Seasonal cycle of C 16 O 16 O, C 16 O 17 O, and C 16 O 18 O in the middle atmosphere: Implications for mesospheric dynamics and biogeochemical sources and sinks of CO 2 (original) (raw)
Related papers
Journal of Geophysical Research, 2003
1] Stratospheric CO 2 is found to be enriched in heavy oxygen isotopes relative to tropospheric CO 2 values caused by photo-induced isotopic exchange with stratospheric ozone, a gas highly enriched in heavy isotopes relative to the ambient oxygen. The transfer of isotopic enrichment from the ozone pool to the CO 2 pool takes place through CO* 3 formed by reaction of CO 2 with O( 1 D) derived from O 3 photolysis. Recent stratospheric measurements indicate that the transfer process results in CO 2 getting more enriched in 17 O compared to 18 O. Experiments, reported here, investigate this exchange and determine the effects of the initial isotopic composition of CO 2 and O 3 on the final isotopic composition of the exchanged CO 2 . It is seen that the slope relating 17 O and 18 O enrichments in CO 2 depends on the initial isotopic composition of the two gases. Interestingly, with CO 2 of tropospheric composition and O 3 of stratospheric composition the observed slope (1.82 ± 0.04) is close to the value (1.71) found for stratospheric CO 2 . The apparent extra 17 O enrichment in CO 2 resulting in slope higher than that of O 3 can be explained by simple mixing if one assumes that O( 1 D) originates mainly from the asymmetric ozone species and if additional fractionation occurs because of differences in collision frequencies of isotopic species and possibly also during dissociation of CO* 3 to CO 2 and O atom.
For quantification and prediction of global warming due to anthropogenic carbon dioxide (CO 2 ) emissions, high-resolution determination of carbon cycle reservoir size and rates of exchange is vital. Isotope ratio measurements of 13 C and 18 O in CO 2 have proven their utility in resolving the carbon cycle. Here we report a long-term record of triple oxygen isotopic composition of tropospheric CO 2 . A presence of a steady state stratospheric component (Δ 17 O = ln[(δ 17 O + 1) À 0.522 ln(δ 18 O + 1)] = 0.08 ± 0.04‰) is observed in tropospheric CO 2 after modeling via a long-term spline fit of the record, adding an additional technique for quantification of CO 2 fluxes. The δ 18 O of CO 2 (measured as O 2 for oxygen triple isotopic composition) from the present study is compared to the measurements of the Scripps Institute of Oceanography CO 2 network (measured as CO 2 for δ 13 C and δ 18 O) and found to be comparable. We note that the triple oxygen isotopic signal (Δ 17 O) significantly decreased to 0.02 ± 0.02‰ from the steady state value of 0.08 ± 0.04‰ during the 1997-1999 time period. The possible causes for this depletion are evaluated and discussed. An enhanced role of global primary productivity, facilitating water-CO 2 isotope exchange is possible, and future measurements and modeling strategies may develop this further.
Geophysical Research Letters, 2005
Stratospheric photochemistry leads to anomalous oxygen isotope enrichments in CO 2 (for which D 17 O = d 17 O À 0.516 Â d 18 O 6 ¼ 0). This isotope anomaly is not lost until air returns to the troposphere and CO 2 undergoes isotope exchange with water primarily in the terrestrial biosphere and oceans. A two-box model is used to investigate the contribution of stratospheric production and contemporary surface carbon fluxes to tropospheric D 17 O CO2. The predicted magnitude of $0.15% is large enough that measurement of a globally averaged tropospheric D 17 O CO2 should provide a new constraint for gross carbon exchanges between the biosphere and atmosphere in terrestrial carbon cycle models. Importantly, D 17 O CO2 should be complementary to the primary isotopic tracer of gross carbon exchanges, d 18 O CO2 , but is not dependent on numerous hydrologic variables. Furthermore, with improved measurement precision, D 17 O CO2 could serve as a direct tracer of gross carbon exchanges and their variations.
Large and unexpected enrichment in stratospheric 16O13C18O and its meridional variation
Proceedings of The National Academy of Sciences, 2009
The stratospheric CO2 oxygen isotope budget is thought to be governed primarily by the O( 1 D)؉CO2 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 CO2 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 CO2 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)؉CO2 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)؉CO2. We believe the most plausible explanation for this meridional variation is either an unrecognized isotopic fractionation associated with the mesospheric photochemistry of CO2 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.
Unexpected variations in the triple oxygen isotope composition of stratospheric carbon dioxide
Proceedings of the National Academy of Sciences, 2013
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 nonmass-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. F or most materials containing oxygen, the relative abundances of its three stable isotopes (16 O, 17 O, and 18 O) fall on a "mass-dependent" fractionation line (1) with a ln 17 O-ln 18 O three-isotope slope † near 0.5, which is well-predicted by statistical thermodynamics (3) and chemical reaction rate theories (4).
CO 2 +O( 1 D ) isotopic exchange: Laboratory and modeling studies
Journal of Geophysical Research, 2000
Carbon dioxide in the middle atmosphere is mass independently enriched in the hea _vy oxygen isotopes relative to trot>ost•heric values. That is, increasing with altitude, the •70/•60 ratio shows an additional enl•an•ement over what is expected on the basis of the •80/•60 increase. As tropospheric CO2 has a mass-dependent isotopic composition that varies by less ß 18 ...... than 3%0 •n/5 O, •sotop•c measurements of m•ddle atmospheric CO2, combined w•th a quantitative understanding of the enrichment mechanism, could provide valuable information regarding processes such as stratosphere-troposphere exchange and the mean age of an air mass. It is known that the mass-independent enrichment in stratospheric CO: occurs when CO,_ quenches an O(•D) atom formed by the photolysis of 03, but the details of this process remain uncertain. Here a series of laboratory and numerical experiments are presented which have been performed to study the time evolution and final equilibrium values of the CO2 + O(•D) reaction in an effort to reach a better understanding of the CO2 enrichment mechanism. Results show that while the isotopic composition of the CO2 reservoir is qualitatively controlled by the isotopic composition of the O(D) reservoir, there are a number of comphcat•ng factors. The simple mixing model discussed here consistently overpredicts the measured isotopic enrichment, thus indicating the CO2 + O(1D) isotopic exchange is more complicated than has generally been recognized. 1. Introduction Although CO2 is chemically inert throughout most of the atmosphere, it is a vital atmospheric constituent, inextricably linked to the Earth's biosphere. The largest fluxes of the global carbon cycle are those that link atmospheric CO2 to the oceans and to land vegetation. Atmospheric concentrations of CO2 have grown by approximately 30% over the past 2 centuries, to a current level of 360 ppmv, presently increasing at a rate of about 1.5 ppmv yr -• [Houghton et al. , 1996]. This is largely the result of human activities, particularly the combustion of fossil fuels and land use conversion. The atmospheric CO2 concentration is affected by processes that operate on different timescales, including interaction with the silicate cycle, dissolution in the oceans, and the annual cycles of photosynthesis and respiration.
2000
We characterize the spring and fall stratospheric distribution of CO at 49øN-55øS latitude from ATMOS profiles measured during 4 shuttle flights. Measured mixing ratios increase with potential temperature (0) from 12 ppbv (10-9 per unit volume) at 525 K, to 30-40 ppbv at 1750 K with only minor variations with latitude and season at a 0 level. Evidence for some confinement near 1150 K in the developing November 1994 vortex is indicated from comparison of CO and N20 horizontal gradients. Measured CO mixing ratios at the tropical tropopause are a factor of 10 higher than values calculated with a steadystate model using standard photochemistry constrained by correlative temperatures and pressures, and ATMOS measurements including CH4 as inputs. Differences decrease with latitude at constant 0 and are <20% at 800 K and all latitudes, where the CO photochemical lifetime is 40-50 days. 1. Introduction Carbon monoxide (CO) is a minor constituent in the tropical and mid-latitude stratosphere where mixing ratios are determined by a combination of chemistry and transport (e.g., Brasseur and Solomon, [1986]; Solomon, et al. [1985]; Herman et al. [1999]). Measurements of the stratospheric CO distribution are of interest because the CO lifetime (a few months) is of the same order of magnitude as the time required for pollutants entering the stratosphere at the tropical tropopause to be distributed in the lower stratosphere, as indicated by measured and modeled mean age distributions [Hall et al., 1999]. The CO mixing ratio at the tropical tropopause is much higher than its steady-state value as a result of CO transport across the tropopause [Herman et al., 1999]. Hence, measured and calculated CO profiles and the corresponding calculated CO lifetimes are useful for assessing where photochemical steady state (PSS) is a valid assumption. The assumption of PSS is invoked in many stratospheric chemistry studies (e.g., McElroy et al., [1992]). Profiles of stratospheric CO have been measured by a variety of observational techniques. The methods include solar occultation spectroscopy from space (e.g. ATMOS (Atmospheric Trace
CO 2 in the upper troposphere: Influence of stratosphere-troposphere exchange
Geophysical Research Letters, 2006
1] A two-dimensional transport model constrained to measured surface CO 2 concentrations was used to simulate the spatial and temporal variation of CO 2 in the atmosphere for the period from 1975 to 2004. We find that the amplitude, phase and shape of the CO 2 seasonal cycle vary as a function of both altitude and latitude. Cross tropopause exchanges, especially the downward branch of the Brewer-Dobson circulation, which brings stratospheric air to the upper troposphere at middle and high latitudes, change the CO 2 concentration and seasonal cycle in the extra-tropics. The model results match recent aircraft measurements of CO 2 in the upper troposphere remarkably well. We conclude that upper tropospheric CO 2 volume mixing ratios will provide a valuable tool for validating vertical transport. The implications of the CO 2 variation caused by the stratosphere-troposphere exchange for remote sensing of CO 2 are discussed.
Measurements of18O18O and17O18O in the atmosphere and the role of isotope-exchange reactions
Journal of Geophysical Research: Atmospheres, 2012
Of the six stable isotopic variants of O 2 , only three are measured routinely. Observations of natural variations in 16 O 18 O/ 16 O 16 O and 16 O 17 O/ 16 O 16 O ratios have led to insights in atmospheric, oceanographic, and paleoclimate research. Complementary measurements of the exceedingly rare 18 O 18 O and 17 O 18 O isotopic variants might therefore broaden our understanding of oxygen cycling. Here we describe a method to measure natural variations in these multiply substituted isotopologues of O 2. Its accuracy is demonstrated by measuring isotopic effects for Knudsen diffusion and O 2 electrolysis in the laboratory that are consistent with theoretical predictions. We then report the first measurements of 18 O 18 O and 17 O 18 O proportions relative to the stochastic distribution of isotopes (i.e., D 36 and D 35 values, respectively) in tropospheric air. Measured enrichments in 18 O 18 O and 17 O 18 O yield D 36 = 2.05 AE 0.24‰ and D 35 = 1.4 AE 0.5‰ (2s). Based on the results of our electrolysis experiment, we suggest that autocatalytic O(3 P) + O 2 isotope exchange reactions play an important role in regulating the distribution of 18 O 18 O and 17 O 18 O in air. We constructed a box model of the atmosphere and biosphere that includes the effects of these isotope exchange reactions, and we find that the biosphere exerts only a minor influence on atmospheric D 36 and D 35 values. O(3 P) + O 2 isotope exchange in the stratosphere and troposphere is therefore expected to govern atmospheric D 36 and D 35 values on decadal timescales. These results suggest that the 'clumped' isotopic composition of atmospheric O 2 in ice core records is sensitive to past variations in atmospheric dynamics and free-radical chemistry.