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Abstract Tropospheric18O18O 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 drive18O18O abundances toward isotopic equilibrium. However, previous work used an offline box‐model framework to explain the18O18O 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 the18O18O proxy, we developed a scheme to simulate atmospheric18O18O abundances (quantified as ∆36values) 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 O2isotopic equilibration scheme within GEOS‐Chem show quantitative agreement with measurements only in the middle stratosphere; modeled ∆36values are too high elsewhere. Investigations using a comprehensive model of the O‐O2‐O3isotopic photochemical system and proof‐of‐principle experiments suggest that the simple equilibration scheme omits an important pressure dependence to ∆36values: the anomalously efficient titration of18O18O to form ozone. Incorporating these effects into the online ∆36calculation scheme in GEOS‐Chem yields quantitative agreement for all available observations. While this previously unidentified bias affects the atmospheric budget of18O18O in O2, the modeled change in the mean tropospheric ∆36value since 1850 CE is only slightly altered; it is still quantitatively consistent with the ice‐core ∆36record, implying that the tropospheric ozone burden increased less than 40% over the twentieth century.
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Localized and delocalized bound states of the main isotopologue 48 O 3 and of 18 O-enriched 50 O 3 isotopomers of the ozone molecule near the dissociation threshold
Knowledge of highly excited rovibrational states of ozone isotopologues is of key importance for modelling the dynamics of exchange reactions, for understanding longstanding problems related to isotopic anomalies of the ozone formation, and for analyses of extra-sensitive laser spectral experiments currently in progress. This work is devoted to new theoretical study of high-energy states for the main isotopologue 48 O 3 = 16 O 16 O 16 O and for the family of 18 O-enriched isotopomers 50 O 3 = { 16 O 16 O 18 O, 16 O 18 O 16 O, 18 O 16 O 16 O} of the ozone molecule considered using a full-symmetry approach. Energies and wave functions of bound states near the dissociation threshold are computed in hyperspherical coordinates accounting for the permutation symmetry of three identical nuclei in 48 O 3 and of two identical nuclei in 50 O 3 , using the most accurate potential energy surface available now. The obtained vibrational band centers agree with observed ones with the root-mean-squares deviation of about 1 cm −1 , making the results appropriate for assignments and analyses of future experimental spectra. The levels delocalized between the three potential wells of ozone isomers are computed and analyzed. The states situated deep in the three (for 48 O 3 ) or two (for 50 O 3 ) equivalent potential wells have similar energies with negligible splitting. However, the states situated just below the potential barriers separating the wells, are split due to the tunneling between the wells resulting in the splitting of rovibrational sub-bands. We evaluate the amplitudes of the corresponding effects and consider possible perturbations in vibration–rotation bands due to interactions between three potential wells. Theoretical predictions for the splitting of observable band centers are provided for the first time.
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- Award ID(s):
- 1806915
- PAR ID:
- 10170504
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- ISSN:
- 1463-9076
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0CP02177F
