In situ measurement of dissolved methane and carbon dioxide in freshwater ecosystems by off-axis integrated cavity output spectroscopy

Methane dynamics altered by reservoir operations in a typical tributary of the Three Gorges Reservoir

Substantial nutrient inputs from reservoir impoundment typically increase sedimentation rate and primary production. This can greatly enhance methane (CH4) production, making reservoirs potentially significant sources of atmospheric CH4. Consequently, elucidating CH4 emissions from reservoirs is crucial for assessing their role in the global methane budget. Reservoir operations can also influence hydrodynamic and biogeochemical processes, potentially leading to pronounced spatiotemporal heterogeneity, especially in reservoirs with complex tributaries, such as the Three Gorges Reservoir (TGR). Although several studies have investigated the spatial and temporal variations in CH4 emissions in the TGR and its tributaries, considerable uncertainties remain regarding the impact of reservoir operations on CH4 dynamics. These uncertainties primarily arise from the limited spatial and temporal resolutions of previous measurements and the complex underlying mechanisms of CH4 dynamics in reservoirs. In this study, we employed a fast-response automated gas equilibrator to measure the spatial distribution and seasonal variations of dissolved CH4 concentrations in XXB, a representative area significantly impacted by TGR operations and known for severe algal blooms. Additionally, we measured CH4 production rates in sediments and diffusive CH4 flux in the surface water. Our multiple campaigns suggest substantial spatial and temporal variability in CH4 concentrations across XXB. Specifically, dissolved CH4 concentrations were generally higher upstream than downstream and exhibited a vertical stratification, with greater concentrations in bottom water compared to surface water. The peak dissolved CH4 concentration was observed in May during the drained period. Our results suggest that the interplay between aquatic organic matter, which promotes CH4 production, and the dilution process caused by intrusion flows from the mainstream primarily drives this spatiotemporal variability. Importantly, our study indicates the feasibility of using strategic reservoir operations to regulate these factors and mitigate CH4 emissions. This eco-environmental approach could also be a pivotal management strategy to reduce greenhouse gas emissions from other reservoirs.

Quantifying the CO2 sink intensity of large and small saline lakes on the Tibetan Plateau

This study quantitatively evaluates the carbon dioxide (CO2) sink intensity of a large saline lake (area > 2000 km2) and a small saline lake (area 1.4 km2) on the Tibetan Plateau (TP), alongside an alpine meadow, by analysing their net ecosystem exchange (NEE) figures obtained by eddy covariance (EC) measurements. Specifically, the "large lake" exhibits an NEE value of -122.51 g C m-2 yr-1, whereas the small lake has an NEE value of -47.17 g C m-2 yr-1. The alpine meadow, in contrast, demonstrates an NEE value of -128.18 g C m-2 yr-1. Through standardization of the eddy flux data processing and accounting for site-specific conditions with a wind direction filter and footprint analysis, the study provides robust estimates of CO2 sink intensity. The "large lake" was found to absorb CO2 primarily during non-icing cold periods with minimal exchange occurring during ice-covered season, whereas the "small lake" showed no significant CO2 exchange throughout the year. On the other hand, alpine meadows engaged in CO2 uptake during the vegetative growth season but showed weak CO2 release in winter. CO2 uptake in lakes is mainly controlled by ice barrier and chemical processes, while biological processes dominate the alpine meadow. The carbon sink intensity of the TP's saline lakes is estimated to be 1.87-3.01 Tg C yr-1, smaller than the previous reported estimations. By evaluating the CO2 sink intensity of different lakes, the study highlights the importance of saline lakes in regional carbon balance assessments. It specifically points out the differential roles lakes of various sizes play in the carbon cycle, thereby enriching our understanding of carbon dynamics in high-altitude lacustrine ecosystems.

Real-Time and Online Monitoring of Hazardous Volatile Organic Compounds in Environmental Water by an Unmanned Shipborne Mass Spectrometer System

Hazardous volatile organic compounds (VOCs) are one of the critical concerns in environmental water due to their toxicity to aquatic organisms and drinking water. Therefore, rapid detection of hazardous VOCs in environmental water is highly needed as many analytical methods are limited to on-site monitoring. In this work, we designed a novel unmanned shipborne mass spectrometer (US-MS) system for the real-time and online monitoring of hazardous VOCs in environmental water. The US-MS system consists of a miniaturized mass spectrometer, an automatic sampling device, a robust unmanned ship, and other monitoring and control devices. Along with the navigation route of the US-MS system, environmental water was continuously introduced into the MS system for the online and real-time detection of hazardous VOCs via a liquid/gas exchange membrane. Analytical performances of the US-MS system were investigated by a mixture of 10 VOCs showing low limits of detection (LODs: 0.31-1.26 ng/mL), good reproducibility (RSDs: 2.93-11.03%, n = 7), and excellent quantitative ability (R2 > 0.99). Furthermore, on-site detection and online monitoring of hazardous volatile contaminants such as benzene, chloroprene, and toluene in different aquatic environments such as rivers and lakes were successfully demonstrated, showing excellent field applicability of the US-MS system. Overall, the newly developed US-MS system could perform on-site, online, and real-time monitoring of complex VOCs in environmental water, showing good performances and versatile applications in water analysis.

Biogeography of microbial communities in high-latitude ecosystems: Contrasting drivers for methanogens, methanotrophs and global prokaryotes

Methane-cycling is becoming more important in high-latitude ecosystems as global warming makes permafrost organic carbon increasingly available. We explored 387 samples from three high-latitudes regions (Siberia, Alaska and Patagonia) focusing on mineral/organic soils (wetlands, peatlands, forest), lake/pond sediment and water. Physicochemical, climatic and geographic variables were integrated with 16S rDNA amplicon sequences to determine the structure of the overall microbial communities and of specific methanogenic and methanotrophic guilds. Physicochemistry (especially pH) explained the largest proportion of variation in guild composition, confirming species sorting (i.e., environmental filtering) as a key mechanism in microbial assembly. Geographic distance impacted more strongly beta diversity for (i) methanogens and methanotrophs than the overall prokaryotes and, (ii) the sediment habitat, suggesting that dispersal limitation contributed to shape the communities of methane-cycling microorganisms. Bioindicator taxa characterising different ecological niches (i.e., specific combinations of geographic, climatic and physicochemical variables) were identified, highlighting the importance of Methanoregula as generalist methanogens. Methylocystis and Methylocapsa were key methanotrophs in low pH niches while Methylobacter and Methylomonadaceae in neutral environments. This work gives insight into the present and projected distribution of methane-cycling microbes at high latitudes under climate change predictions, which is crucial for constraining their impact on greenhouse gas budgets.

Simultaneous detection of greenhouse gases CH4 and CO2 based on a dual differential photoacoustic spectroscopy system

In addition to the atmospheric measurement, detection of dissolved carbon oxides and hydrocarbons in a water region is also an important aspect of greenhouse gas monitoring, such as CH4 and CO2. The first step of measuring dissolved gases is the separation process of water and gases. However, slow degassing efficiency is a big challenge which requires the gas detection technology itself with low gas consumption. Photoacoustic spectroscopy (PAS) is a good choice with advantages of high sensitivity, low gas consumption, and zero background, which has been rapidly developed in recent years and is expected to be applied in the field of dissolved gas detection. In this study, a miniaturized differential photoacoustic cell with a volume of 7.9 mL is designed for CH4 and CO2 detection, and a dual differential method with four microphones is proposed to enhance the photoacoustic signal. What we believe to be a new method increases photoacoustic signal by 4 times and improves the signal to noise ratio (SNR) over 10 times compared with the conventional single-microphone mode. Two distributed feedback (DFB) lasers at 1651 nm and 2004nm are employed to construct the PAS system for CH4 and CO2 detection respectively. Wavelength modulation spectroscopy (WMS) and 2nd harmonic demodulation techniques are applied to further improve the SNR. As a result, sensitivity of 0.44 ppm and 7.39 ppm for CH4 and CO2 are achieved respectively with an integration time of 10 s. Allan deviation analysis indicates that the sensitivity can be further improved to 42 ppb (NNEA=4.7×10-10cm-1WHz-1/2) for CH4 and 0.86 ppm (NNEA=5.3×10-10cm-1WHz-1/2) for CO2 when the integration time is extended to 1000 s.

Seconds-Scale Response Sensor for In Situ Oceanic Carbon Dioxide Detection

Research on the transient variation processes of oceanic dissolved CO₂ makes significant sense because of the complexity and dynamics of the marine environment. Yet, it is inherently challenging due to the limitation of the response performance of in situ sensors. Here, we report a novel system solution capable of providing high-performance detection with a seconds-scale response, sub-ppmv level precision, and 3000 m rated depth. Through the proposed strategy, we break the limitation of the membrane on the response performance of the sensor and improve it by 2 orders of magnitude to the τ₁₀₀ of 3.5 s (τ₉₀ = 2.7 s). By taking water temperature and CO₂ concentration as the tracer, we succeed in portraying the water mixing process and reveal the microstructure of the concentration variation profile. By enabling in situ detection at an unprecedented response speed, this instrument can provide new insights and prospects into the research on the carbon cycle in deep-sea unstable regions, such as hydrothermal vents and cold seeps.

Open-path anti-pollution multi-pass cell-based TDLAS sensor for the online measurement of atmospheric H2O and CO2 fluxes

We report an open-path and anti-pollution multi-pass cell based tunable diode laser absorption spectroscopy (TDLAS) sensor, which was designed for online measurement of atmospheric H₂O and CO₂ fluxes. It is mainly composed of two plano-convex mirrors coated on a convex surface, which makes it different from traditional multi-pass cells. This design does not allow a direct contact between the coating layer of the lens and air, thereby realizing the anti-pollution effect of the coating layer. Two DFB lasers operating at 1392 nm and 2004 nm were employed to target H₂O and CO₂ absorption lines, respectively. Allan analysis of variance indicated that detection limits of H₂O and CO₂ were 5.98 ppm and 0.68 ppm, respectively, at an average time of 0.1 s. The sensor performance was demonstrated by measuring CO₂ and H₂O flux emissions at Jiangdu Agricultural Monitoring Station in Jiangsu Province. The results were compared with those obtained using the commercial instrument LI-7500, which is based on non-dispersive infrared technology. The developed gas analysis instrument exhibited good consistency with commercial instruments, and its accuracy was comparable; thus, it has strong application prospects for flux measurements in any ecosystem.

Methane and carbon dioxide cycles in lakes of the King George Island, maritime Antarctica

Freshwater ecosystems are important contributors to the global greenhouse gas budget and a comprehensive assessment of their role in the context of global warming is essential. Despite many reports on freshwater ecosystems, relatively little attention has been given so far to those located in the southern hemisphere and our current knowledge is particularly poor regarding the methane cycle in non-perennially glaciated lakes of the maritime Antarctica. We conducted a high-resolution study of the methane and carbon dioxide cycle in a lake of the Fildes Peninsula, King George Island (Lat. 62°S), and a succinct characterization of 10 additional lakes and ponds of the region. The study, done during the ice-free and the ice-seasons, included methane and carbon dioxide exchanges with the atmosphere (both from water and surrounding soils) and the dissolved concentration of these two gases throughout the water column. This characterization was complemented with an ex-situ analysis of the microbial activities involved in the methane cycle, including methanotrophic and methanogenic activities as well as the methane-related marker gene abundance, in water, sediments and surrounding soils. The results showed that, over an annual cycle, the freshwater ecosystems of the region are dominantly autotrophic and that, despite low but omnipresent atmospheric methane emissions, they act as greenhouse gas sinks.

A combined microbial and biogeochemical dataset from high-latitude ecosystems with respect to methane cycle

High latitudes are experiencing intense ecosystem changes with climate warming. The underlying methane (CH₄) cycling dynamics remain unresolved, despite its crucial climatic feedback. Atmospheric CH₄ emissions are heterogeneous, resulting from local geochemical drivers, global climatic factors, and microbial production/consumption balance. Holistic studies are mandatory to capture CH₄ cycling complexity. Here, we report a large set of integrated microbial and biogeochemical data from 387 samples, using a concerted sampling strategy and experimental protocols. The study followed international standards to ensure inter-comparisons of data amongst three high-latitude regions: Alaska, Siberia, and Patagonia. The dataset encompasses different representative environmental features (e.g. lake, wetland, tundra, forest soil) of these high-latitude sites and their respective heterogeneity (e.g. characteristic microtopographic patterns). The data included physicochemical parameters, greenhouse gas concentrations and emissions, organic matter characterization, trace elements and nutrients, isotopes, microbial quantification and composition. This dataset addresses the need for a robust physicochemical framework to conduct and contextualize future research on the interactions between climate change, biogeochemical cycles and microbial communities at high-latitudes.

Measurement(s)	microbial diversity • microbial abundances • cations and anions • trace elements
Technology Type(s)	MiSeq sequencing 16S rRNA gene • Real Time PCR • HPLC • ICP-MS
Sample Characteristic - Organism	bacteria • archaea
Sample Characteristic - Environment	lake water • lake sediment • wetland • peatland • soil • pond
Sample Characteristic - Location	Western Siberia • Alaska • Patagonia • Cape Horn province • Magellanic subantarctic ecoregion

Eutrophic levels and algae growth increase emissions of methane and volatile sulfur compounds from lakes

Eutrophic lakes are hot spots of CH₄ and volatile sulfur compound (VSC) emissions, especially during algal blooms and decay. However, the response of CH₄ and VSC emissions to lake eutrophication and algae growth as well as the underlying mechanisms remain unclear. In this study, the emissions of CH₄ and VSCs from four regions of Lake Taihu with different eutrophic levels were investigated in four months (i.e., March, May, August and December). The CH₄ emissions ranged from 20.4 to 126.9 mg m^-2 d^-1 in the investigated sites and increased with eutrophic levels and temperature. H₂S and CS₂ were the dominant volatile sulfur compounds (VSCs) emitted from the lake. The CH₄ oxidation potential of water ranged from 2.1 to 14.9 μg h^-1 L^-1, which had positive correlations with trophic level index and the environmental variables except for the NH₄⁺-N concentration. Eutrophic levels could increase the abundances of bacteria and methanotrophs in lake water. α-Proteobacteria methanotroph Methylocystis was more abundant than γ-Proteobacteria methanotrophs in March and May, while the latter was more abundant in August and November. The relative abundance of Cyanobacteria, including Microcystis, A. granulata var. angustissima and Cyanobium had significantly positive correlations with temperature, turbidity, SO₄^2--S, and total sulfur. Partial least squares path modelling revealed that the algal growth could promote VSC emissions, which had a positive correlation with CH₄ oxidation potential, likely due to the positive correlation between the CH₄ and VSC emissions from lakes. These findings indicate that water eutrophication and algae growth could increase the emissions of CH₄ and VSCs from lakes. Controlling algae growth might be an effective way to mitigate the emissions of CH₄ and VSCs from freshwater lakes.

Spatial and seasonal dynamics of the methane cycle in a tropical coastal lagoon and its tributary river

Coastal aquatic ecosystems such as estuaries and coastal lagoons are important atmospheric methane sources that must be better constrained. This work presents a detailed characterization of the methane cycle in a tropical coastal lagoon (La Mancha, Veracruz, Mexico) and its tributary river over three distinct seasons, along a transect from the river to the sea connection. In addition to several physicochemical parameters, the dissolved methane, carbon dioxide, and oxygen concentrations were measured with high resolution in the sediments and the water column, combined with production/uptake rates. Methane and carbon dioxide cycles were further constrained by determining atmospheric flux over the entire river and lagoon sections. The results indicate that La Mancha is a highly contrasted ecosystem. The river section is characterized by a strong pycnocline, relatively high methane concentration, and active methanogenesis and methanotrophy, discharging into a relatively homogeneous lagoon section where the methane and carbon cycles are less active. Overall, both the river and the lagoon were a net source of methane and carbon dioxide, with an annual emission of 2.9 metric tons of methane and 2757 metric tons of carbon dioxide. The spatial structure of the main components of the methane, carbon dioxide, and oxygen cycles was established, and it was observed that depthwise heterogeneities predominated in the river section. In contrast, lengthwise heterogeneities dominated in the lagoon section.

Spatial and temporal variability of dissolved methane concentrations and diffusive emissions in the Three Gorges Reservoir

Methane (CH₄) emissions from freshwater aquatic systems such as rivers and reservoirs are an important component of the global methane budget. However, the estimation can be largely affected by the spatial and temporal resolutions of measurements. Especially, the lack of high-resolution studies in the Three Gorges Reservoir (TGR), one of the largest reservoirs in the world, has led to a longstanding debate on its CH₄ emissions. In this study, the spatial distribution and seasonal variations of dissolved CH₄ concentrations were measured using a fast-response automated gas equilibrator in the TGR. We observed large spatiotemporal variations of dissolved CH₄ (mean ± SD: 0.26 ± 0.19 μM in summer and 0.24 ± 0.17 μM in winter). Higher concentrations with stronger variations were found in the upstream than in the section close to the Three Gorges Dam. The dissolved CH₄ concentration in the TGR was mainly influenced by sewage discharge, sedimentation, topographical conditions, tributaries, and spatial and seasonal variations in hydrodynamics. Regression analyses suggest that the concentration can be characterized by sewage discharge, water depth, and electrical conductivity to a certain extent. Mean diffusive CH₄ fluxes from the TGR in summer and winter were 16.2 mg m^-2 d^-1 and 3.1 mg m^-2 d^-1, respectively. Downsampling simulations show that scaling dissolved CH₄ in the TGR from one site likely involves large errors, and at least ∼38 sites and ∼52-58 sites are needed to achieve an accurate estimate in summer and winter, respectively. Due to the large spatial and temporal heterogeneity, high-resolution measurements are key to improving the reliability of CH₄ estimates and assessing the contribution of the TGR to regional and global CH₄ budgets.

Quantifying groundwater carbon dioxide and methane fluxes to an urban freshwater lake using radon measurements

Freshwater lakes can play a significant role in greenhouse gas budgets as they can be sources or sinks of carbon to the atmosphere. However, there is limited information on groundwater discharge being a source of carbon to freshwater lakes. Here, we measure CO₂ and CH₄ in the largest urban freshwater lake in the metropolitan area of Sydney (Australia) and quantify groundwater discharge rates into the lake using radon (²²²Rn, a natural groundwater tracer). We also assess the spatial variability of radon, CO₂ and CH₄ in the lake, in addition to surface water and groundwater nutrient and carbon concentrations. Results revealed that the lake system was a source of CO₂ and CH₄ to the atmosphere with fluxes of 113 ± 81 and 0.3 ± 0.1 mmol/m²/d, respectively. These calculated CO₂ fluxes were larger than commonly observed lake fluxes and the global average flux from lakes. However, CH₄ fluxes were lower than the average global value. Based on the radon mass balance model, groundwater discharge to the lake was 16 ± 10 cm/d, which resulted in groundwater-derived CO₂ and CH₄ fluxes contributing 25 and 13% to the overall greenhouse gas emissions from the lake, respectively. Radon, CO₂ and CH₄ maps showed similar spatial distribution trends in the lake and a strong relationship between radon, NO₃ and NH₄ suggested groundwater flow was also a driver of nitrogen into the lake from the western side of the lake, following the general regional groundwater flow. This work provides insights into groundwater and greenhouse gas dynamics in Sydney's largest urban freshwater lake with two implications for carbon budgets: to incorporate urban lakes in global carbon budgets and to account for, the often ignored, groundwater discharge as a source of carbon to lakes.

Temperature differently affected methanogenic pathways and microbial communities in sub-Antarctic freshwater ecosystems

Freshwater ecosystems are responsible for an important part of the methane (CH₄) emissions which are likely to change with global warming. This study aims to evaluate temperature-induced (from 5 to 20 °C) changes on microbial community structure and methanogenic pathways in five sub-Antarctic lake sediments from Magallanes strait to Cape Horn, Chile. We combined in situ CH₄ flux measurements, CH₄ production rates (MPRs), gene abundance quantification and microbial community structure analysis (metabarcoding of the 16S rRNA gene). Under unamended conditions, a temperature increase of 5 °C doubled MPR while microbial community structure was not affected. Stimulation of methanogenesis by methanogenic precursors as acetate and H₂/CO₂, resulted in an increase of MPRs up to 127-fold and 19-fold, respectively, as well as an enrichment of mcrA-carriers strikingly stronger under acetate amendment. At low temperatures, H₂/CO₂-derived MPRs were considerably lower (down to 160-fold lower) than the acetate-derived MPRs, but the contribution of hydrogenotrophic methanogenesis increased with temperature. Temperature dependence of MPRs was significantly higher in incubations spiked with H₂/CO₂ (c. 1.9 eV) compared to incubations spiked with acetate or unamended (c. 0.8 eV). Temperature was not found to shape the total microbial community structure, that rather exhibited a site-specific variability among the studied lakes. However, the methanogenic archaeal community structure was driven by amended methanogenic precursors with a dominance of Methanobacterium in H₂/CO₂-based incubations and Methanosarcina in acetate-based incubations. We also suggested the importance of acetogenic H₂-production outcompeting hydrogenotrohic methanogenesis especially at low temperatures, further supported by homoacetogen proportion in the microcosm communities. The combination of in situ-, and laboratory-based measurements and molecular approaches indicates that the hydrogenotrophic pathway may become more important with increasing temperatures than the acetoclastic pathway. In a continuously warming environment driven by climate change, such issues are crucial and may receive more attention.

Overall spatiotemporal dynamics of greenhouse gasses and oxygen in two subtropical reservoirs with contrasting trophic states

The impact of cultural eutrophication on carbon cycling in subtropical reservoirs was assessed using high-resolution measurements of dissolved gas concentration, atmospheric exchange, and uptake/production rates of methane, carbon dioxide, and oxygen. Seasonal measurements were performed in two reservoirs that pertain to the same hydrological basin but are drastically different in terms of allochthonous carbon input. These results were used to feed a mass balance model, from which a large number of overall parameters were determined to explicitly describe the dynamics and spatial attributes of the carbon cycle in the reservoirs. A single graphical representation of each reservoir was created to facilitate an overall appraisal of the carbon cycle. The impact of cultural eutrophication was profound and resulted in a complete redistribution of how the various bioprocesses participated in the methane, carbon dioxide, and oxygen cycles. Among several identified impacts of eutrophication, it was observed that while eutrophication triggered increased methane production, this effect was followed by a similar increase in methane emissions and methanotrophic rates, while gross primary production was depleted.

Review of Dissolved CO and H2 Measurement Methods for Syngas Fermentation

Syngas fermentation is a promising technique to produce biofuels using syngas obtained through gasified biomass and other carbonaceous materials or collected from industrial CO-rich off-gases. The primary components of syngas, carbon monoxide (CO) and hydrogen (H2), are converted to alcohols and other chemicals through an anaerobic fermentation process by acetogenic bacteria. Dissolved CO and H2 concentrations in fermentation media are among the most important parameters for successful and stable operation. However, the difficulties in timely and precise dissolved CO and H2 measurements hinder the industrial-scale commercialization of this technique. The purpose of this article is to provide a comprehensive review of available dissolved CO and H2 measurement methods, focusing on their detection mechanisms, CO and H2 cross interference and operations in syngas fermentation process. This paper further discusses potential novel methods by providing a critical review of gas phase CO and H2 detection methods with regard to their capability to be modified for measuring dissolved CO and H2 in syngas fermentation conditions.

Anaerobic oxidation of methane and associated microbiome in anoxic water of Northwestern Siberian lakes

Arctic lakes emit methane (CH₄) to the atmosphere. The magnitude of this flux could increase with permafrost thaw but might also be mitigated by microbial CH₄ oxidation. Methane oxidation in oxic water has been extensively studied, while the contribution of anaerobic oxidation of methane (AOM) to CH₄ mitigation is not fully understood. We have investigated four Northern Siberian stratified lakes in an area of discontinuous permafrost nearby Igarka, Russia. Analyses of CH₄ concentrations in the water column demonstrated that 60 to 100% of upward diffusing CH₄ was oxidized in the anoxic layers of the four lakes. A combination of pmoA and mcrA gene qPCR and 16S rRNA gene metabarcoding showed that the same taxa, all within Methylomonadaceae and including the predominant genus Methylobacter as well as Crenothrix, could be the major methane-oxidizing bacteria (MOB) in the anoxic water of the four lakes. Correlation between Methylomonadaceae and OTUs within Methylotenera, Geothrix and Geobacter genera indicated that AOM might occur in an interaction between MOB, denitrifiers and iron-cycling partners. We conclude that MOB within Methylomonadaceae could have a crucial impact on CH₄ cycling in these Siberian Arctic lakes by mitigating the majority of produced CH₄ before it leaves the anoxic zone. This finding emphasizes the importance of AOM by Methylomonadaceae and extends our knowledge about CH₄ cycle in lakes, a crucial component of the global CH₄ cycle.

Impact of H2O on atmospheric CH4 measurement in near-infrared absorption spectroscopy

A distributed-feedback (DFB) diode laser based near-infrared absorption spectrometer was used to study H₂O broadening coefficients for methane (CH₄) transitions at 1.653 μm, which contains well-characterized and relatively isolated transitions of appropriate line strengths for sensitive atmospheric CH₄ detection. The influence of H₂O broadening on R₃ transitions of CH₄ at 6046.9 cm^-1 was experimentally investigated in detail. The results indicate that H₂O broadening coefficients are approximately 1.3 times higher than the corresponding air-broadening parameters. Based on the measured H₂O induced broadening coefficients, the influence of H₂O on actual measurement of atmospheric CH₄ in tunable diode laser absorption spectroscopy was theoretically and experimentally discussed and compared, and a good agreement was obtained. The experimental results proved that the difference between air- and H₂O-broadenings cannot be neglected for high precision gas concentration measurement, especially in a highly humid environment.

Development of an in situ analysis system for methane dissolved in seawater based on cavity ringdown spectroscopy

This paper reports the development of a compact in situ real-time concentration analysis system for methane dissolved in seawater by using a continuous-wave cavity ringdown spectroscopy (CRDS) technique. The miniaturized design of the system, including optical resonance cavity and control and data acquisition-analysis electronics, has a cylindrical dimension of 550 mm in length and 100 mm in diameter. Ringdown signal generation, data acquisition and storage, current driver, and temperature controller of the diode laser are all integrated in the miniaturized system circuits, with an electrical power consumption of less than 12 W. Fitting algorithms of the ringdown signal and spectral line are implemented in a digital signal processor, which is the main control chip of the system circuit. The detection sensitivity for methane concentration can reach 0.4 ppbv with an approximate averaging time of 240 s (or 4 min). Comparing the system's measurement of ambient air against a high-quality commercial CRDS instrument has demonstrated a good agreement in results. In addition, as a "proof of concept" for measuring dissolved methane, the developed instrument was tested in an actual underwater environment. The results showed the potential of this miniaturized portable instrument for in situ gas sensing applications.

Sub-oxycline methane oxidation can fully uptake CH4 produced in sediments: case study of a lake in Siberia

It is commonly assumed that methane (CH₄) released by lakes into the atmosphere is mainly produced in anoxic sediment and transported by diffusion or ebullition through the water column to the surface of the lake. In contrast to that prevailing idea, it has been gradually established that the epilimnetic CH₄ does not originate exclusively from sediments but is also locally produced or laterally transported from the littoral zone. Therefore, CH₄ cycling in the epilimnion and the hypolimnion might not be as closely linked as previously thought. We utilized a high-resolution method used to determine dissolved CH₄ concentration to analyze a Siberian lake in which epilimnetic and hypolimnetic CH₄ cycles were fully segregated by a section of the water column where CH₄ was not detected. This layer, with no detected CH₄, was well below the oxycline and the photic zone and thus assumed to be anaerobic. However, on the basis of a diffusion-reaction model, molecular biology, and stable isotope analyses, we determined that this layer takes up all the CH₄ produced in the sediments and the deepest section of the hypolimnion. We concluded that there was no CH₄ exchange between the hypolimnion (dominated by methanotrophy and methanogenesis) and the epilimnion (dominated by methane lateral transport and/or oxic production), resulting in a vertically segregated lake internal CH₄ cycle.

Enhancing off-axis integrated cavity output spectroscopy (OA-ICOS) with radio frequency white noise for gas sensing

Injecting radio frequency (RF) white noise to the current driving of the laser can broaden the laser emission linewidth and efficiently suppress cavity-mode noise in off-axis integrated cavity output spectroscopy (OA-ICOS). The effect of the injected RF noise level on the cavity-mode noise and the deformation of the absorption line shape in off-axis integrated cavity output spectroscopy (OA-ICOS) with a distributed feedback laser (DFB) at 1.65 µm were investigated. We measured methane at different concentrations between 0.1 ppmv and 2 ppmv associated with a -20 dBm RF noise injection. A linear spectral response of the intensity of the cavity output spectra with the CH₄ concentration was observed. A threefold improvement in the detection limit was achieved compared to unperturbed OA-ICOS. The response time of the improved OA-ICOS system is about 30 s and the minimum detectable concentration (MDC) of CH₄ is 7.6 ppbv, which corresponds to a minimum detectable fractional absorption scaled to the path length of 7.3 × 10^-10 cm^-1. The noise equivalent absorption sensitivity of the system is 5.51 × 10^-9 cm^-1Hz^-1/2.

Development of a compact tunable diode laser absorption spectroscopy based system for continuous measurements of dissolved carbon dioxide in seawater

Robust marine carbon sensors with small size, low power consumption, and high sensitivity provide greater insight into the carbon cycle studies and resolve environmental variability. We report here the development of a diminutively integrated tunable diode laser absorption spectroscopy (TDLAS) system with a specially designed multipass gas cell for small amounts of dissolved gas extractions and measurements. It was used to detect and monitor carbon dioxide (CO₂) dissolved in water and seawater. Systematic experiments have been carried out for system evaluation in the lab. Extracted CO₂ was determined via its 4989.9 cm^-1 optical absorption line. The achieved TDLAS measurement precision was 4.18 ppm for CO₂, measured by averaging up to 88 s. The integrated absorbance was found to be linear to gas concentrations over a wide range. Comparison measurements of the atmospheric CO₂ values with a commercial instrument confirmed a good accuracy of our TDLAS-based system. The first test campaign was also accomplished with a hollow fiber membrane contactor, and concentrations of CO₂ were quantitatively detected with partial degasification operations. The results clearly show the ability to continuously measure dissolved gases and highlight the potential of the system to help us better understand physical and geochemical processes in a marine environment.

Methane dynamics in the subsaline ponds of the Chihuahuan Desert: A first assessment

The Cuatro Cienegas Basin (CCB) in the Chihuahuan desert is characterized by the presence of over 500 ponds located in an endorheic basin. These ponds are subsaline ecosystems characterized by a low productivity and a particularly high sulfate concentration, comparable to marine environments. This study focused on assessing the main physicochemical parameters in these ponds along with the characterization of the CH₄ dynamics through the determination of fluxes, dissolved CH₄ concentrations, and net methanotrophic and methanogenic activity. Despite a sulfate concentration ranging from 1.06 to 4.73 g L^-1, the studied ponds showed moderate but clear CH₄ production and emission, which suggests that methanogenesis is not completely outcompeted by sulfate reduction. CH₄ fluxes ranged from 0.12 to 0.98 mg m^-2 d^-1, which falls within the higher range of marine emissions and within the lower range reported for coastal saline lagoons and saline ponds. During summer, significant CH₄ production in the oxic water column was observed. In addition to CH₄, CO₂ fluxes were determined at levels from 0.2 to 53 g m^-2 d^-1, which is within the range recorded for saline lakes in other parts of the world. Our results provide additional evidence that subsaline/saline aquatic ecosystems play an important role in the emission of greenhouse gases to the atmosphere.

Continuous Measurement of Diffusive and Ebullitive Fluxes of Methane in Aquatic Ecosystems by an Open Dynamic Chamber Method

An open dynamic chamber for the continuous monitoring of diffusive and ebullitive fluxes of methane (CH₄) in aquatic ecosystems was designed and developed. This method is based on a standard floating chamber in which a well-defined carrier gas flows. The concentration of CH₄ is measured continuously at the outlet of the chamber, and the flux is determined from a mass balance equation. The method was carefully tested in a laboratory and was subsequently applied to two lakes, in Mexico, with contrasting trophic states. We show here that the method allows for the continuous quantification of CH₄ diffusive flux higher than 25 × 10^-6 g m^-2 h^-1, the determination of ebullitive flux, and the individual characterization of bubbles larger than 1.50-1.72 mm in diameter. The method was also applied to determine carbon dioxide emissions (CO₂). In that case, the method was less sensitive but allowed for the characterization of diffusive fluxes higher than 10 mg CO₂ m^-2 h^-1 and of bubbles larger than 5.3-8.4 mm in diameter. This high-throughput method can be adapted to any gas detector at low cost, making it a convenient tool to better constrain greenhouse gas emission from freshwater ecosystems.

Greenhouse gas emissions from lakes and impoundments: upscaling in the face of global change

Lakes and impoundments are important sources of greenhouse gases (GHG: i.e., CO2, CH4, N2O), yet global emission estimates are based on regionally-biased averages and elementary upscaling. We assembled the largest global dataset to date on emission rates of all three GHGs and found they covary with lake size and trophic state. Fitted models were upscaled to estimate global emission using global lake size inventories and a remotely-sensed global lake productivity distribution. Traditional upscaling approaches overestimated CO2 and N2O emission but underestimated CH4 by half. Our upscaled size-productivity weighted estimates (1.25-2.30 Pg of CO2-equivalents annually) are nearly 20% of global CO2 fossil fuel emission with ~75% of the climate impact due to CH4. Moderate global increases in eutrophication could translate to 5-40% increases in the GHG effects in the atmosphere, adding the equivalent effect of another 13% of fossil fuel combustion or an effect equal to GHG emissions from current land use change.

Sub-Ocean: Subsea Dissolved Methane Measurements Using an Embedded Laser Spectrometer Technology

We present a novel instrument, the Sub-Ocean probe, allowing in situ and continuous measurements of dissolved methane in seawater. It relies on an optical feedback cavity enhanced absorption technique designed for trace gas measurements and coupled to a patent-pending sample extraction method. The considerable advantage of the instrument compared with existing ones lies in its fast response time of the order of 30 s, that makes this probe ideal for fast and continuous 3D-mapping of dissolved methane in water. It could work up to 40 MPa of external pressure, and it provides a large dynamic range, from subnmol of CH₄ per liter of seawater to mmol L^-1. In this work, we present laboratory calibration of the instrument, intercomparison with standard method and field results on methane detection. The good agreement with the headspace equilibration technique followed by gas-chromatography analysis supports the utility and accuracy of the instrument. A continuous 620-m depth vertical profile in the Mediterranean Sea was obtained within only 10 min, and it indicates background dissolved CH₄ values between 1 and 2 nmol L^-1 below the pycnocline, similar to previous observations conducted in different ocean settings. It also reveals a methane maximum at around 6 m of depth, that may reflect local production from bacterial transformation of dissolved organic matter.

Large but variable methane production in anoxic freshwater sediment upon addition of allochthonous and autochthonous organic matter

An important question in the context of climate change is to understand how CH4 production is regulated in anoxic sediments of lakes and reservoirs. The type of organic carbon (OC) present in lakes is a key factor controlling CH4 production at anoxic conditions, but the studies investigating the methanogenic potential of the main OC types are fragmented. We incubated different types of allochthonous OC (alloOC; terrestrial plant leaves) and autochthonous OC (autoOC; phytoplankton and two aquatic plants species) in an anoxic sediment during 130 d. We tested if (1) the supply of fresh alloOC and autoOC to an anoxic refractory sediment would fuel CH4 production and if (2) autoOC would decompose faster than alloOC. The addition of fresh OC greatly increased CH4 production and the δ13C-CH4 partitioning indicated that CH4 originated exclusively from the fresh OC. The large CH4 production in an anoxic sediment fueled by alloOC is a new finding which indicates that all systems with anoxic conditions and high sedimentation rates have the potential to be CH4 emitters. The autoOC decomposed faster than alloOC, but the total CH4 production was not higher for all autoOC types, one aquatic plant species having values as low as the terrestrial leaves, and the other one having values as high as phytoplankton. Our study is the first to report such variability, suggesting that the extent to which C fixed by aquatic plants is emitted as greenhouse gases or buried as OC in sediment could more generally differ between aquatic vegetation types.

Modeling the optical field in off-axis integrated-cavity-output spectroscopy using the decentered Gaussian beam model

Off-axis integrated-cavity-output spectroscopy (OA-ICOS) is widely used in trace gas detection and isotopic analysis for its enhanced sensitivity as well as robustness against optical instability. However, its sensitivity is ultimately limited by the spurious coupling noise formed in the cavity, and much of the design and optimization process relies on empirical iterations while quantitative analysis is lacking. In this paper, we develop a method to model the optical field in OA-ICOS based on the decentered Gaussian beam model, which is a generalization for large tilting angles as compared with previously developed models. From the optical field, the cavity transmission spectrum for different cavity configurations or input beam conditions can be calculated, and the fringe noise level can be derived. Results show that an optimum combination of input laser beam and off-axis alignment exists to fully suppress the interference fringes. Factors affecting the fringe noise level, including a mismatch between the input beam and the cavity, optical alignment conditions, and deviation from the re-entrant condition, are studied thoroughly. The developed method can serve to guide the design and optimization of OA-ICOS systems.

Measuring CO2 and CH4 with a portable gas analyzer: Closed-loop operation, optimization and assessment

The use of cavity ring-down spectrometer (CRDS) based portable greenhouse gas analyzers (PGAs) in closed-loop configuration to measure small sample volumes (< 1 l) for CH₄ and CO₂ concentrations is increasing and offers certain advantages over conventional measurement methods in terms of speed as well as the ability to measure directly in field locations. This first systematic assessment of the uncertainties, problems and issues associated with achieving reliable and repeatable measurement with this technique presents the adaptation, measurement range, calibration and maintenance, accuracy and issues of efficient operation, for one example instrument. Regular open-loop calibration, a precise loop volume estimate, leak free system, and a high standard of injection practices are necessary for accurate results. For 100 μl injections, measured values ranging from 4.5 to 9 x10⁴ ppm (CH₄), and 1000 ppm to 1 x10⁶ ppm (CO₂) are possible with uncertainties ±5.9% and ±3.0%, respectively, beyond 100 ppm CH₄ correction may be necessary. Uncertainty arising from variations water vapour content and atmospheric pressure are small (0.24% and -0.9% to +0.5%, respectively). With good practice, individual operator repeatability of 1.9% (CH₄) and 2.48% (CO₂) can be achieved. Between operator injection error was around 3% for both gases for four operators. Slow syringe plunger operation (> 1s) is recommended; generally delivered more (ca. 3–4%) sample into the closed instrument loop than did rapid operation. Automated value retrieval is recommended; we achieved a 3 to 5-fold time reduction for each injection cycle (ca. <2 min), and operator reading, recording, and digitization errors are eliminated.

Spatially Resolved Measurements of CO2 and CH4 Concentration and Gas-Exchange Velocity Highly Influence Carbon-Emission Estimates of Reservoirs

The magnitude of diffusive carbon dioxide (CO2) and methane (CH4) emission from man-made reservoirs is uncertain because the spatial variability generally is not well-represented. Here, we examine the spatial variability and its drivers for partial pressure, gas-exchange velocity (k), and diffusive flux of CO2 and CH4 in three tropical reservoirs using spatially resolved measurements of both gas concentrations and k. We observed high spatial variability in CO2 and CH4 concentrations and flux within all three reservoirs, with river inflow areas generally displaying elevated CH4 concentrations. Conversely, areas close to the dam are generally characterized by low concentrations and are therefore not likely to be representative for the whole system. A large share (44-83%) of the within-reservoir variability of gas concentration was explained by dissolved oxygen, pH, chlorophyll, water depth, and within-reservoir location. High spatial variability in k was observed, and kCH4 was persistently higher (on average, 2.5 times more) than kCO2. Not accounting for the within-reservoir variability in concentrations and k may lead to up to 80% underestimation of whole-system diffusive emission of CO2 and CH4. Our findings provide valuable information on how to develop field-sampling strategies to reliably capture the spatial heterogeneity of diffusive carbon fluxes from reservoirs.

Initial report on methane and carbon dioxide emission dynamics from sub-Antarctic freshwater ecosystems: A seasonal study of a lake and a reservoir

The sub-Antarctic Magellanic ecoregion is a part of the world where ecosystems have been understudied and where the CH₄ cycling and emissions in lakes has not ever been reported. To fill that knowledge gap, a lake and a reservoir located at 53°S were selected and studied during three campaigns equally distributed over one year. Among the parameters measured were CH₄ and CO₂ emissions, as well their dissolved concentrations in the water column, which were determined with high spatial resolution. No ebullition was observed and the CH₄ flux ranged from 0.0094 to 4.47mmolm^-2d^-1 while the CO₂ flux ranged from -22.95 to 35.68mmolm^-2d^-1. Dissolved CH₄ concentrations varied over more than four orders of magnitude (0.025-128.75μmolL^-1), and the dissolved carbon dioxide ranged from below the detection limit of our method (i.e., 0.15μmolL^-1) to 379.09μmolL^-1. The high spatial resolution of the methods used enabled the construction of bathymetric maps, surface contour maps of CH₄ and CO₂ fluxes, and transect contour maps of dissolved oxygen, temperature, and dissolved greenhouse gases. Overall, both lakes were net greenhouse gas producers and were not significantly different from temperate lakes located at a similar northern latitudes (53°N), except that ebullition was never observed in the studied sub-Antarctic lakes.

Export of Dissolved Methane and Carbon Dioxide with Effluents from Municipal Wastewater Treatment Plants

Inland waters play an important role for regional and global scale carbon cycling and are significant sources of the atmospheric greenhouse gases methane (CH4) and carbon dioxide (CO2). Although most studies considered the input of terrestrially derived organic and inorganic carbon as the main sources for these emissions, anthropogenic sources have rarely been investigated. Municipal wastewater treatment plants (WWTPs) could be additional sources of carbon by discharging the treated wastewater into the surrounding aquatic ecosystems. Here we analyze seasonally resolved measurements of dissolved CH4 and CO2 concentrations in effluents and receiving streams at nine WWTPs in Germany. We found that effluent addition significantly altered the physicochemical properties of the streamwater. Downstream of the WWTPs, the concentrations of dissolved CH4 and CO2 were enhanced and the atmospheric fluxes of both gases increased by a factor of 1.2 and 8.6, respectively. The CH4 exported with discharged effluent, however, accounted for only a negligible fraction (0.02%) of the estimated total CH4 emissions during the treatment process. The CH4 concentration in the effluent water was linearly related to the organic load of the wastewater, which can provide an empirical basis for future attempts to add WWTPs inputs to regional-scale models for inland water-carbon fluxes.

A Quantum Cascade Laser-Based Optical Sensor for Continuous Monitoring of Environmental Methane in Dunkirk (France)

A room-temperature continuous-wave (CW) quantum cascade laser (QCL)-based methane (CH₄) sensor operating in the mid-infrared near 8 μm was developed for continuous measurement of CH₄ concentrations in ambient air. The well-isolated absorption line (7F_2,4 ← 8F_1,2) of the ν₄ fundamental band of CH₄ located at 1255.0004 cm⁻¹ was used for optical measurement of CH₄ concentration by direct absorption in a White-type multipass cell with an effective path-length of 175 m. A 1σ (SNR = 1) detection limit of 33.3 ppb in 218 s was achieved with a measurement precision of 1.13%. The developed sensor was deployed in a campaign of measurements of time series CH₄ concentration on a site near a suburban traffic road in Dunkirk (France) from 9 to 22 January 2013. An episode of high CH₄ concentration of up to ~3 ppm has been observed and analyzed with the help of meteorological parameters combined with back trajectory calculation using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model of NOAA.