An improved CeO2 method for high-precision measurements of 17O/16O ratios for atmospheric carbon dioxide

Preparation of high-precision CO2 with known triple oxygen isotope for oxygen isotope analysis

The objective of this work is to propose a more effective way to prepare an in-house CO₂ with known triple oxygen isotope compositions. The major experimental steps include: (1) the O₂ is combusted to CO₂ on a graphite rod at 750 °C with Pt-catalyst for 3-4 min; and (2) converted CO₂ is subsequently purified by two cryogenic traps. The results show high reproducibility of δ¹³C and δ¹⁸O values of the converted CO₂ within 0.010-0.020 ‰ and 0.006-0.010 ‰ (1σ, SD), and the identical δ¹⁸O value within error with that of the original O₂. Additionally, we have measured the triple oxygen isotope compositions of converted CO₂ using an O₂-CO₂ Pt-catalyzed oxygen-isotope equilibration method. The measured δ¹⁷O values of CO₂ show high reproducibility within 0.006 ‰ (1σ, SD), and are identical within error with the original O₂ as well. Notably, our experiments also found that the O₂ with heavier oxygen isotope ratios (δ¹⁸O > 40 ‰, VSMOW) might have a lesser conversion efficiency, and this effect, combined with the lighter isotope preferential fractionations during the reaction processes of O₂ to CO and CO to CO₂, may explain the observed lower ¹⁷O/¹⁶O and ¹⁸O/¹⁶O ratios of the converted CO₂ relative to the original O₂.

An optimized multicopter UAV sounding technique (MUST) for probing comprehensive atmospheric variables

The unique maneuverability, ease of deployment, simplicity in logistics, and relatively low costs of multicopters render them effective vehicles for low atmospheric research. While many efforts have contributed to the fundamental success of atmospheric applications of multicopters in the past, several challenges remain, including limited measurable variables, possible response-delay in real-time observations, insufficient measurement accuracy, endurance of harsh conditions and tolerance towards interferences. To address these challenges and further fortify the applicability in diversified research disciplines, this study developed an optimized multicopter UAV sounding technique (MUST). The MUST serves as an integrated platform by combining self-developed algorithms, optimized working environments for sensors/monitors, and retrofitted sampling devices to probe a comprehensive set of atmospheric variables. These variables of interest include meteorological parameters (temperature, relative humidity, pressure, wind direction and speed), the chemical composition (speciated VOCs, CO, CO₂, CH₄, CO₂ isotopologues, O₃, PM2.5, and black carbon), and the radiation flux, as well as visible and thermal images. The aim of this study is to achieve the following objectives: 1. to easily probe a comprehensive set of near-surface atmospheric variables; 2. to improve data quality by correcting for sensors' delay in real-time observations and minimizing environmental interferences; and 3. to enhance the versatility and applicability of aerial measurements by incorporating necessary hardware and software. Field launching cases from the surface to a maximum height of 1000 m were conducted to validate the robustness of the integrated MUST platform with sufficient speed, accuracy and resolution for the target variables.

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.

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.

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.

Development of a multicopter-carried whole air sampling apparatus and its applications in environmental studies

To advance the capabilities of probing chemical composition aloft, we designed a lightweight remote-controlled whole air sampling component (WASC) and integrated it into a multicopter drone with agile maneuverability to perform aerial whole air sampling. A field mission hovering over an exhaust shaft of a roadway tunnel to collect air samples was performed to demonstrate the applicability of the multicopter-carried WASC apparatus. Ten aerial air samples surrounding the shaft vent were collected by the multicopter-carried WASC. Additional five samples were collected manually inside the shaft for comparison. These samples were then analyzed in the laboratory for the chemical composition of 109 volatile organic compounds (VOCs), CH4, CO, CO2, or CO2 isotopologues. Most of the VOCs in the upwind samples (the least affected by shaft exhaust) were low in concentrations (5.9 ppbv for total 109 VOCs), posting a strong contrast to those in the shaft exhaust (235.8 ppbv for total 109 VOCs). By comparing the aerial samples with the in-shaft samples for chemical compositions, the influence of the shaft exhaust on the surrounding natural air was estimated. Through the aerial measurements, three major advantages of the multicopter-carried WASC were demonstrated: 1. The highly maneuverable multicopter-carried WASC can be readily deployed for three-dimensional environmental studies at a local scale (0-1.5 km); 2. Aerial sampling with superior sample integrity and preservation conditions can now be performed with ease; and 3. Data with spatial resolution for a large array of gaseous species with high precision can be easily obtained.

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.

Oxygen anomaly in near surface carbon dioxide reveals deep stratospheric intrusion

Stratosphere-troposphere exchange could be enhanced by tropopause folding, linked to variability in the subtropical jet stream. Relevant to tropospheric biogeochemistry is irreversible transport from the stratosphere, associated with deep intrusions. Here, oxygen anomalies in near surface air CO₂ are used to study the irreversible transport from the stratosphere, where the triple oxygen isotopes of CO₂ are distinct from those originating from the Earth’s surface. We show that the oxygen anomaly in CO₂ is observable at sea level and the magnitude of the signal increases during the course of our sampling period (September 2013-February 2014), concordant with the strengthening of the subtropical jet system and the East Asia winter monsoon. The trend of the anomaly is found to be 0.1‰/month (R² = 0.6) during the jet development period in October. Implications for utilizing the oxygen anomaly in CO₂ for CO₂ biogeochemical cycle study and stratospheric intrusion flux at the surface are discussed.