.

Recent field studies have shown that there are habitats in the subarctic tundra emitting N₂O at exceptionally high rates. In this study, stable isotope techniques were applied to characterize the processes responsible for these high N₂O emissions which have been found from bare peat surfaces in permafrost peatlands. The results include the first data on the nitrogen and oxygen isotopic composition of N₂O emitted from arctic tundra. The emission‐weighted average δ¹⁵N^bulk value for N₂O of −13.0‰ ± 2.0‰ (mean ± SD; n = 8) from the bare peat surfaces falls within the range of the emission‐weighted average values from other natural ecosystems but is distinct from those for managed/agricultural ecosystems. This implies that if in the future, a smaller rate in the overall decreasing trend of δ¹⁵N^bulk N₂O tropospheric isotopic composition is found, it cannot be attributed only to agricultural N₂O emission reductions from mitigation actions but also to soils in natural ecosystems that may be emitting more N₂O to the atmosphere due to warmer conditions. The site preference (SP) values from emitted N₂O range from −30‰ to 58‰, indicating a temporal shift of microbial production and consumption of N₂O during the sampling period. Soil emission SP data suggest that the N₂O emission in subarctic tundra are more likely to be produced by nitrifier denitrification in the drier study year, but due to variable published SP values for N₂O production processes in soils, this interpretation has to be taken with caution. According to SP values at depth, denitrification was the main N₂O production pathway. To better address the usefulness of SP in partitioning microbial mechanisms in soils, further studies in soils mesocosms are required.

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 k_d 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 k_d and kSO₂ are consistent with previously reported relative rate coefficients k_d/kSO₂ of (CH₃)₂COO formed from ozonolysis of tetramethyl ethylene. Quantum chemical calculations of k_d 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.

The dynamics of the O(¹D) + 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(³P) 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.