Isotope Effects and the Atmosphere Carlstad, Julia M; Boering, Kristie A
Annual review of physical chemistry,
04/2023, Letnik:
74, Številka:
1
Journal Article
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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.
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, k SO2 , was also determined in the temperature range 283 to 303 K. Our temperature-dependent values for k d and k SO2 are consistent with previously reported relative rate coefficients k d/k SO2 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 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(−E a/RT) with an activation energy of −8.1 ± 0.6 kcal mol–1 and k(298 K) = (7.4 ± 0.6) × 10–12 cm3 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.
We report observations of stratospheric CO 2 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–CO 2 isotope exchange demonstrates that non–mass-dependent isotope effects in ozone formation alone quantitatively account for the 17 O anomaly in CO 2 in the laboratory, resolving long-standing discrepancies between models and laboratory measurements. Model sensitivities to hypothetical mass-dependent isotope effects in reactions involving O 3 , O( 1 D), or CO 2 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 CO 2 , O 2 , mineral sulfates, and fossil bones and teeth, which all derive from stratospheric CO 2 .
Oxygen has three naturally occurring isotopes, of mass numbers 16, 17 and 18. Their ratio in atmospheric O2 depends primarily on the isotopic composition of photosynthetically produced O2 from ...terrestrial and aquatic plants, and on isotopic fractionation due to respiration. These processes fractionate isotopes in a mass-dependent way, such that 17O enrichment would be approximately half of the 18O enrichment relative to 16O. But some photochemical reactions in the stratosphere give rise to a mass-independent isotope fractionation, producing approximately equal 17O and 18O enrichments in stratospheric ozone and carbon dioxide,, and consequently driving an atmospheric O2 isotope anomaly. Here we present an experimentally based estimate of the size of the 17O/16O anomaly in tropospheric O2, and argue that it largely reflects the influences of biospheric cycling and stratospheric photochemical processes. We propose that because the biosphere removes the isotopically anomalous stratosphere-derived O2 by respiration, and replaces it with isotopically 'normal' oxygen by photosynthesis, the magnitude of the tropospheric 17O anomaly can be used as a tracer of global biosphere production. We use measurements of the triple-isotope composition of O2 trapped in bubbles in polar ice to estimate global biosphere productivity at various times over the past 82,000 years. In a second application, we use the isotopic signature of oxygen dissolved in aquatic systems to estimate gross primary production on broad time and space scales.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The distribution of isotopes within O2 molecules can be rapidly altered when they react with atomic oxygen. This mechanism is globally important: while other contributions to the global budget of O2 ...impart isotopic signatures, the O(3P) + O2 reaction resets all such signatures in the atmosphere on subdecadal timescales. Consequently, the isotopic distribution within O2 is determined by O3 photochemistry and the circulation patterns that control where that photochemistry occurs. The variability of isotopic ordering in O2 has not been established, however. We present new measurements of 18O18O in air (reported as delta36 values) from the surface to 33 km altitude. They confirm the basic features of the clumped-isotope budget of O2: Stratospheric air has higher delta36 values than tropospheric air (i.e., more 18O18O), reflecting colder temperatures and fast photochemical cycling of O3. Lower delta36 values in the troposphere arise from photochemistry at warmer temperatures balanced by the influx of high-delta36 air from the stratosphere. These observations agree with predictions derived from the GEOS-Chem chemical transport model, which provides additional insight. We find a link between tropical circulation patterns and regions where delta36 values are reset in the troposphere. The dynamics of these regions influences lapse rates, vertical and horizontal patterns of O2 reordering, and thus the isotopic distribution toward which O2 is driven in the troposphere. Temporal variations in delta36 values at the surface should therefore reflect changes in tropospheric temperatures, photochemistry, and circulation. Our results suggest that the tropospheric O3 burden has remained within a +/-10 percent range since 1978.
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.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
8.
Evaluation of transport in stratospheric models Hall, Timothy M.; Waugh, Darryn W.; Boering, Kristie A. ...
Journal of Geophysical Research, Washington, DC,
20 August 1999, Letnik:
104, Številka:
D15
Journal Article
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We evaluate transport characteristics of two‐ and three‐dimensional chemical transport models of the stratosphere by comparing their simulations of the mean age of stratospheric air and the ...propagation of annually periodic oscillations in tracer mixing ratio at the tropical tropopause into the stratosphere to inferences from in situ and satellite observations of CO2, SF6, and water vapor. The models, participants in the recent NASA “Models and Measurements II” study, display a wide range of performance. Most models propagate annual oscillations too rapidly in the vertical and overattenuate the signal. Most models also significantly underestimate mean age throughout the stratosphere, and most have at least one of several unrealistic features in their mean age contour shapes. In the lower stratosphere, model‐to‐model variation in N2O, NOy, and Cly is well correlated with variation in mean age, and the magnitude of NOy and Cly variation is large. We conclude that model transport inaccuracies significantly affect simulations of important long‐lived chemical species in the lower stratosphere.
Stratospheric N2O is known to be enriched in the heavy isotopes 15N and 18O relative to tropospheric N2O, primarily because of the preferential photolysis of light isotopologues. We present ...measurements of δ15N, δ18O, and site‐specific δ15N on N2O from 32 stratospheric whole air samples collected by the NASA ER‐2 aircraft between 1997 and 2000 from 62°N to 89°N with N2O mixing ratios ranging from 51 to 313 ppbv. The relationships between the isotopic compositions and N2O mixing ratios show significant differences between aircraft deployments and with previous measurements for N2O < 200 ppbv. The differences between ER‐2 deployments at low N2O are significant at the 3σ level and are due to the effects of transport and mixing. The ratios of enrichment factors for the different isotopologues, however, are the same to within their 1σ uncertainties for N2O > 200 ppbv and N2O < 200 ppbv. The observed isotope:N2O relationships are also used to estimate the fluxes of the N2O isotopologues from the stratosphere to the troposphere given independent estimates of the N2O loss rate. On the basis of the robustness of isotope:N2O relationships for N2O > 200 ppbv we conclude that the fluxes to the troposphere estimated from these relationships and, therefore, the influence of stratosphere‐to‐troposphere transport on the isotopic compositions of N2O in the free troposphere are now relatively well quantified, leaving the isotopic compositions of the N2O sources as the remaining largest uncertainties in the global N2O isotope budget.
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