Oxygen anomaly in near surface carbon dioxide reveals deep stratospheric intrusion

Isotope ratios of total C, N, and S in particulate matter simultaneously calibrated by mixed USGS and IAEA reference materials

This study presents a method for calibrating the isotope ratios of the total carbon, nitrogen, and sulfur in particulate matter (PM) collected from the Seoul metro using an elemental analyzer-isotope ratio mass spectrometer (EA-IRMS). Mixtures of isotope reference materials (MRMs) from the U.S. Geological Survey (USGS) and International Atomic Energy Agency (IAEA) reference materials formed an input dataset for generalized least squares (GLS) regression to yield calibration lines. The analytical method proposed in this study enabled the measurement of stable isotope ratios of total carbon, nitrogen, and sulfur simultaneously. Results showed good linearity and repeatability for carbon and nitrogen isotopes, but poor results for sulfur isotopes due to peak broadening. Reference values with uncertainties for the isotope ratios of total carbon, nitrogen, and sulfur were determined for the collected PM, demonstrating twice as much uncertainty as that of the USGS and IAEA reference materials. Homogeneity was the biggest uncertainty source for the calibrated values.

Temperature dependence of isotopic fractionation in the CO2 -O2 isotope exchange reaction

RATIONALE

Oxygen isotope exchange between O₂ and CO₂ in the presence of heated platinum (Pt) is an established technique for determining the δ¹⁷ O value of CO₂ . However, there is not yet a consensus on the associated fractionation factors at the steady state.

METHODS

We determined experimentally the steady-state α¹⁷ and α¹⁸ fractionation factors for Pt-catalyzed CO₂ -O₂ oxygen isotope exchange at temperatures ranging from 500 to 1200°C. For comparison, the theoretical α¹⁸ equilibrium exchange values reported by Richet et al. (1997) have been updated using the direct sum method for CO₂ and the corresponding α¹⁷ values were determined. Finally, we examined whether the steady-state fractionation factors depend on the isotopic composition of the reactants, by using CO₂ and O₂ differing in δ¹⁸ O value from -66 ‰ to +4 ‰.

RESULTS

The experimentally determined steady-state fractionation factors α¹⁷ and α¹⁸ are lower than those obtained from the updated theoretical calculations (of CO₂ -O₂ isotope exchange under equilibrium conditions) by 0.0024 ± 0.0001 and 0.0048 ± 0.0002, respectively. The offset is not due to scale incompatibilities between isotope measurements of O₂ and CO₂ nor to the neglect of non-Born-Oppenheimer effects in the calculations. There is a crossover temperature at which enrichment in the minor isotopes switches from CO₂ to O₂ . The direct sum evaluation yields a θ value of ~0.54, i.e. higher than the canonical range maximum for a mass-dependent fractionation process.

CONCLUSIONS

Updated theoretical values of α¹⁸ for equilibrium isotope exchange are lower than those derived from previous work by Richet et al. (1997). The direct sum evaluation for CO₂ yields θ values higher than the canonical range maximum for mass-dependent fractionation processes. This demonstrates the need to include anharmonic effects in the calculation and definition of mass-dependent fractionation processes for poly-atomic molecules. The discrepancy between the theory and the experimental α¹⁷ and α¹⁸ values may be due to thermal diffusion associated with the temperature gradient in the reactor.

Leaf scale quantification of the effect of photosynthetic gas exchange on Δ47 of CO2

The clumped isotope composition (Δ₄₇, the anomaly of the mass 47 isotopologue relative to the abundance expected from a random isotope distribution) of CO₂ has been suggested as an additional tracer for gross CO₂ fluxes. However, the effect of photosynthetic gas exchange on Δ₄₇ has not been directly determined and two indirect/conceptual studies reported contradicting results. In this study, we quantify the effect of photosynthetic gas exchange on Δ₄₇ of CO₂ using leaf cuvette experiments with one C₄ and two C₃ plants. The experimental results are supported by calculations with a leaf cuvette model. Our results demonstrate the important roles of the Δ₄₇ value of CO₂ entering the leaf, kinetic fractionation as CO₂ diffuses into, and out of the leaf and CO₂–H₂O isotope exchange with leaf water. We experimentally confirm the previously suggested dependence of Δ₄₇ of CO₂ in the air surrounding a leaf on the stomatal conductance and back-diffusion flux. Gas exchange can enrich or deplete the Δ₄₇ of CO₂ depending on the Δ₄₇ of CO₂ entering the leaf and the fraction of CO₂ exchanged with leaf water and diffused back to the atmosphere, but under typical ambient conditions, it will lead to a decrease in Δ₄₇.

Techniques for measuring carbon and oxygen isotope compositions of atmospheric CO2 via isotope ratio mass spectrometry

Measuring the stable isotope compositions of atmospheric CO₂ is common in earth and atmospheric sciences, and various analytical methods have been developed utilizing continuous-flow (CF) or dual-inlet (DI) isotope ratio mass spectrometry (IRMS). Air is typically collected via passive, manual, or automated collection methods and the volume of the air sample ranges from 10 to 300 mL for CF-IRMS to >1 L for DI-IRMS to yield a measurable amount of atmospheric CO₂ gas. It has been determined that the integrity of vials and flasks for air sample storage can be compromised after 3 days of air collection for δ¹³ C values and within 10 hours for δ¹⁸ O values. Air samples must be purified after collection to remove constituents of air, such as Ar, O₂ , N₂ , N₂ O, and water vapor, to avoid isobaric interferences during mass spectrometric measurement. Purification is generally undertaken by utilizing commercial or custom-made preconcentration devices, the blanking method for CF-IRMS, or an offline/online cryogenic separation using a vacuum line for DI-IRMS. Ambient N₂ O is a component of air that may affect analytical results and thus must either be corrected for or be removed using a gas chromatographic column. In some cases, water is removed during air collection by using a common chemical desiccant, magnesium perchlorate (Mg(ClO₄ )₂ ), or by a dry ice/alcohol mixture (-78°C). Lastly, a linearity issue for IRMS due to the low amount of purified CO₂ from a typical ambient air sample must be considered. In general, analytical precisions of 0.02-0.21‰ and 0.04-0.34‰ for CF-IRMS and 0.01-0.02‰ and 0.01-0.02‰ for DI-IRMS are expected for δ¹³ C and δ¹⁸ O measurements, respectively.

A new perspective of probing the level of pollution in the megacity Delhi affected by crop residue burning using the triple oxygen isotope technique in atmospheric CO2

Air quality in the megacity Delhi is affected not only by local emissions but also by pollutants from crop residue burning in the surrounding areas of the city, particularly the rice straw burning in the post monsoon season. As a major burning product, gaseous CO₂, which is rather inert in the polluted atmosphere, provides an alternative solution to characterize the impact of biomass burning from a new perspective that other common tracers such as particulate matters are limited because of their physical and chemical reactiveness. Here, we report conventional ([CO₂], δ¹³C, and δ¹⁸O) and unconventional (Δ¹⁷O) isotope data for CO₂ collected at Connaught Place (CP), a core area in the megacity Delhi, and two surrounding remote regions during a field campaign in October 18-20, 2017. We also measured the isotopic ratios near a rice straw burning site in Taiwan to constrain their end member isotopic compositions. Rice straw burning produces CO₂ with δ¹³C, δ¹⁸O, and Δ¹⁷O values of -29.02 ± 0.65, 19.63 ± 1.16, and 0.05 ± 0.02‰, respectively. The first two isotopic tracers are less distinguishable from those emitted by fossil fuel combustion but the last one is significantly different. We then utilize these end member isotopic ratios, with emphasis on Δ¹⁷O for the reason given above, for partitioning sources that affect the CO₂ level in Delhi. Anthropogenic fraction of CO₂ at CP ranges from 4 to 40%. Further analysis done by employing a three-component (background, rice straw burning, and fuel combustion) mixing model with constraints from the Δ¹⁷O values yields that rice straw burning contributes as much as ∼70% of the total anthropogenic CO₂, which is more than double of the fossil fuel contribution (∼30%), during the study days.

A Stratospheric Intrusion-Influenced Ozone Pollution Episode Associated with an Intense Horizontal-Trough Event

Ozone pollution is currently a serious issue in China. As an important source of tropospheric ozone, the stratospheric ozone has received less concern. This study uses a combination of ground-based ozone measurements, the latest ERA5 reanalysis data as well as chemistry-climate model and Lagrangian Particle Dispersion Modeling (LPDM) simulations to investigate the potential impacts of stratospheric intrusion (SI) on surface ozone pollution episodes in eastern China. Station-based observations indicate that severe ozone pollution occurred from 27 April to 28 April 2018 in eastern China, with maximal values over 140 ppbv. ERA5 meteorological and ozone data suggest that a strong horizontal-trough exists at the same time, which leads to an evident SI event and brings ozone-rich air from the stratosphere to the troposphere. Using a stratospheric ozone tracer defined by NCAR’s Community Atmosphere Model with Chemistry (CAM-Chem), we conclude that this SI event contributed about 15 ppbv (15%) to the surface ozone pollution episode during 27–28 April in eastern China. The potential impacts of SI events on surface ozone variations should be therefore considered in ozone forecast and control.

Determination of the triple oxygen and carbon isotopic composition of CO2 from atomic ion fragments formed in the ion source of the 253 Ultra high-resolution isotope ratio mass spectrometer

RATIONALE

Determination of δ¹⁷ O values directly from CO₂ with traditional gas source isotope ratio mass spectrometry is not possible due to isobaric interference of ¹³ C¹⁶ O¹⁶ O on ¹² C¹⁷ O¹⁶ O. The methods developed so far use either chemical conversion or isotope equilibration to determine the δ¹⁷ O value of CO₂ . In addition, δ¹³ C measurements require correction for the interference from ¹² C¹⁷ O¹⁶ O on ¹³ C¹⁶ O¹⁶ O since it is not possible to resolve the two isotopologues.

METHODS

We present a technique to determine the δ¹⁷ O, δ¹⁸ O and δ¹³ C values of CO₂ from the fragment ions that are formed upon electron ionization in the ion source of the Thermo Scientific 253 Ultra high-resolution isotope ratio mass spectrometer (hereafter 253 Ultra). The new technique is compared with the CO₂ -O₂ exchange method and the ¹⁷ O-correction algorithm for δ¹⁷ O and δ¹³ C values, respectively.

RESULTS

The scale contractions for δ¹³ C and δ¹⁸ O values are slightly larger for fragment ion measurements than for molecular ion measurements. The δ¹⁷ O and Δ¹⁷ O values of CO₂ can be measured on the ¹⁷ O⁺ fragment with an internal error that is a factor 1-2 above the counting statistics limit. The ultimate precision depends on the signal intensity and on the total time that the ¹⁷ O⁺ beam is monitored; a precision of 14 ppm (parts per million) (standard error of the mean) was achieved in 20 hours at the University of Göttingen. The Δ¹⁷ O measurements with the O-fragment method agree with the CO₂ -O₂ exchange method over a range of Δ¹⁷ O values of -0.3 to +0.7‰.

CONCLUSIONS

Isotope measurements on atom fragment ions of CO₂ can be used as an alternative method to determine the carbon and oxygen isotopic composition of CO₂ without chemical processing or corrections for mass interferences.

Global 3-D Simulations of the Triple Oxygen Isotope Signature Δ17O in Atmospheric CO2

The triple oxygen isotope signature Δ17O in atmospheric CO2, also known as its "17O excess," has been proposed as a tracer for gross primary production (the gross uptake of CO2 by vegetation through photosynthesis). We present the first global 3-D model simulations for Δ17O in atmospheric CO2 together with a detailed model description and sensitivity analyses. In our 3-D model framework we include the stratospheric source of Δ17O in CO2 and the surface sinks from vegetation, soils, ocean, biomass burning, and fossil fuel combustion. The effect of oxidation of atmospheric CO on Δ17O in CO2 is also included in our model. We estimate that the global mean Δ17O (defined as Δ 17 O = ln ( δ 17 O + 1 ) - λ RL · ln ( δ 18 O + 1 ) with λ RL = 0.5229) of CO2 in the lowest 500 m of the atmosphere is 39.6 per meg, which is ∼20 per meg lower than estimates from existing box models. We compare our model results with a measured stratospheric Δ17O in CO2 profile from Sodankylä (Finland), which shows good agreement. In addition, we compare our model results with tropospheric measurements of Δ17O in CO2 from Göttingen (Germany) and Taipei (Taiwan), which shows some agreement but we also find substantial discrepancies that are subsequently discussed. Finally, we show model results for Zotino (Russia), Mauna Loa (United States), Manaus (Brazil), and South Pole, which we propose as possible locations for future measurements of Δ17O in tropospheric CO2 that can help to further increase our understanding of the global budget of Δ17O in atmospheric CO2.

Relating Δ17O Values of Animal Body Water to Exogenous Water Inputs and Metabolism

The dynamics of animal body water and metabolism are integral aspects of biological function but are difficult to measure, particularly in free-ranging individuals. We demonstrate a new method to estimate inputs to body water via analysis of Δ17O, a measure of 17O/16O relative to 18O/16O. Animal body water is primarily a mixture of drinking or food water (meteoric water; Δ17O ≈ 0.030 per mille [‰]) and metabolic water synthesized from atmospheric oxygen (Δ17O ≈ –0.450‰). Greater drinking or food water intake should increase Δ17O toward 0.030‰, whereas greater metabolic rate should decrease Δ17O toward –0.450‰. We found that wild mammal Δ17O values generally increased with body mass, consistent with both a decline in mass-specific metabolic rate and an increase in water intake. Captive mouse (Peromyscus maniculatus) Δ17O values were higher than predicted but exhibited the expected relative change based on metabolic rate and water intake. Measurements of Δ17O may enable novel ecophysiological studies.

Distribution of CO2 in Western Pacific, Studied Using Isotope Data Made in Taiwan, OCO-2 Satellite Retrievals, and CarbonTracker Products

To assess sources and processes that affect the variability of CO2 at local to regional scales, we have analyzed the mixing ratio [CO2] and stable isotopic compositions (δ13C and δ18O) of atmospheric CO2 for three years (2014-2016) in urban and sub-urban areas in Taipei, Taiwan. The data are compared with those from some background sites, viz., Lulin, Mauna Loa, and Minamitorishima, to evaluate how local emissions affect CO2 level regionally. [CO2] over the urban and sub-urban stations are significantly higher than that observed at the three aforementioned remote sites mainly due to local emissions, which partly mask the seasonal cycle caused by photosynthesis and respiration. Likewise, significantly low δ13C and δ18O values observed at two Taipei stations also point to anthropogenic emissions. The seasonal cycles in [CO2] and in the isotopic compositions are retrieved using the ensemble empirical mode decomposition method. Regional impact is assessed using CO2 products from the Orbiting Carbon Observatory-2 satellite, the NOAA/EARL CarbonTracker project, and meteorological data from European Centre for Medium range Weather Forecast-Interim. We found that besides local emissions, Taiwan is largely affected by external CO2 in winter and spring originated from north, west and southwest landmasses. In winter air masses with elevated CO2 concentrations, originated in eastern China influence Taipei. In spring season, about 2 ppmv enhancement in CO2 observed at the top of Lulin, a high mountain station (2.8 km), could be linked to CO2 produced by biomass burning in the southeast Asian countries and transported to the region by easterly winds.

Oxygen isotope anomaly in tropospheric CO2 and implications for CO2 residence time in the atmosphere and gross primary productivity

The abundance variations of near surface atmospheric CO₂ isotopologues (primarily ¹⁶O¹²C¹⁶O, ¹⁶O¹³C¹⁶O, ¹⁷O¹²C¹⁶O, and ¹⁸O¹²C¹⁶O) represent an integrated signal from anthropogenic/biogeochemical processes, including fossil fuel burning, biospheric photosynthesis and respiration, hydrospheric isotope exchange with water, and stratospheric photochemistry. Oxygen isotopes, in particular, are affected by the carbon and water cycles. Being a useful tracer that directly probes governing processes in CO₂ biogeochemical cycles, Δ¹⁷O (=ln(1 + δ¹⁷O) - 0.516 × ln(1 + δ¹⁸O)) provides an alternative constraint on the strengths of the associated cycles involving CO₂. Here, we analyze Δ¹⁷O data from four places (Taipei, Taiwan; South China Sea; La Jolla, United States; Jerusalem, Israel) in the northern hemisphere (with a total of 455 measurements) and find a rather narrow range (0.326 ± 0.005‰). A conservative estimate places a lower limit of 345 ± 70 PgC year^-1 on the cycling flux between the terrestrial biosphere and atmosphere and infers a residence time of CO₂ of 1.9 ± 0.3 years (upper limit) in the atmosphere. A Monte Carlo simulation that takes various plant uptake scenarios into account yields a terrestrial gross primary productivity of 120 ± 30 PgC year^-1 and soil invasion of 110 ± 30 PgC year^-1, providing a quantitative assessment utilizing the oxygen isotope anomaly for quantifying CO₂ cycling.

Direct, Precise Measurements of Isotopologue Abundance Ratios in CO2 Using Molecular Absorption Spectroscopy: Application to Δ17O

We present an ultrasensitive absorption spectrometer based on a 30 Hz/s stability, sub-kHz line width laser source coupled to a high-stability cavity-ring-down-spectroscopy setup. It provides direct and precise measurements of the isotopic ratios δ¹⁷O and δ¹⁸O in CO₂. We demonstrate the first optical absorption measurements of ¹⁷O anomalies in CO₂ with a precision better than 10 ppm, matching the requirements for paleo-environmental applications. This illustrates how optical absorption methods have become a competitive alternative to state-of-the-art isotopic ratio mass spectrometry techniques.

Identification of Anthropogenic CO2 Using Triple Oxygen and Clumped Isotopes

Quantification of contributions from various sources of CO₂ is important for understanding the atmospheric CO₂ budget. Considering the number and diversity of sources and sinks, the widely used proxies such as concentration and conventional isotopic compositions (δ¹³C and δ¹⁸O) are not always sufficient to fully constrain the CO₂ budget. Additional constraints may help in understanding the mechanisms of CO₂ production and consumption. The anomaly in triple oxygen isotopes or ¹⁷O excess (denoted by Δ¹⁷O) and molecules containing two rare isotopes, called clumped isotopes, are two recently developed tracers with potentials to independently constrain some important processes that regulate CO₂ in the atmosphere. The clumped isotope for CO₂, denoted by Δ₄₇, is the excess of ¹³C¹⁶O¹⁸O over a random distribution of isotopes in a CO₂ molecule. We measured the concentrations of δ¹³C, δ¹⁸O, Δ¹⁷O, and Δ₄₇ in air CO₂ samples collected from the Hsuehshan tunnel (length: 12.9 km), and applied linear and polynomial regressions to obtain the fossil fuel end-members for all these isotope proxies. The other end-members, the values of all these proxies for background air CO₂, are either assumed or taken as the values obtained over the tunnel and ocean. The fossil fuel (anthropogenic) CO₂ end-member values for δ¹³C, δ¹⁸O, Δ¹⁷O, and Δ₄₇ are estimated using the two component mixing approach: the derived values are -26.76 ± 0.25‰, 24.57 ± 0.33‰, -0.219 ± 0.021‰, and 0.267 ± 0.036‰, respectively. These four major CO₂ isotope tracers along with the concentration were used to estimate the anthropogenic contribution in the atmospheric CO₂ in urban and suburban locations. We demonstrate that Δ¹⁷O and Δ₄₇ have the potential to independently estimate anthropogenic contribution, and the advantages of these two over the conventional isotope proxies are discussed.

An improved method of high-precision determination of Δ(17)O of CO2 by catalyzed exchange with O2 using hot platinum

RATIONALE

CO2 and O2 can exchange their oxygen isotopes rapidly in the presence of hot (~670 °C) platinum and this has led to a method for determining the δ(17)O value of a CO2 sample. We have improved the method to achieve a precision of 0.008 ‰ (1-σ standard deviation) in the determination of δ(17)O values. Such high precision is essential to identify the stratospheric component in tropospheric CO2 and use it for global carbon flux studies. The crucial issue in the accurate determination of the δ(17)O value is estimation of a correction factor, which depends on the amount ratio CO2/O2. An attempt was also made to investigate the mechanism of exchange with their controlling parameters.

METHODS

The oxygen isotopes of a CO2 sample gas are exchanged with those of an appropriate amount of tank O2 in the presence of hot platinum. The pre-exchange CO2 and O2 gas samples as well as the post-exchange O2 sample are analyzed by isotope ratio mass spectrometry. A mixing model was developed involving the δ(18)O value of the CO2 and δ(17)O and δ(18)O values of pre- and post-exchange O2 to obtain the δ(17)O value of the CO2 sample. A correction to the measured value was determined to obtain the actual value with high accuracy and precision.

RESULTS

To obtain a precision better than 0.01 ‰ requires the amount ratio CO2/O2 to be controlled to better than ~15 %. We also find that the oxygen isotopes are nearly homogeneously distributed between the O2 and the CO2 molecules. In addition, determination of the (16) O(13)C(18)O/(16)O(12)C(16)O isotopologue ratio in the CO2 shows that the abundance of (16)O(13)C(18)O is close to that expected for random partitioning of the isotopes among the CO2 isotopologues.

CONCLUSIONS

The isotopic scrambling between O2 and CO2 that occurs on hot platinum allows one to accurately determine the δ(17)O values of CO2 through isotopic analysis of O2.