CO2and CH4 Research Articles

Seamless mapping of long-term (2010–2020) daily global XCO2 and XCH4 from the Greenhouse Gases Observing Satellite (GOSAT), Orbiting Carbon Observatory 2 (OCO-2), and CAMS global greenhouse gas reanalysis (CAMS-EGG4) with a spatiotemporally self-supervised fusion method

Abstract. Precise and continuous monitoring of long-term carbon dioxide (CO2) and methane (CH4) over the globe is of great importance, which can help study global warming and achieve the goal of carbon neutrality. Nevertheless, the available observations of CO2 and CH4 from satellites are generally sparse, and current fusion methods to reconstruct their long-term values on a global scale are few. To address this problem, we propose a novel spatiotemporally self-supervised fusion method to establish long-term daily seamless XCO2 and XCH4 products from 2010 to 2020 over the globe on grids of 0.25∘. A total of three datasets are applied in our study, including the Greenhouse Gases Observing Satellite (GOSAT), the Orbiting Carbon Observatory 2 (OCO-2), and CAMS global greenhouse gas reanalysis (CAMS-EGG4). Attributed to the significant sparsity of data from GOSAT and OCO-2, the spatiotemporal discrete cosine transform is considered for our fusion task. Validation results show that the proposed method achieves a satisfactory accuracy, with standard deviations of bias (σ) of ∼1.18 ppm for XCO2 and 11.3 ppb for XCH4 against Total Carbon Column Observing Network (TCCON) measurements from 2010 to 2020. Meanwhile, the determination coefficients (R2) of XCO2 and XCH4 reach 0.91 or 0.95 (2010–2014 or 2015–2020) and 0.9 (2010–2020), respectively, after fusion. Overall, the performance of fused results distinctly exceeds that of CAMS-EGG4, which is also superior or close to those of GOSAT and OCO-2. In particular, our fusion method can effectively correct the large biases in CAMS-EGG4 due to the issues from assimilation data, such as the unadjusted anthropogenic emission inventories for COVID-19 lockdowns in 2020. Moreover, the fused results present coincident spatial patterns with GOSAT and OCO-2, which accurately display the long-term and seasonal changes in globally distributed XCO2 and XCH4. The daily global seamless gridded (0.25∘) XCO2 and XCH4 from 2010 to 2020 can be freely accessed at https://doi.org/10.5281/zenodo.7388893 (Wang et al., 2022a).

Comparison of shale depth functions in contrasting offshore basins and sealing behaviour for CH 4 and CO 2 containment systems

Mudrock compaction trends from the Rovuma Basin offshore Mozambique are compared with those of the Norwegian North Sea, the Gulf of Mexico and the Kutai Basin offshore Indonesia. The comparison reveals that burial rates and timing of rifting are the dominant causes for the differences observed. The compaction trend for the Rovuma Basin is broadly similar to the trends for the Kutai Basin and the Gulf of Mexico, but very different from those for the Norwegian North Sea data, which show higher porosity and shallower onset of overpressure than those from the other three basins. The relationships for seismic velocities as a function of depth show strong similarities between the Rovuma and Gulf of Mexico basins. These comparisons are used to make a general assessment of the capillary sealing potential of Cretaceous mudrocks in the Rovuma Basin, using a mudstone permeability-prediction function and a method for mapping permeability to threshold pressure, allowing estimation of maximum column heights for CO 2 and CH 4 , with uncertainty ranges. Predicted CO 2 column heights are slightly less than the equivalent CH 4 column heights. The observed CH 4 column height at one of the wells is significantly lower than that predicted from mudstone permeability, which is probably due to other factors such as fracturing or gas migration out of the structure. The comparison indicates generally good capillary sealing potential for the Rovuma Basin Cretaceous shales and offers a general approach for assessing CO 2 storage potential from hydrocarbon sealing datasets from multiple offshore basins. This article is part of the Energy Geoscience Series available at https://www.lyellcollection.org/cc/energy-geoscience-series

Using atmospheric trace gas vertical profiles to evaluate model fluxes: a case study of Arctic-CAP observations and GEOS simulations for the ABoVE domain

Abstract. Accurate estimates of carbon–climate feedbacks require an independent means for evaluating surface flux models at regional scales. The altitude-integrated enhancement (AIE) derived from the Arctic Carbon Atmospheric Profiles (Arctic-CAP) project demonstrates the utility of this bulk quantity for surface flux model evaluation. This bulk quantity leverages background mole fraction values from the middle free troposphere, is agnostic to uncertainties in boundary layer height, and can be derived from model estimates of mole fractions and vertical gradients. To demonstrate the utility of the bulk quantity, six airborne profiling surveys of atmospheric carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO) throughout Alaska and northwestern Canada between April and November 2017 were completed as part of NASA's Arctic–Boreal Vulnerability Experiment (ABoVE). The Arctic-CAP sampling strategy involved acquiring vertical profiles of CO2, CH4, and CO from the surface to 5 km altitude at 25 sites around the ABoVE domain on a 4- to 6-week time interval. All Arctic-CAP measurements were compared to a global simulation using the Goddard Earth Observing System (GEOS) modeling system. Comparisons of the AIE bulk quantity from aircraft observations and GEOS simulations of atmospheric CO2, CH4, and CO highlight the fidelity of the modeled surface fluxes. The model–data comparison over the ABoVE domain reveals that while current state-of-the-art models and flux estimates are able to capture broad-scale spatial and temporal patterns in near-surface CO2 and CH4 concentrations, more work is needed to resolve fine-scale flux features that are captured in CO observations.

GFIT3: a full physics retrieval algorithm for remote sensing of greenhouse gases in the presence of aerosols

Abstract. Remote sensing of greenhouse gases (GHGs) in cities, where high GHG emissions are typically associated with heavy aerosol loading, is challenging due to retrieval uncertainties caused by the imperfect characterization of scattering by aerosols. We investigate this problem by developing GFIT3, a full physics algorithm to retrieve GHGs (CO2 and CH4) by accounting for aerosol scattering effects in polluted urban atmospheres. In particular, the algorithm includes coarse- (including sea salt and dust) and fine- (including organic carbon, black carbon, and sulfate) mode aerosols in the radiative transfer model. The performance of GFIT3 is assessed using high-spectral-resolution observations over the Los Angeles (LA) megacity made by the California Laboratory for Atmospheric Remote Sensing Fourier transform spectrometer (CLARS-FTS). CLARS-FTS is located on Mt. Wilson, California, at 1.67 km a.s.l. overlooking the LA Basin, and it makes observations of reflected sunlight in the near-infrared spectral range. The first set of evaluations are performed by conducting retrieval experiments using synthetic spectra. We find that errors in the retrievals of column-averaged dry air mole fractions of CO2 (XCO2) and CH4 (XCH4) due to uncertainties in the aerosol optical properties and atmospheric a priori profiles are less than 1 % on average. This indicates that atmospheric scattering does not induce a large bias in the retrievals when the aerosols are properly characterized. The methodology is then further evaluated by comparing GHG retrievals using GFIT3 with those obtained from the CLARS-GFIT algorithm (used for currently operational CLARS retrievals) that does not account for aerosol scattering. We find a significant correlation between retrieval bias and aerosol optical depth (AOD). A comparison of GFIT3 AOD retrievals with collocated ground-based observations from AErosol RObotic NETwork (AERONET) shows that the developed algorithm produces very accurate results, with biases in AOD estimates of about 0.02. Finally, we assess the uncertainty in the widely used tracer–tracer ratio method to obtain CH4 emissions based on CO2 emissions and find that using the CH4/CO2 ratio effectively cancels out biases due to aerosol scattering. Overall, this study of applying GFIT3 to CLARS-FTS observations improves our understanding of the impact of aerosol scattering on the remote sensing of GHGs in polluted urban atmospheric environments. GHG retrievals from CLARS-FTS are potentially complementary to existing ground-based and spaceborne observations to monitor anthropogenic GHG fluxes in megacities.

AtmosphericCO2/CH4permeability ofEVAcopolymer/SiO2composite membrane for biogas purification

AbstractThe CO2and CH4permeabilities of poly(ethylene‐co‐vinyl acetate) (EVA)/SiO2composite membrane were investigated at atmospheric pressure. The membranes were fabricated by compression molding and characterized by Fourier transformed infrared spectroscopy, differential scanning calorimetry, a universal testing machine, and a contact angle analyzer. The effect of vinyl acetate content (18–33 wt%) was evaluated for both single‐gas and mixed‐gas permeation systems. A non‐pressurized homemade‐permeation cell was used for the single‐gas permeation of CO2and CH4, while a tubular membrane was utilized for a continuous separation of CO2/CH4mixture. CO2flux was readily increased (from 0.7 to 2.0 ml/m2.s) with vinyl acetate content (18–33 wt%). The enhanced CO2permeability is attributed to the increase in polarity and also the decrease in crystallinity of the membrane. A satisfied gas separation selectivity (CO2/CH4) of 4.31 could be obtained from tubular membrane with 28 wt% VA content. The incorporation of SiO2as a filler (0.5–2.0 wt%) especially increased the membrane polarity and hence the CO2flux up to 6.0 ml/m2.s. However, the CH4flux was not affected by VA and SiO2contents.

The Seismotectonic Significance of Geofluids in Italy

There is growing interest in how geofluid emissions are released in the atmosphere by the planet’s geodynamic activity, and how much they contribute to the global budget of greenhouse gases. Many workers are addressing this issue with studies conducted at global scale, so as to get the required global-scale answers. The data available at the global scale on geofluids, faults, earthquakes and volcanoes, however, are generally too coarse to provide these answers. We investigate the relationships between geofluid emissions and tectonics at a more detailed scale. Building on over a century of data on geofluid emissions and on an extensive knowledge of the region’s tectonics and seismicity, we focused on Italy, one of the areas of the globe that experience the largest release of natural CO2and CH4. We systematically overlaid and compared data collected by a number of workers into 13 published countrywide databases concerning geofluid emissions, carbon-bearing deposits, seismogenic faults, historical and instrumentally documented earthquakes, and heat flow observations. Our results indicate that 1) thermal springs and CO2emissions dominate in areas of mantle upwelling and crustal stretching, but also that 2) some of them occur in the extending inner Apennines, generally along major lithospheric chain-perpendicular lineaments that bound the largest normal faults. Conversely, 3) CH4emissions and mud volcanoes dominate in areas undergoing active contraction, where no CO2emissions are observed; in particular, we find 4) that mud volcanoes concentrate where the crests of active anticlines intersect major lithospheric chain-perpendicular lineaments. An overarching conclusion is that, in Italy, the release of geofluids is primarily controlled by deep crustal discontinuities that developed over the course of 5–10 My, and is only mildly affected by ongoing crustal strains. Geofluid emissions bring information on processes that occur primarily in the lower crust, marking the surface projection of generally hidden discontinuities that control the geometry and modes of seismic release. As such they may also provide valuable insight for improving the assessment of seismic hazard in hard-to-investigate seismically active regions, such as Italy.

Calixarene-based porous 3D polymers and copolymers with high capacity and binding energy for CO2, CH4and Xe capture

Calixarene-based porous polymers and copolymers with surface area of 3000 m2g−1selectively capture CO2and CH4(DH of 35 and 24 kJ mol−1, respectively) in specific sites, as revealed by microcalorimetry, breakthrough experiments and HP129Xe NMR.

A Study on the High Load Operation of a Natural Gas-Diesel Dual-Fuel Engine

Diesel fueled compression ignition engines are widely used in power generation and freight transport owing to their high fuel conversion efficiency and ability to operate reliably for long periods of time at high loads. However, such engines generate significant amounts of carbon dioxide (CO2), nitrogen oxides (NOx), and particulate matter (PM) emissions. One solution to reduce the CO2and particulate matter emissions of diesel engines while maintaining their efficiency and reliability is natural gas (NG)-diesel dual-fuel combustion. In addition to methane emissions, the temperatures of the diesel injector tip and exhaust gas can also be concerns for dual-fuel engines at medium and high load operating conditions. In this study, a single cylinder NG-diesel dual-fuel research engine is operated at two high load conditions (75% and 100% load). NG fraction and diesel direct injection (DI) timing are two of the simplest control parameters for optimization of diesel engines converted to dual-fuel engines. In addition to studying the combined impact of these parameters on combustion and emissions performance, another unique aspect of this research is the measurement of the diesel injector tip temperature which can predict potential coking issues in dual-fuel engines. Results show that increasing NG fraction and advancing diesel direct injection timing can increase the injector tip temperature. With increasing NG fraction, while the methane emissions increase, the equivalent CO2emissions (cumulative greenhouse gas effect of CO2and CH4) of the engine decrease. Increasing NG fraction also improves the brake thermal efficiency of the engine though NOx emissions increase. By optimizing the combustion phasing through control of the DI timing, brake thermal efficiencies of the order of ∼42% can be achieved. At high loads, advanced diesel DI timings typically correspond to the higher maximum cylinder pressure, maximum pressure rise rate, brake thermal efficiency and NOx emissions, and lower soot, CO, and CO2-equivalent emissions.

CLAYEY SOIL AMENDMENT BY HYDROPHILIC NANO BENTONITE FOR LANDFILL COVER BARRIER: A CASE STUDY

Methane and carbon dioxide are of major concern as greenhouse gases; the landfills have the problem of controlling these gases. Al Akaider in Jordan is the second biggest landfill suffers controlling gases as it lacks a cover design system. In this work, the main goal is to investigate the appropriateness of amended expansive clayey soil in Irbid as a cover barrier. The expansive soil is unwanted in construction projects, thus the modification of this expelled soil enables using it as a low cost landfill cover barrier. In this research, the effect of adding nano-clay material (Hydrophilic Nano Bentonite) on the geotechnical characteristics, hydraulic conductivity, and gas transport coefficients of the clayey soil are studied. The soil samples were obtained from Irbid city. Unconfined compressive strength and free swelling tests were performed on soil samples with different percentages of nano-clay added in the range (0.1% to 1.2%) by weight. The results indicated that the addition of nano-clay at low percentages increases the strength of expansive soil up to 315 kPa at 0.6% of nano-clay and the swelling potential decreased dramatically with the addition of nano-clay. The optimal percent of nano-clay was found to be 0.6%. The intrinsic permeability of the amended soil was 6.03×10–15 m2. The average values of fluid transport coefficients were determined at 25 °C. The hydraulic conductivity for water was about 6.5×10–10 m/s. Gas conductivity coefficients for CO2 and CH4 were 5×10–9 m/s and 2.5×10–9 m/s respectively. Gas diffusion coefficients for CO2 and CH4 were 3×10–6 m2/s and 4×10–6 m2/s respectively.

Evaluating Radiometric Responsivity Degradations of TANSO-FTS/GOSAT Using Principal Component Analysis

AbstractThe short-wavelength infrared bands of the Thermal and Near-Infrared Sensor for Carbon Observation (TANSO)–Fourier transform spectrometer (FTS) instrument on board theGreenhouse Gas Observing Satellite(GOSAT) have degraded, which affects the retrieval of data for CO2and CH4. Herein, a new algorithm that uses principal component analysis (PCA) to evaluate these degradations from on-orbit solar calibration spectra has been developed. The datasets of the spectra were decomposed using PCA, and the temporal variations of their components were fitted using the appropriate functions. Our results show that PCA is effective to construct a suitable degradation model for TANSO-FTS. Comparisons of CO2data retrieved using the new degradation model with that using the ground-based FTS indicate that the new model improves the measurement biases.

In situ measurement of CO<sub>2</sub> and CH<sub>4</sub> from aircraft over northeast China and comparison with OCO-2 data

Abstract. Several satellites have been launched to monitor the increasing concentrations of greenhouse gases, especially CO2 and CH4 in the atmosphere, through backscattered hyperspectral radiance in the shortwave infrared (SWIR) band. The vertical profiles of greenhouse gases and aerosols could strongly affect the results from these instruments. To investigate the effects of the vertical distribution of CO2 on the uncertainty of SWIR satellite retrieval results, we conducted observations of the vertical profiles of CO2, CH4 and aerosol particles at 0.6–7 km above sea level using a Beechcraft King Air 350ER in Jiansanjiang (46.77∘ N, 131.99∘ E), Heilongjiang Province, northeast China, on 7–12 August 2018. The profiles from this aircraft captured a decrease in CO2 from 2 km to the minimum altitude due to the absorption of vegetation at the surface in summer. CH4 measurements showed about a 0.2 ppm increase from 2.0 to 0.6 km on 10 August, which may result from emissions from the large area of paddy fields below, and a constant mole fraction between 1.951 and 1.976 ppm was recorded at 2 km and above. Comparison of CO2 profiles from a new version of the carbon cycle data assimilation system Tan-Tracker (v1), retrievals from OCO-2 and aircraft measurements was conducted. The results from OCO-2 and the assimilation model system Tan-Tracker captured the vertical structure of CO2 above 3 km, whereas below 3 km the values from OCO-2 and the Tan-Tracker model were lower than those from in situ measurements. Column-averaged CO2 volume mole fractions calculated from in situ measurements showed biases of -4.68±0.44 ppm (-1.18±0.11%) compared to OCO-2 retrievals.

The use of the 1.27 µm O<sub>2</sub> absorption band for greenhouse gas monitoring from space and application to MicroCarb

Abstract. Monitoring CO2 from space is essential to characterize the spatiotemporal distribution of this major greenhouse gas and quantify its sources and sinks. The mixing ratio of CO2 to dry air can be derived from the CO2∕O2 column ratio. The O2 column is usually derived from its absorption signature on the solar reflected spectra over the O2 A band (e.g. Orbiting Carbon Observatory-2 (OCO-2), Thermal And Near infrared Sensor for carbon Observation (TANSO)/Greenhouse Gases Observing Satellite (GOSAT), TanSat). As a result of atmospheric scattering, the atmospheric path length varies with the aerosols' load, their vertical distribution, and their optical properties. The spectral distance between the O2 A band (0.76 µm) and the CO2 absorption band (1.6 µm) results in significant uncertainties due to the varying spectral properties of the aerosols over the globe. There is another O2 absorption band at 1.27 µm with weaker lines than in the A band. As the wavelength is much closer to the CO2 and CH4 bands, there is less uncertainty when using it as a proxy of the atmospheric path length to the CO2 and CH4 bands. This O2 band is used by the Total Carbon Column Observing Network (TCCON) implemented for the validation of space-based greenhouse gas (GHG) observations. However, this absorption band is contaminated by the spontaneous emission of the excited molecule O2*, which is produced by the photo-dissociation of O3 molecules in the stratosphere and mesosphere. From a satellite looking nadir, this emission has a similar shape to the absorption signal that is used. In the frame of the CNES (Centre National d'Études Spatiales – the French National Centre for Space Studies) MicroCarb project, scientific studies have been performed in 2016–2018 to explore the problems associated with this O2* airglow contamination and methods to correct it. A theoretical synthetic spectrum of the emission was derived from an approach based on A21 Einstein coefficient information contained in the line-by-line high-resolution transmission molecular absorption (HITRAN) 2016 database. The shape of our synthetic spectrum is validated when compared to O2* airglow spectra observed by the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY)/Envisat in limb viewing. We have designed an inversion scheme of SCIAMACHY limb-viewing spectra, allowing to determine the vertical distribution of the volume emission rate (VER) of the O2* airglow. The VER profiles and corresponding integrated nadir intensities were both compared to a model of the emission based on the Reactive Processes Ruling the Ozone Budget in the Stratosphere (REPROBUS) chemical transport model. The airglow intensities depend mostly on the solar zenith angle (both in model and data), and the model underestimates the observed emission by ∼15 %. This is confirmed with SCIAMACHY nadir-viewing measurements over the oceans: in such conditions, we have disentangled and retrieved the nadir O2* emission in spite of the moderate spectral resolving power (∼860) and found that the nadir SCIAMACHY intensities are mostly dictated by solar zenith angle (SZA) and are larger than the model intensities by a factor of ∼1.13. At a fixed SZA, the model airglow intensities show very little horizontal structure, in spite of ozone variations. It is shown that with the MicroCarb spectral resolution power (25 000) and signal-to-noise ratio (SNR), the contribution of the O2* emission at 1.27 µm to the observed spectral radiance in nadir viewing may be disentangled from the lower atmosphere/ground absorption signature with a great accuracy. Indeed, simulations with 4ARCTIC radiative transfer inversion tool have shown that the CO2 mixing ratio may be retrieved with the accuracy required for quantifying the CO2 natural sources and sinks (pressure-level error ≤1 hPa; XCO2 accuracy better than 0.4 ppmv) with the O2 1.27 µm band only as the air proxy (without the A band). As a result of these studies (at an intermediate phase), it was decided to include this band (B4) in the MicroCarb design, while keeping the O2 A band for reference (B1). Our approach is consistent with the approach of Sun et al. (2018), who also analysed the potential of the O2 1.27 µm band and concluded favourably for GHG monitoring from space. We advocate for the inclusion of this O2 band on other GHG monitoring future space missions, such as GOSAT-3 and EU/European Space Agency (ESA) CO2-M missions, for a better GHG retrieval.

Warming enhances carbon dioxide and methane fluxes from Red Sea seagrass (<i>Halophila stipulacea</i>) sediments

Abstract. Seagrass meadows are autotrophic ecosystems acting as carbon sinks, but they have also been shown to be sources of carbon dioxide (CO2) and methane (CH4). Seagrasses can be negatively affected by increasing seawater temperatures, but the effects of warming on CO2 and CH4 fluxes in seagrass meadows have not yet been reported. Here, we examine the effect of two disturbances on air–seawater fluxes of CO2 and CH4 in Red Sea Halophila stipulacea communities compared to adjacent unvegetated sediments using cavity ring-down spectroscopy. We first characterized CO2 and CH4 fluxes in vegetated and adjacent unvegetated sediments, and then experimentally examined their response, along with that of the carbon (C) isotopic signature of CO2 and CH4, to gradual warming from 25 ∘C (winter seawater temperature) to 37 ∘C, 2 ∘C above current maximum temperature. In addition, we assessed the response to prolonged darkness, thereby providing insights into the possible role of suppressing plant photosynthesis in supporting CO2 and CH4 fluxes. We detected 6-fold-higher CO2 fluxes in vegetated compared to bare sediments, as well as 10- to 100-fold-higher CH4 fluxes. Warming led to an increase in net CO2 and CH4 fluxes, reaching average fluxes of 10 422.18 ± 2570.12 µmol CO2 m−2 d−1 and 88.11±15.19 µmol CH4 m−2 d−1, while CO2 and CH4 fluxes decreased over time in sediments maintained at 25 ∘C. Prolonged darkness led to an increase in CO2 fluxes but a decrease in CH4 fluxes in vegetated sediments. These results add to previous research identifying Red Sea seagrass meadows as a significant source of CH4, while also indicating that sublethal warming may lead to increased emissions of greenhouse gases from seagrass meadows, providing a feedback mechanism that may contribute to further enhancing global warming.

Dry Reforming of Methane Using Cold Plasma; Kinetic Model Study

In this work, the dry reforming of methane was studied using a corona and gliding discharge plasma microreactors. A chemical kinetic model was developed to describe the experimental behavior observed. The kinetic model is proposed based on the assumption that the reactant molecules CH4 or CO2 are attacked by active species produced by the plasma discharges, and the production of this active species are a function of the plasma power. The modelization allowed the prediction of the conversion of the reactants (CH4 and CO2) according to the energy transfer to the gas (). The value is characteristic of the energy cost; the lower β value indicated better efficiency. The value of CH4 was found to be 10.42 and 58.25 J and for CO2 equal to 12.24 and 27.77 J for corona and gliding discharge plasma, respectively. The kinetic model also demonstrated that the methane and carbon dioxide conversion was an exponential function of the plasma energy, and were a linear function of the input energy for a CH4 and CO2 inlet concentration. Our Model also implied that a plasma reactor with a smaller input discharge power has better energy efficiency for CO2 andCH4 conversion.

On the zero-level offset in the GOSAT TANSO-FTS O<sub>2</sub> A band and the quality of solar-induced chlorophyll fluorescence (SIF): comparison of SIF between GOSAT and OCO-2

Abstract. Satellite remote sensing of solar-induced chlorophyll fluorescence (SIF) has attracted attention as a method for improving the estimation accuracy of the photosynthetic production of terrestrial vegetation in recent years. The Greenhouse gases Observing Satellite (GOSAT) has the ability to observe both SIF and the concentrations of CO2 and CH4 and thus is expected to contribute to the understanding of the global carbon budget. Evaluating artefact signals (e.g. zero-level offset caused by non-linearity in the analogue circuit in the case of GOSAT) is effective for inferring the instrument status and important for retrieving SIF from satellite measurements. Here we investigate the characteristics of the zero-level offset and the consistency of satellite-derived SIFs by comparing the derived SIF with the Orbiting Carbon Observatory-2 (OCO-2) SIF at multiple spatial scales (footprint to global). The zero-level offset was evaluated using filling-in signals over bare soil while investigating the criteria for identifying barren areas. An analysis of the temporal variation of the zero-level offset over a period of 9 years suggests that the radiometric sensitivity of the GOSAT spectrometer changed after switching the optics path selector in January 2015. The GOSAT SIF was highly consistent with the OCO-2 SIF, with a bias within 0.1 mW m−2 nm−1 sr−1 for most months and an inter-region bias of about 0.2 mW m−2 nm−1 sr−1. Our results agree with the previous comparisons and support the consistency among the present satellite SIF data, which is important for the utilization of those data.

Variations in dissolved greenhouse gases (CO<sub>2</sub>, CH<sub>4</sub>, N<sub>2</sub>O) in the Congo River network overwhelmingly driven by fluvial-wetland connectivity

Abstract. We report the spatial variations of dissolved carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) concentrations in the lowland part of the Congo River network obtained during ten field expeditions carried out between 2010 and 2015, in the eastern part of the basin (Democratic Republic of Congo). Two transects of 1,650 km were carried out from the city of Kisangani to the city of Kinshasa, along the longest possible navigable section of the river, and corresponding to 41 % of the total length of the mainstem. Additionally, three time series of CH4 and N2O were obtained at fixed points in the mainstem of the middle Congo (2013–2018, biweekly sampling), in the mainstem of the lower Kasai (2015–2017, monthly sampling), and in the mainstem of the middle Oubangui (2010–2012, biweekly sampling). The variations of dissolved N2O concentrations were modest, with values oscillating around the concentration corresponding to saturation with the atmosphere, with N2O saturation level (%N2O) ranging between 0 % and 561 % (average 142 %). The relatively narrow range of %N2O variations was consistent with low NH4+ (2.3±1.3 µmol L−1) and NO3− (5.6±5.1 µmol L−1) levels in these near pristine rivers and streams with low agriculture pressure on the catchment (croplands correspond to 0.1 % of catchment land cover of sampled rivers), dominated by forests (~ 70 % of land cover). The co-variations of %N2O, NH4+, NO3−, and dissolved oxygen saturation level (%O2) indicate N2O removal by sedimentary denitrification in low O2, high NH4+ and low NO3− environments (typically small and organic matter rich streams) and N2O production by nitrification in high O2, low NH4+ and high NO3− (typical of larger rivers that are poor in organic matter). Surface waters were very strongly over-saturated in CO2 and CH4 with respect to atmospheric equilibrium, with values of the partial pressure of CO2 (pCO2) ranging between 1,087 and 22,899 ppm (equilibrium ~ 400 ppm), and dissolved CH4 concentrations ranging between 22 and 71,428 nmol L−1 (equilibrium ~ 2 nmol L−1). Spatial variations were overwhelmingly more important than seasonal variations for pCO2, CH4 and %N2O, and than diurnal (day-night) variations for pCO2. The wide range of pCO2 and CH4 variations was consistent with the equally wide range of %O2 (0.3–122.8 %) and of dissolved organic carbon (DOC) (1.8–67.8 mg L−1), indicative of intense processing of organic matter that generated these two greenhouse gases. However, the emission rate of CO2 to the atmosphere from riverine surface waters was on average about 10 times higher than the flux of CO2 produced by aquatic net heterotrophy (as evaluated from measurements of pelagic respiration and primary production). This indicates that the CO2 emissions from the river network were sustained by lateral inputs of CO2 (either from terra firme or from wetlands). The pCO2 and CH4 values decreased and %O2 increased with increasing Strahler order, showing that stream size explains part of the spatial variability of these quantities. In addition, several lines of evidence indicate that lateral inputs of carbon from wetlands (flooded forest and aquatic macrophytes) were of paramount importance in sustaining high CO2 and CH4 concentrations in the Congo river network, as well as driving spatial variations: the rivers draining the Cuvette Centrale Congolaise (CCC) (a giant wetland of flooded forest in the core of the Congo basin) were characterized by significantly higher pCO2 and CH4 and significantly lower %O2 and %N2O values than those not draining the CCC; pCO2 and %O2 values were correlated to the coverage of flooded forest on the catchment. The flux of GHGs between rivers and the atmosphere averaged 2,469 mmol m−2 d−1 for CO2 (range 86 and 7,110 mmol m−2 d−1), 12,553 µmol m−2 d−1 for CH4 (range 65 and 597,260 µmol m−2 d−1), 22 µmol m−2 d−1 for N2O (range −52 and 319 µmol m−2 d−1). The estimate of integrated CO2 emission from the Congo River network (251 TgC (1012 gC) yr−1) corresponded to nearly half the CO2 emissions from tropical oceans globally (565 TgC yr−1) and was nearly two times the CO2 emissions from the tropical Atlantic Ocean (137 TgC yr−1). Moreover, the integrated CO2 emission from the Congo River network is more than three times higher than the estimate of terrestrial net ecosystem exchange (NEE) on the whole catchment (77 TgC yr−1). This shows that it is unlikely that the CO2 emissions from the river network were sustained by the hydrological carbon export from terra firme soils (typically very small compared to terrestrial NEE), but most likely, to a large extent, they were sustained by wetlands (with a much higher hydrological connectivity with rivers and streams).

Evaluating long‐term ivermectin use and the role of dung beetles in reducing short‐term CH4and CO2emissions from livestock faeces: a mesocosm design under Mediterranean conditions

1. Greenhouse gas (GHG) emissions from livestock contribute significantly to global warming, and a reduction of this source of emissions is crucial in achieving the goal of mitigating global warming.2. CO2and CH4emissions from dung pats were analysed by means of a mesocosm experiment in a Mediterranean ecosystem. The experiment consisted of a total of 30 mesocosms distributed across three treatments: a well‐preserved, undisturbed dung beetle assemblage associated with organic livestock; a dung beetle assemblage that was impoverished as a result of the long‐term use of veterinary medical products; and a control treatment without dung beetles.3. Corrections related to insect respiration allow researchers to provide more precise measurements of CO2emissions from dung, especially in the initial and final phases of dung exposure, when the percentage of CO2emitted by dung beetles can become greater than the emissions from the dung pats themselves.4. The effects of dung beetles on CO2and CH4emissions are much more accentuated in warm‐temperate conditions than in northern temperate areas previously studied. Mediterranean assemblages remove and spread dung faster and more effectively than do northern dung beetle assemblages characterised by a lower functional richness and beetle abundance and biomass.5. From a livestock management viewpoint, mesocosms representing areas with impoverished dung beetle assemblages, due to the long‐term use of veterinary medical products, such as ivermectin, emitted 1.6‐ and 2.8‐fold higher total CO2and CH4, respectively, than mesocosms mimicking sites with untreated livestock.

Atmospheric CO<sub>2</sub>, CH<sub>4</sub>, and CO with the CRDS technique at the Izaña Global GAW station: instrumental tests, developments, and first measurement results

Abstract. At the end of 2015, a CO2/CH4/CO cavity ring-down spectrometer (CRDS) was installed at the Izaña Global Atmosphere Watch (GAW) station (Tenerife, Spain) to improve the Izaña Greenhouse Gases GAW Measurement Programme, and to guarantee the renewal of the instrumentation and the long-term maintenance of this program. We present the results of the CRDS acceptance tests, the raw data processing scheme applied, and the response functions used. Also, the calibration results, the implemented water vapor correction, the target gas injection statistics, the ambient measurements performed from December 2015 to July 2017, and their comparison with other continuous in situ measurements are described. The agreement with other in situ continuous measurements is good most of the time for CO2 and CH4, but for CO it is just outside the GAW 2 ppb objective. It seems the disagreement is not produced by significant drifts in the CRDS CO World Meteorological Organization (WMO) tertiary standards. The more relevant contributions of the present article are (1) determination of linear relationships between flow rate, CRDS inlet pressure, and CRDS outlet valve aperture; (2) determination of a slight CO2 correction that takes into account changes in the inlet pressure/flow rate (as well as its stability over the years), and attributing it to the existence of a small spatial inhomogeneity in the pressure field inside the CRDS cavity due to the gas dynamics; (3) drift rate determination for the pressure and temperature sensors located inside the CRDS cavity from the CO2 and CH4 response function drift trends; (4) the determination of the H2O correction for CO has been performed using raw spectral peak data instead of the raw CO provided by the CRDS and using a running mean to smooth random noise in a long water-droplet test (12 h) before performing the least square fit; and (5) the existence of a small H2O dependence in the CRDS flow and of a small spatial inhomogeneity in the temperature field inside the CRDS cavity are pointed out and their origin discussed.

Calibration of isotopologue-specific optical trace gas analysers: a practical guide

Abstract. The isotopic composition of atmospheric trace gases such as CO2 and CH4 provides a valuable tracer for the sources and sinks that contribute to atmospheric trace gas budgets. In the past, isotopic composition has typically been measured with high precision and accuracy by isotope ratio mass spectrometry (IRMS) offline and separately from real-time or flask-based measurements of concentrations or mole fractions. In recent years, development of infrared optical spectroscopic techniques based on laser and Fourier-transform infrared spectroscopy (FTIR) has provided high-precision measurements of the concentrations of one or more individual isotopologues of atmospheric trace gas species in continuous field and laboratory measurements, thus providing both concentration and isotopic measurements simultaneously. Several approaches have been taken to the calibration of optical isotopologue-specific analysers to derive both total trace gas amounts and isotopic ratios, converging into two different approaches: calibration via the individual isotopologues as measured by the optical device and calibration via isotope ratios, analogous to IRMS. This paper sets out a practical guide to the calculations required to perform calibrations of isotopologue-specific optical analysers, applicable to both laser and broadband FTIR spectroscopy. Equations to calculate the relevant isotopic and total concentration quantities without approximation are presented, together with worked numerical examples from actual measurements. Potential systematic errors, which may occur when all required isotopic information is not available, or is approximated, are assessed. Fortunately, in most such realistic cases, these systematic errors incurred are acceptably small and within the compatibility limits specified by the World Meteorological Organisation – Global Atmosphere Watch. Isotopologue-based and ratio-based calibration schemes are compared. Calibration based on individual isotopologues is simpler because the analysers fundamentally measure amounts of individual isotopologues, not ratios. Isotopologue calibration does not require a range of isotopic ratios in the reference standards used for the calibration, only a range of concentrations or mole fractions covering the target range. Ratio-based calibration leads to concentration dependence, which must also be characterised.

Spatial-Temporal Variations of CO2 and CH4 Flux Through a Water-air Interface Under the Effect of Primary Productivity in Wulixia Reservoir

In recent years, the effect of greenhouse gas has been a focus. In order to reveal the spatial-temporal variations of CO2 and CH4 flux through a water-air interface, Wulixia reservoir was selected as a typical case for measuring CO2and CH4 flux using a floating chamber during four study periods in 2016.The primary productivity of the water was also studied. The results indicate that surface-layer water in Wulixia reservoir is a CO2 sink in summer, varying between -30.14 and -3.47 mg·(m2·h)-1. However, it is the source of CO2 in autumn and winter, varying between 15.57 mg·(m2·h)-1 and 115.06 mg·(m2·h)-1. The variation of methane flux is obvious in summer, but it is stable in autumn and winter. The spatial distribution of CO2 and CH4 show that they are higher in the typical bay area and lower in the tailing dam as well as in the drawdown zone of the reservoir. In addition, the spatial and temporal variation of CO2 and CH4 are negatively and positively correlated with primary productivity (r -0.477 and 0.771), respectively. Due to the high activity of photosynthetic micro- organisms in summer, CO2 can be sequestered, allowing the surface-layer water in Wulixia reservoir to be a CO2 sink. In addition, methanogenic bacteria can oxidize organic carbon produced by photosynthetic micro-organisms. Therefore, CH4 has a negative correlation with primary productivity. The results provide important information for the study of microorganism functions in karst water systems.