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Increased effectiveness of methane drainage from sealed post-mining goaves in hard coal mines contributes to reduced methane emission from goaves into the mine ventilation system. This paper focuses on issues concerning the assessment of the additional amount of methane released from the goaf into mine workings during periods of atmospheric pressure drops, which can be captured with a methane drainage system. Thanks to the solutions presented in the paper, it is possible to control the efficiency of the goaf drainage system, which in turn leads to the reduction of methane emission from the mine ventilation air into Earth's atmosphere. These solutions are of great added value for both the environment and coal mines as they reduce the costs arising from greenhouse gas emissions that are incurred by mining companies, increasing the efficiency of methane capture and its use in gas engines or district heating systems. The paper uses relationships relating to the influence of atmospheric pressure changes on the process of gas release from the goaf according to the hysteresis loop of methane release during atmospheric pressure changes, which was developed based on conducted research. The analysis and conclusions presented in this paper may facilitate the development of strategies aimed at reducing methane emissions from a mine's ventilated air into Earth's atmosphere. [ABSTRACT FROM AUTHOR]

A review of works of the last 20 years devoted to the development in our country and abroad of methods and means of measuring the concentration fields of long-lived carbon-containing greenhouse gases in the atmosphere—carbon dioxide CO2 and methane CH4—from satellites has been carried out. Physical and mathematical foundations for interpreting measurements from modern satellite spectrometers in the near-infrared and infrared spectral ranges are briefly reviewed. Information is provided on programs for the development of domestic and foreign satellite systems for monitoring the content of CO2 and CH4 in the atmosphere, as well as on ground-based observation networks, the data of which can be used for calibrating and validating satellite information products. [ABSTRACT FROM AUTHOR]

Lakes process large volumes of organic carbon (OC), are important sources of methane (CH4), and contribute to climatic warming. However, there is a lack of data from large lakes >500 km2, which creates uncertainty in global budgets. In this data article, we present dissolved CH4, OC bioreactivity measurements, water chemistry, and algal biovolumes at 11 stations across Lake Mälaren, the third largest (1,074 km2) Swedish lake. Total phosphorus concentrations show that during the study period the lake was classed as mesotrophic/eutrophic. Overall mean CH4 concentration from all stations, sampled five times to cover seasonal variation, was 2.51 μg l−1 (0.98–5.39 μg l−1). There was no significant seasonal variation although ranges were greatest during summer. Concentrations of CH4 were greatest in shallow waters close to anthropogenic nutrient sources, whilst deeper, central basins had lower concentrations. Methane correlated positively with measures of lake productivity (chlorophyll a, total phosphorus), and negatively to water depth and oxygen concentration, with oxygen emerging as the sole significant driver in a linear mixed effects model. We collated data from other lakes >500 km2 (n = 21) and found a significant negative relationship between surface area and average CH4 concentration. Large lakes remain an understudied contributor to the global CH4 cycle and future research efforts should aim to quantify the spatial and temporal variation in their diffusive and ebullitive emissions, and associated drivers. Plain Language Summary: Lakes contribute to climatic warming, because they emit large amounts of the powerful greenhouse gas methane into the atmosphere. This occurs because lake bottom sediments and lake waters are home to microbes that produce methane, which then travels diffusively in a dissolved form, or as bubbles, through the lake water and into the air. There is large uncertainty about how much methane is released by lakes on a global scale, and more measurements are required to reduce this uncertainty, particularly from very large lakes. In our study, we measured dissolved methane from 11 sampling locations across a very large Swedish lake, and repeated this five times over a year. Levels of methane within the lake were generally low, but they varied over space and time. Higher methane levels occurred in shallower waters near large towns and cities, and were associated with greater concentrations of nutrients such as phosphorus, which act as food for the methane‐producing microbes. Key Points: Lakes are globally important sources of atmospheric methane but there is a lack of data from large lakes >500 km2We measured methane concentrations across a large (1,074 km2) Swedish lake on five occasions over an annual periodMethane varied seasonally and spatially, and higher concentrations were associated with nutrient inputs, lower dissolved oxygen, and shallower waters [ABSTRACT FROM AUTHOR]

Taxis pose a higher threat to global climate change and human health through air emissions. However, the evidence on this topic is scarce, especially, in developing countries. Therefore, this study conducted estimation of fuel consumption (FC) and emission inventories on Tabriz taxi fleet (TTF), Iran. A structured questionnaire to obtain operational data of TTF, municipality organizations, and literature review were used as data sources. Then modeling was used to estimate fuel consumption ratio (FCR), emission factors (EFs), annual FC, and emissions of TTF using uncertainty analysis. Also, the impact of COVID-19 pandemic period was considered on the studied parameters. The results showed that TTF have high FCRs of 18.68 L/100 km (95% CI=17.67–19.69 L/100 km), which are not affected by age or mileage of taxis, significantly. The estimated EFs for TTF are higher than Euro standards, but the differences are not significant. However, it is critical as can be an indication of inefficiency of periodic regulatory technical inspection tests for TTF. COVID-19 pandemic caused significant decrease in annual total FC and emissions (9.03–15.6%), but significant increase in EFs of per-passenger-kilometer traveled (47.9–57.3%). Annual vehicle-kilometer-traveled by TTF and the estimated EFs for gasoline-compressed natural gas bi-fueled TTF are the main influential parameters in the variability of annual FC and emission levels. More studies on sustainable FC and emissions mitigation strategies are needed for TTF. [ABSTRACT FROM AUTHOR]

A comprehensive methane (CH4) budget, including spatial and temporal patterns of sources and sinks, is pivotal for effective climatic mitigation and prediction. This study used a bottom‐up approach to evaluate the CH4 budget of Asia during 1970–2021. Natural sources and sinks were evaluated using a process‐based biogeochemical model and with ground and satellite observations. Anthropogenic sources were evaluated using emission inventories produced by socioeconomic studies. During the 1970s and 2010s, anthropogenic emissions accounted for 78% and 84% of the emitted total CH4 budget of 140.2 and 207.6 Tg CH4 yr−1, respectively. Anthropogenic sources were responsible for a 49% increase in total emissions. During 2001–2021, Central, East, South, Southeast, and West Asia accounted for 3.3%, 35.1%, 25.6%, 23.1%, and 13.0%, respectively, of Asian emissions. Sectoral compositions and temporal variations differed among subregions. Emissions from paddy field soils in Asia were smaller in 2021 (33.6 Tg CH4 yr−1) than in 1970 (39.2 Tg CH4 yr−1) because of changes in agricultural production. Among natural sources, wetlands accounted for 63% of total emissions in East Asia, but geological emissions predominated in Central and West Asia. Interannual variability of the net CH4 budget was mainly due to wetland and biomass burning emissions, whereas decadal variability was due to anthropogenic sectors. δ13C‐CH4 and isoflux analysis of sector‐specific fluxes and isotopic ratios indicated the effects of the regional budget on atmospheric trends. Bottom‐up evaluations of regional budgets can provide information critical to the Global Stocktake of the Paris Agreement, the Global Methane Pledge, and other related actions. Key Points: We used a bottom‐up approach to estimate methane emissions from Asia and their spatial distribution during 1970–2021Asia was estimated to be a net source of methane of 207.6 Tg CH4 yr−1 in the 2010s, 84% of which came from anthropogenic sourcesThe estimated net emissions and their δ13C signature suggested that Asia made a large contribution to atmospheric methane trends [ABSTRACT FROM AUTHOR]

Measurements of carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO) are of great importance in the Qinghai-Tibetan region, as it is the highest and largest plateau in the world affecting global weather and climate systems. In this study, for the first time, we present CO2, CH4, and CO column measurements carried out by a Bruker EM27/SUN Fourier-transform infrared spectrometer (FTIR) at Golmud (36.42°E, 94.91°N, 2808 m) in August 2021. The mean and standard deviation of the column-average dry-air mixing ratio of CO2, CH4, and CO (XCO2, XCH4, and XCO) are 409.3 ± 0.4 ppm, 1905.5 ± 19.4 ppb, and 103.1 ± 7.7 ppb, respectively. The differences between the FTIR co-located TROPOMI/S5P satellite measurements at Golmud are 0.68 ± 0.64% (13.1 ± 12.2 ppb) for XCH4 and 9.81 ± 3.48% (−10.7 ± 3.8 ppb) for XCO, which are within their retrieval uncertainties. High correlations for both XCH4 and XCO are observed between the FTIR and S5P satellite measurements. Using the FLEXPART model and satellite measurements, we find that enhanced CH4 and CO columns in Golmud are affected by anthropogenic emissions transported from North India. This study provides an insight into the variations of the CO2, CH4, and CO columns in the Qinghai-Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Beach wrack of marine macrophytes is a natural component of many beaches. To test if such wrack emits the potent greenhouse gas methane, field measurements were made at different seasons on beach wrack depositions of different ages, exposure, and distance from the water. Methane emissions varied greatly, from 0 to 176 mg CH4-C m−2 day−1, with a clear positive correlation between emission and temperature. Dry wrack had lower emissions than wet. Using temperature data from 2016 to 2020, seasonal changes in fluxes were calculated for a natural wrack accumulation area. Such calculated average emissions were close to zero during winter, but peaked in summer, with very high emissions when daily temperatures exceeded 20 °C. We conclude that waterlogged beach wrack significantly contributes to greenhouse gas emissions and that emissions might drastically increase with increasing global temperatures. When beach wrack is collected into heaps away from the water, the emissions are however close to zero. [ABSTRACT FROM AUTHOR]

Keywords: bottom-up methane estimates; greenhouse gas; methane flux; modeling; top-down methane inventories; wetland EN bottom-up methane estimates greenhouse gas methane flux modeling top-down methane inventories wetland 4175 4177 3 07/06/23 20230801 NES 230801 The wetland at Old Woman Creek National Estuarine Research Reserve, with flux tower US-OWC, which is characterized by high methane fluxes, high spatial heterogeneity, dynamic hydrology and water level fluctuations, and high lateral transport of dissolved organic carbon and nutrients, embodies many of the challenges in modeling methane fluxes. Bottom-up methane estimates, greenhouse gas, modeling, top-down methane inventories, wetland, methane flux Observational constraints reduce model spread but not uncertainty in global wetland methane emission estimates. [Extracted from the article]

Carbon dioxide and methane are the two most important greenhouse gases and are closely related to global warming and extreme weather events. To master their spatial and temporal variations, the CO2 and CH4 concentration data monitored by the GOSAT satellite in 2020 and 2021 were used to map and analyse the annual, seasonal and monthly changes in CO2 and CH4 concentrations in the world and major countries/regions. The results demonstrate that (1) in 2021, the average annual CO2 concentration over the global land area was 412.74 ppm, an increase in 0.64% compared with the same period last year, and there were spatial differences in the distribution of CO2 concentration, with high values mostly concentrated in the middle latitudes of the Northern Hemisphere; (2) compared with 2020, the CO2 concentration in China, the United States, India, the European Union and other countries/regions increased significantly; (3) in 2020 and 2021, the quarterly CO2 trend of the global and major countries/regions was the same, which was higher in the first (January, February, March) and second (April, May, June) quarters, significantly lower in the third (July, August, September) quarter, and gradually increased in the fourth (October, November, December) quarter. Further work on long time series and validation needs to be conducted. [ABSTRACT FROM AUTHOR]

A kerosene fuelled aircraft was modelled within a performance tool and fuel burn and the emissions of nitrogen oxides (NOx) and water vapour at different stages of flight throughout the mission were estimated. Adaptions were made to engine and aircraft parameters within the performance tool to accommodate a liquid hydrogen fuel over the same given mission. Once an iterative design process had been completed to ensure the aircraft could perform the given mission, the performance tool was again used to calculate total fuel burn. Fuel burn results alongside predicted emission indices were used to estimate the emissions of NOx, water vapour from hydrogen-fuelled aircraft. The use of hydrogen fuel over kerosene fuel in the modelled aircraft resulted in the removal of carbon-based emission species alongside 86% reduction in NOx and 4.3 times increase in water vapour emission. The climate impact of this switch with the reduction in NOx emission was assessed by a 3D global atmospheric chemistry and transport model, STOCHEM-CRI, which found a significant reduction in the concentration of a potent greenhouse gas, ozone, and an oxidizing agent, OH, by up to 6% and 25%, respectively. The reduction of OH levels could induce a positive radiative forcing effect as the lifetime of another important greenhouse gas, methane, is increased. However, the magnitude of this increase is very small (~0.3%), thus the overall impact of the reduction in NOx emissions is likely to have a net negative radiative forcing effect, improving aviation's impact on the environment. However, further work is warranted on effects of other emission species, specifically water vapour, particulate matter and unburned hydrogen. [ABSTRACT FROM AUTHOR]

Improving the mitigation of global greenhouse effects requires drastic reductions in carbon emissions. Effective carbon management at different spatial scales is therefore crucial to a more sustainable path of economic development. Regional and local carbon management currently rely on the traditional bottom-up method of emission reports, which are known for their opaque and complex procedures. Novel top-down carbon monitoring methods including space-based, aerial and ground-based observations are tantalising alternatives to more reliable data sources of carbon emissions as well as sinks. Unmanned Aerial Vehicles (UAVs) based carbon monitoring is an upcoming field because of its lower costs, higher spatial-temporal resolution and operability at a wider range of weather conditions compared to satellite observations. UAVs equipped with Non-dispersive Infrared (NDIR) carbon dioxide (CO 2) sensors can measure in the air with faster response and can intelligently navigate themselves within areas of greater interest. With the help of lighter NDIR payloads and multiple UAVs flying in synchrony, larger cross sections or areas for maximising information acquisition of emission plumes can be covered. In this article, we first introduce the design of a miniaturised NDIR payload, which is compatible with a lightweight UAV to achieve a single system weighing less than 4 kg. Then, we test our design via comparisons against another NDIR payload to determine its accuracy (−0.82 ppm and −0.44 ppm) and precision (0.12 ppm and −0.03 ppm) in measurements of two vertical concentration profiles. Finally, we deployed four UAVs on two major anthropogenic emission source monitoring campaigns with coordinated flight patterns. To fully understand the benefit of multi-UAV carbon monitoring, we attempted two flight patterns and computed power plant emission rates via the cross-sectional flux method. While all flights successfully captured the concentration enhancements downwind of the power plants, the single-sectional flight pattern measured an emission rate of 10ktCO 2 /day which is 44% less than that reported monthly and the multi-sectional flight pattern yielded relatively varied results (8.5ktCO 2 /day on average, which is 54% less) for the respective sections due to wind underestimations, plume section undersampling and wind fluctuations. • Miniaturised Non-dispersive infrared carbon dioxide measurement payload compatible with a lightweight drone. • First attempt of monitoring man-made emissions with four drones flying in unison. • We acquired high resolution 3D structure of carbon dioxide plumes within time spans of 15 min. [ABSTRACT FROM AUTHOR]

Continuous-wave cavity ring-down spectroscopy (CW-CRDS) is an important technical means to monitor greenhouse gases in atmospheric environment. In this paper, a CW-CRDS system is built to meet the needs of atmospheric methane monitoring. The problem of mode matching is explained from the perspective of transverse mode and longitudinal mode, and the influence of laser injection efficiency on measurement precision is further analyzed. The results of cavity ring-down time measurement show that the measurement precision is higher when the laser is coupled with the fundamental mode. In the experiment, DFB laser is used to calibrate the system with standard methane concentration, and the measurement residual is less than ±4 Ă— 10âˆ'4 ÎĽsâˆ'1. The methane concentration in the air is monitored in real time for two days. The results show the consistency of the concentration changes over the two days, which further demonstrates the reliability of the system for the measurement of trace methane. By analyzing the influence of mode matching, it not only assists the adjustment of the optical path, but also further improves the sensitivity of the system measurement. [ABSTRACT FROM AUTHOR]

Through an analysis of CH4 data observed at different altitudes at the atmospheric background station in Lin'an from 2016 to 2020, in combination with back-trajectory and distribution characteristics of potential source areas, the CH4 concentration variations at higher and lower altitudes and their relationships with sources and sinks were studied. The results showed that the CH4 concentration was characterized by notable diurnal variations. The largest concentration difference occurred between 5 and 7 a.m.; the concentration difference in summer was higher than that in the other three seasons. Background filtering of the hourly CH4 concentration was carried out using a numerical method. The results showed that the difference in the CH4 background concentration between the two altitudes was 4.6 ppb (SD = 7.9). The CH4 background concentrations at the two altitudes had the same seasonal variation: double peaks and valleys. The peaks appeared in May and December, and the valleys appeared in March and July. In spring and summer, the potential CH4 source areas were mainly distributed in the rice planting and wetland discharge regions. In autumn, they were mainly distributed in regions affected by fugitive emissions from rice planting and coal mining. In winter, they were mainly distributed in livestock and poultry management regions. [ABSTRACT FROM AUTHOR]

Atmospheric methane (CH4) concentrations have shown a puzzling resumption in growth since 2007 following a period of stabilization from 2000 to 2006. Multiple hypotheses have been proposed to explain the temporal variations in CH4 growth, and attribute the rise of atmospheric CH4 either to increases in emissions from fossil fuel activities, agriculture and natural wetlands, or to a decrease in the atmospheric chemical sink. Here, we use a comprehensive ensemble of CH4 source estimates and isotopic δ13C-CH4 source signature data to show that the resumption of CH4 growth is most likely due to increased anthropogenic emissions. Our emission scenarios that have the fewest biases with respect to isotopic composition suggest that the agriculture, landfill and waste sectors were responsible for 53 ± 13% of the renewed growth over the period 2007–2017 compared to 2000–2006; industrial fossil fuel sources explained an additional 34 ± 24%, and wetland sources contributed the least at 13 ± 9%. The hypothesis that a large increase in emissions from natural wetlands drove the decrease in atmospheric δ13C-CH4 values cannot be reconciled with current process-based wetland CH4 models. This finding suggests the need for increased wetland measurements to better understand the contemporary and future role of wetlands in the rise of atmospheric methane and climate feedback. Our findings highlight the predominant role of anthropogenic activities in driving the growth of atmospheric CH4 concentrations. [ABSTRACT FROM AUTHOR]

The atmospheric methane (CH4) burden is rising sharply, but the causes are still not well understood. One factor of uncertainty is the importance of tropical CH4 emissions into the global mix. Isotopic signatures of major sources remain poorly constrained, despite their usefulness in constraining the global methane budget. Here, a collection of new δ13CCH4 signatures is presented for a range of tropical wetlands and rice fields determined from air samples collected during campaigns from 2016 to 2020. Long-term monitoring of δ13CCH4 in ambient air has been conducted at the Chacaltaya observatory, Bolivia and Southern Botswana. Both long-term records are dominated by biogenic CH4 sources, with isotopic signatures expected from wetland sources. From the longer-term Bolivian record, a seasonal isotopic shift is observed corresponding to wetland extent suggesting that there is input of relatively isotopically light CH4 to the atmosphere during periods of reduced wetland extent. This new data expands the geographical extent and range of measurements of tropical wetland and rice δ13CCH4 sources and hints at significant seasonal variation in tropical wetland δ13CCH4 signatures which may be important to capture in future global and regional models. [ABSTRACT FROM AUTHOR]

Various countries have rapidly implemented strict actions to slow the blowout of COVID-19. Many events were dis-regarded, and anthropogenic activities such as industrial and transport systems were at a stoppage. Many countries were on lockdown, including the largest emitters of carbon dioxide. Due to these lockdowns, anthropogenic activities have been reduced and reported that air quality improves at a regional scale in many countries, including India. Therefore, the current study using Greenhouse Gases Observing Satellite (GOSAT/IBUKI) datasets to monitor the fluctuation of the average global concentration of dry mole fractions of atmospheric Methane (CH4) and Carbon Dioxide (CO2) during these pandemic lockdowns from January to June 2020. Outputs emphasize no significant reduction in the average concentration of dry mole fractions of atmospheric CH4 over the globe, but a minor reduction was observed in global CO2 engagement. The average concentration of both gases compares at each ten-degree latitude. The study reveals that, against the regional breakdowns, these short time lockdowns are not enough to control the concentration of these greenhouse gases at a larger scale, such as 10˚ latitude and globally, except for a minor reduction in CO2 concentration. [ABSTRACT FROM AUTHOR]

Seasonal methane emissions from natural wetlands in Nakhon Ratchasima province were estimated based on 12-month field works obtained from the actual methane flux measurements at a natural wetland. Methane gas was measured monthly with a static closed chamber technique and later analyzed by a gas chromatography equipped with a flame ionization detector (GC-FID). Results showed that methane fluxes varied widely in the range of 1.9-22.7 mg m-2day-1 with the median ± SD of 10.1 ± 5.4 mg m-2day-1. Seasonally, the methane fluxes during wet season ranged from 5.2 to 22.7 mg m-2day-1 with the median ± SD of 14.1 ± 5.0 mg m-2day-1 while the methane fluxes during dry season were between 1.9 and 21.9 mg m-2day-1 with the median ± SD of 8.8 ± 5.2 mg m-2day-1. The estimate methane fluxes of the wetland in wet and dry seasons were 1.5-3.1 kg m-2 and 0.7-2.9 kg m-2, respectively. The estimated methane emission factor from the natural wetland in the province was 1.7 to 5.7 kg m-2year-1 compared to the default methane emission factor from IPCC, 0.0136 kg m-2year-1. When considering global warming potential (GWP) of methane based on 100-year time horizon, the natural wetlands in the province may emit about 15.48 to 52.16 million ton CO2 equivalent a year based on the emission factor derived locally. With the IPCC default emission factor, the methane emission was as low as 0.03 to 0.22 million ton CO2 equivalent a year. [ABSTRACT FROM AUTHOR]

We study the drivers behind the global atmospheric methane (CH4) increase observed after 2006. Candidate emission and sink scenarios are constructed based on proposed hypotheses in the literature. These scenarios are simulated in the TM5 tracer transport model for 1984–2016 to produce three‐dimensional fields of CH4 and δ13C‐CH4, which are compared with observations to test the competing hypotheses in the literature in one common model framework. We find that the fossil fuel (FF) CH4 emission trend from the Emissions Database for Global Atmospheric Research 4.3.2 inventory does not agree with observed δ13C‐CH4. Increased FF CH4 emissions are unlikely to be the dominant driver for the post‐2006 global CH4 increase despite the possibility for a small FF emission increase. We also find that a significant decrease in the abundance of hydroxyl radicals (OH) cannot explain the post‐2006 global CH4 increase since it does not track the observed decrease in global mean δ13C‐CH4. Different CH4 sinks have different fractionation factors for δ13C‐CH4, thus we can investigate the uncertainty introduced by the reaction of CH4 with tropospheric chlorine (Cl), a CH4 sink whose abundance, spatial distribution, and temporal changes remain uncertain. Our results show that including or excluding tropospheric Cl as a 13 Tg/year CH4 sink in our model changes the magnitude of estimated fossil emissions by ∼20%. We also found that by using different wetland emissions based on a static versus a dynamic wetland area map, the partitioning between FF and microbial sources differs by 20 Tg/year, ∼12% of estimated fossil emissions. Key Points: Increased fossil fuel emissions are unlikely the dominant driver for post‐2006 global CH4 increaseA significant decrease in the abundance of hydroxyl radicals (OH) cannot explain the post‐2006 global CH4 increaseCH4 emission attributions are sensitive to the uncertainties in OH fractionation, tropospheric Cl sink, and wetland areas [ABSTRACT FROM AUTHOR]

In recent times methanotrophs have gained immense interest due to their ability to sequester and utilize methane, which is an inexpensive carbon source as well as a very potent greenhouse gas. Despite their vast potential for both industrial and environmental applications, only a few methanotrophs have been well studied to date. A novel and an efficient obligate methanotroph was isolated from a methanotrophic consortium originated from a municipal solid waste leachate. The isolate belonged to the genus Methylosarcina as confirmed by 16S ribosomal ribonucleic acid (rRNA) phylogenetic analysis and was further characterized by scanning electron microscopy, chemotaxonomic, and carbon, hydrogen, nitrogen, and sulphur (CHNS) elemental analysis. Methane oxidation by the isolate was studied in detail, including kinetics, specific oxygen requirement, and carbon capture efficiency. Further, the fate of oxidized methane was determined, unraveling its potential in the bioremediation of methane. [ABSTRACT FROM AUTHOR]

Methane is an important greenhouse gas with growing atmospheric concentrations. Freshwater lakes and reservoirs contribute substantially to atmospheric methane concentrations, but the magnitude of this contribution is poorly constrained. Uncertainty stems partially from whether the sites currently sampled represent the global population as well as incomplete knowledge of which environmental variables predict methane flux. Thus, determining the main drivers of methane flux across diverse waterbody types will inform more accurate upscaling approaches. Here we use a new database of total, diffusive, and ebullitive areal methane emissions from 313 lakes and reservoirs (ranging in surface area from 6 m2 to 5,400 km2) to identify the best predictors of methane emission. We found that the best predictors of methane emission differed by waterbody type (lakes vs. reservoirs), and that ecosystem morphometric variables (e.g., surface area and maximum depth) were more important predictors in lakes whereas metrics of autochthonous production (e.g., chlorophyll a) were more important in reservoirs. We also found that productivity strongly predicted methane ebullition, whereas ecosystem morphometry and waterbody type were more important predictors of diffusive methane flux. Finally, we identify several knowledge gaps that limit upscaling efforts. First, we need more methane emission measurements in small reservoirs, large lakes, and both natural and artificial ponds. Additionally, more accurate upscaling efforts require improved global information about waterbody surface area, waterbody type (lake vs. reservoir), ice phenology, and the distribution of productivity‐related predictor variables such as total phosphorus, DOC, and chlorophyll a. Plain Language Summary: Methane is an important greenhouse gas, and its atmospheric concentrations are steadily rising. Inland waters, such as lakes and reservoirs, are an important source of methane, but there is uncertainty in the magnitude of emissions as well as the types of systems that emit more methane. Recent work suggests that high ecosystem productivity (e.g., high rates of photosynthesis) leads to high methane emission and that smaller waterbodies tend to emit more methane than larger ones. Still, the extent to which these patterns differ in lakes versus reservoirs (lakes formed behind dams) is not known. Here, we examined the best predictors of lake and reservoir methane flux using a new data set of methane emissions from 313 lakes and reservoirs. The best predictors of methane emission differed between lakes and reservoirs: lake methane flux was more driven by waterbody shape and size, whereas reservoir methane flux was related more to productivity. Future global‐scale estimates of lake and reservoir methane emission should consider these systems separately. With improved understanding of the drivers of methane flux from lakes and reservoirs, we can better inform models of global methane emissions and target future emissions reductions. Key Points: The best predictors of methane emission differ between lakes and reservoirsMorphometric features better predict methane emission in lakes, whereas chlorophyll a is a better predictor in reservoirsTo improve global upscaling, we need more emission measurements from small reservoirs, large lakes, and natural and artificial ponds [ABSTRACT FROM AUTHOR]

Methane emissions from sewer networks are an important source of anthropogenic greenhouse gases (GHGs) but are not currently reflected in the national GHG inventory. We found significant CH 4 emissions of approximately 573 [395–831] CH 4 t y−1 from sewer networks in the old residential and commercial areas of Seoul (Gwanak district) using an electric vehicle-based atmospheric GHG monitoring platform. The majority of ethane-to-methane ratios (<0.005) from the observations further suggest that distinctive CH 4 emissions from sewer networks are likely related to microbial activity rather than to simple natural gas leakage. Because over 90% of the sewer network in Seoul is a gravity drain type of combined sewer network, where both wastewater and stormwater flow through the same pipes, resulting in the generation of methane emissions from the microbial activity and the manholes and rain gutters, which are directly connected to the combined sewer networks are major sources of atmospheric methane emissions. This study suggests that appropriate treatment of sewer networks can mitigate missing methane emissions in cities that were not originally included in GHG inventory of South Korea. [Display omitted] • CH 4 emissions from urban sewer networks have been quantified using mobile measurements. • Most CH 4 hotspots in urban residential areas are microbial rather than fossil fuel sources. • Significant CH4 emissions of approximately 573 CH 4 t y−1 from sewer networks in the old city of Seoul. [ABSTRACT FROM AUTHOR]

An increase in the atmospheric concentration of methane greenhouse gas results in an increase in global warming, surface water evaporation, and amount of the solutes in relevant areas besides, it also causes climate change. In these situations, methanotrophs that are resistant to climate change and deserted conditions could be applied. Methylococcus, as one of the most extensively studied methanotrophs is highly resistant to salinity, a strain of which was isolated in this study from Hoz-e Soltan Salt Lake in Iran as an example influenced by climate change. Isolated strain was identified. To determine the ability of the bacteria for decrease of methane gas, the culture medium was analyzed by gas chromatography. Results showed that isolated strain is able to grow in salt proportion of 3.3% and acidic pH of 3.5 in vitro, reducing more than 75% of total methane gas within 10 days. In addition to nitrogen fixation ability of the strain, positive results obtained regarding drought tolerance and heat shock in this study confirmed that Methylococcus strains may be able to withstand environmental conditions of foreseeable future. Thus, to prevent methane emission in regions such as Hoz-e Soltan Salt Lake in Iran, more compatible methanotroph strains need to be identified and used. [ABSTRACT FROM AUTHOR]

Rice is the main staple food for more than half of the world's population, but rice cultivation is a significant source of atmospheric methane (CH 4). Alternate wetting and drying (AWD) reduces CH 4 emission from paddy field, but the effect on rice yield remains unclear. Organic soil amendment increases grain yield but simultaneously increases CH 4 emission. Therefore, the combination of AWD and organic amendment may compensate for each other's shortcomings. The objective of this study was to assess whether AWD and organic amendment can increase rice yield while mitigating CH 4 emission. The experiments were conducted in a farmer's paddy field in Thua Thien Hue province, Vietnam, during five consecutive rice growing seasons in 2019–2021. Two water management practices, continuous flooding (CF) and AWD, with (+O) or without (−O) the application of a commercially available organic fertilizer were examined under normal cultivation conditions. Compared with CF, AWD significantly reduced CH 4 emission by 34 %, increased nitrous oxide (N 2 O) by 46 %, and increased grain yield by 4.4 %. The +O treatment significantly increased the yield by 3.7 % relative to −O. Relative to CF−O by season, AWD+O significantly increased the yield. The integrated global warming potential of CH 4 and N 2 O emissions was decreased by 33 % and irrigation water use was reduced by 33 % in AWD plots relative to that in CF plots. These results indicate that AWD by itself has the potential to increase rice yield as well as reduce CH 4 emission, and the combination of AWD with organic amendment would ensure yield increase. • AWD increased rice yield, reduced CH 4 emission, but increased N 2 O emission. • Organic fertilizer increased rice yield but did not affect CH 4 and N 2 O emissions. • Combined positive effect of AWD and organic fertilizer on rice yield was additive. • Number of drained days in AWD well explained CH 4 emission reduction. [ABSTRACT FROM AUTHOR]

The vast pan-Arctic wetlands appear to be a large source of methane (CH 4), a potent greenhouse gas. Here, I simulated CH 4 emission from pan-Arctic wetlands (above 60°N) over the period 1901–2016 using a process-based biogeochemical model (VISIT), including two different schemes of wetland CH 4 production, that was forced by historical atmospheric composition, climate, and land-use conditions. The two schemes simulated mean wetland CH 4 emission rates of 10.9 and 11.4 Tg CH 4 yr−1 in the 2000s with slightly different spatial source distributions. Both simulations showed clear seasonal cycles of CH 4 emission, but the two schemes had different amplitudes and peak months. The schemes differed markedly in their simulated long-term patterns (i.e., a stationary trend and an increasing decadal trend), making it difficult to relate them to global atmospheric trends in a consistent manner. Linear regression analysis clarified the agreements and differences in environmental sensitivity and biological control of CH 4 production simulated by the schemes. The time-series of CH 4 emission in northern North America, northern Europe, and Siberia showed different patterns of correlation with two major meteorological teleconnection indices. This model-based study implied that the high-latitude CH 4 emission accounts for 5–7% of global wetland emission and would play increasingly important roles (e.g., positive feedback) under changing climate. To reduce estimation uncertainties, as demonstrated by the comparison of two schemes, we need further studies for improving biogeochemical models in collaboration with field studies. [ABSTRACT FROM AUTHOR]

Karst caves are considered a natural sink of atmospheric methane (CH 4). Studies generally focus on measuring CH 4 within the cave environment, and little information is available about the factors influencing CH 4 abundance, such as the surrounding vadose and saturated zones of the aquifer, inputs of carbon dioxide (CO 2) – a potential precursor of CH 4 – and methanotrophic bacteria, which is likely the main cause of CH 4 consumption. In this paper we report the first study of CH 4 budget in Nerja, one of the main karst caves in Spain, based on seasonal monitoring of CH 4 and CO 2 concentration and stable C isotopic ratio in the cave system, integrated with analogue data from the underlying and overlying saturated/vadose zone, and methanotrophic bacteria activity (rDNA and Polymerase Chain Reaction analyses) in cave sediments and water. The results show that the cave environment consumes, via methanotrophic activity and through γ- and α–Proteobacteria, CH 4 coming from both the atmosphere and the vadose/saturated zones of the aquifer, where it is produced microbially. However, vadose zone methanogenesis may seasonally exceed the methanotrophy capacity of the cave, resulting in a net CH 4 increase in it. This competition process may be an important factor limiting the sink potential of karst caves. [ABSTRACT FROM AUTHOR]

Legislation in the State of California mandates reductions in emissions of short‐lived climate pollutants of 40% from 2013 levels by 2030 for CH4. Identification of the sector(s) responsible for these emissions and their temporal and spatial variability is a key step in achieving these goals. Here, we determine the emissions of CH4 in Los Angeles from 2011–2017 using a mountaintop remote sensing mapping spectrometer. We show that the pattern of CH4 emissions contains both seasonal and nonseasonal contributions. We find that the seasonal component peaks in the winter and is correlated (R2 = 0.58) with utility natural gas consumption from the residential and commercial sectors and not from the industrial and gas‐fired power plant sectors. The nonseasonal component is (22.9 ± 1.4) Gg CH4/month. If the seasonal correlation is causal, about (1.4 ± 0.1)% of the commercial and residential natural gas consumption in Los Angeles is released into the atmosphere. Plain Language Summary: CH4 is a desirable target for greenhouse gas emission reductions because emission controls will have a rapid impact on radiative forcing. However, its emission budget is highly uncertain and poorly quantified. This paper reports new results from a novel mountaintop remote sensing spectrometer overlooking the Los Angeles basin. The study shows that the megacity's methane emissions are strongly correlated with the consumption of natural gas by residential and commercial consumers, with a leakage rate of (1.4 ± 0.1)%, while the nonseasonal component is (22.9 ± 1.4) Gg CH4/month. By identifying a clear relationship between CH4 emissions and natural gas consumption, our results provide strong constraints on the pathways for fugitive CH4 emissions from the natural gas distribution system in Los Angeles. Key Points: A mountaintop remote sensing spectrometer is used to derive the time series and spatial pattern of methane emissions in LA basinThe methane emissions in the LA basin are strongly correlated with the consumption of natural gas by residential and commercial consumersAbout (1.4 ± 0.1)% of the residential and commercial natural gas consumption in LA is released into the atmosphere [ABSTRACT FROM AUTHOR]

Aqueous amines are the most commonly used CO2 absorbents. But they have many shortcomings such as volatile loss, thermal degradation, corrosivity and wetting on polymer membranes. Ionic liquids can avoid the shortcomings and show unique performances. Diethanolamine glycinate, a ionic liquid made of hydroxyl ammonium amino acid salt, was developed to capture CO2 for greenhouse gas emission reduction. In order to evaluate the ionic liquid performance, membrane flux, gas outlet CO2 concentration, appropriate ionic liquid concentration and operational parameters were investigated by membrane gas absorption. Membrane liquid desorption was tested for regeneration of diethanolamine glycinate. Performance of diethanolamine glycinate ionic liquid was also compared with that of glycinate and diethanolamine. Results show that diethanolamine glycinate ionic liquid had a higher membrane flux, e.g., 6.6 × 10−4 mol m−2 s−1, and a lower gas outlet CO2 concentration (yout/yin, e.g., 0.07) in membrane gas absorption. Changes in operational conditions, e.g., gas and liquid flowrates increase, changed the membrane flux of membrane gas absorption, which increased only by 23.9%. Dominant factors affecting the membrane flux were the gas CO2 mole fraction, increasing the flux by 80.0%, and the ionic liquid concentration, increasing the flux more than 2 times. The ionic liquid was much more easily regenerated and possessed large regeneration efficiency and average regeneration velocity. Diethanolamine glycinate ionic liquid is thus an efficient and promising CO2 absorbent. [ABSTRACT FROM AUTHOR]

Measurements of methane () mole fractions and that resolve the diel cycle in the agriculturally dominated Reuss Valley, Switzerland, were used to quantify the contributions of different sources to the atmospheric source mix. Both a nocturnal (NBL) and a diurnal convective boundary layer (CBL) approach were employed. A diel course of mole fractions was found with a daytime minimum (background around 1900 ppb) and a nocturnal maximum (up to 3500 ppb). The C value in only showed small variations during the day (9-21 hours CET, ‰ ) when the atmosphere was well mixed, but decreased by ‰ during the night. Biogenic emissions dominated in both approaches (ranging from 60 to 94%), but non-biogenic sources were rather important (42.2% and 46.0% with CBL, 5.8% and 40% with NBL approach in 2011 and 2012, respectively, of total emissions). The sink, dominated by tropospheric OH oxidation and only to a minor extend by soil surface uptake, was quantified at roughly 4% of local emissions. [ABSTRACT FROM AUTHOR]

Chamber measurements of trace gas fluxes between the land surface and the atmosphere have been conducted for almost a century. Different chamber techniques, including static and dynamic, have been used with varying degrees of success in estimating greenhouse gases (CO2, CH4, N2O) fluxes. However, all of these have certain disadvantages which have either prevented them from providing an adequate estimate of greenhouse gas exchange or restricted them to be used under limited conditions. Generally, chamber methods are relatively low in cost and simple to operate. In combination with the appropriate sample allocations, chamber methods are adaptable for a wide variety of studies from local to global spatial scales, and they are particularly well suited for in situ and laboratory-based studies. Consequently, chamber measurements will play an important role in the portfolio of the Pan-European long-term research infrastructure Integrated Carbon Observation System. The respective working group of the Integrated Carbon Observation System Ecosystem Monitoring Station Assembly has decided to ascertain standards and quality checks for automated and manual chamber systems instead of defining one or several standard systems provided by commercial manufacturers in order to define minimum requirements for chamber measurements. The defined requirements and recommendations related to chamber measurements are described here. [ABSTRACT FROM AUTHOR]

Methane (CH 4) is the second most significant driver of global warming, following carbon dioxide. However, the spatial-temporal variation of CH 4 and its driving factors largely remain unclear. Here we selected the Northern Hemisphere as the study area. We used the data from the Total Column Carbon Observing Network (TCCON) to assess the accuracy of the Greenhouse Gases Observing Satellite (GOSAT) Proxy XCH 4 (column-averaged dry air mixing ratio of CH 4) data. We then analyzed the spatial-temporal distribution of XCH 4 in the Northern Hemisphere, and further quantified the influencing factors using geographic detectors. The results showed that during 2009–2021, the annual mean XCH 4 increased from 2009 (1775.19 ppb) to 2021 (1872.71 ppb), with an increasing rate of 7.50 ppb/year. The monthly average value was the lowest in May (1805.65 ppb) and the highest in September (1825.63 ppb). The XCH 4 in the low-latitude region was higher than that in the high-latitudinal region. The geographic detector showed that anthropogenic activities were the main factors affecting the XCH 4. Our results revealed the spatial-temporal patterns XCH 4 and their driving factors in the Northern Hemisphere, and thus provided a scientific basis for the management of this greenhouse gas in the future. • Spatial-temporal patterns of methane in the Northern Hemisphere are analyzed. • The XCH 4 increased from 1775.19 to 1872.71 ppb during 2009–2021. • Human activity is the main driving factor for the increasing XCH 4 concentration. [ABSTRACT FROM AUTHOR]

Natural gas pipelines are an important source of fugitive methane emissions in lifecycle greenhouse gas assessments but limited monitoring has taken place of UK pipelines to quantify fugitive emissions. This study investigated methane emissions from the UK high-pressure pipeline system (National Transmission System - NTS) for natural gas pipelines. Mobile surveys of CH 4 emissions were conducted across four areas in the UK, with routes bisecting high-pressure pipelines (with a maximum operating pressure of 85 bar) and separate control routes away from the pipelines. A manual survey of soil gas measurements was also conducted along one of the high-pressure pipelines using a tunable diode laser. For the pipeline routes, there were 26 peaks above 2.1 ppmv CH 4 at 0.23 peaks/km, compared with 12 peaks at 0.11 peaks/km on control routes. Three distinct thermogenic emissions were identified on the basis of the isotopic signal from these elevated concentrations with a peak rate of 0.03 peaks/km. A further three thermogenic emissions on pipeline routes were associated with pipeline infrastructure. Methane fluxes from control routes were statistically significantly lower than the fluxes measured on pipeline routes, with an overall pipeline flux of 627 (241–1123 interquartile range) tonnes CH 4 /km/yr. Soil gas CH 4 measurements indicated a total flux of 62.6 kt CH 4 /yr, which equates to 2.9% of total annual CH4 emissions in the UK. We recommend further monitoring of the UK natural gas pipeline network, with assessments of transmission and distribution stations, and distribution pipelines necessary. [ABSTRACT FROM AUTHOR]

The present study examines the spatiotemporal variability and interrelations of the atmospheric methane (CH 4 ), carbon monoxide (CO) and biomass burning (BB) outbreaks retrieved from satellite data over the Amazon region during the 2003–2012 period. In the climatological context, we found consistent seasonal cycles of BB outbreaks and CO in the Amazon, both variables showing a peak during the dry season. The dominant CO variability mode features the largest positive loadings in the southern Amazon, and describes the interannual CO variations related to BB outbreaks along the deforestation arc during the dry season. In line with CO variability and BB outbreaks, the results show strong correspondence with the spatiotemporal variability of CH 4 in the southern Amazon during years of intense drought. Indeed, the areas with the largest positive CH 4 anomalies in southern Amazon overlap the areas with high BB outbreaks and positive CO anomalies. The analyses also showed that high (low) BB outbreaks in the southern Amazon occur during dry (wet) years. In consequence, the interannual climate variability modulates the BB outbreaks in the southern Amazon, which in turn have considerable impacts on CO and CH 4 interannual variability in the region. Therefore, the BB outbreaks might play a major role in modulating the CH 4 and CO variations, at least in the southern Amazon. This study also provides a comparison between the estimate of satellite and aircraft measurements for the CH 4 over the southern Amazon, which indicates relatively small differences from the aircraft measurements in the lower troposphere, with errors ranging from 0.18% to 1.76%. [ABSTRACT FROM AUTHOR]

The air in subterranean karst cavities is often depleted in methane (CH 4 ) relative to the atmosphere. Karst is considered a potential sink for the atmospheric greenhouse gas CH 4 because its subsurface drainage networks and solution-enlarged fractures facilitate atmospheric exchange. Karst landscapes cover about 14% of earth's continental surface, but observations of CH 4 concentrations in cave air are limited to localized studies in Gibraltar, Spain, Indiana (USA), Vietnam, Australia, and by incomplete isotopic data. To test if karst is acting as a global CH 4 sink, we measured the CH 4 concentrations, δ 13 C CH 4 , and δ 2 H CH 4 values of cave air from 33 caves in the USA and three caves in New Zealand. We also measured CO 2 concentrations, δ 13 C CO 2 , and radon (Rn) concentrations to support CH 4 data interpretation by assessing cave air residence times and mixing processes. Among these caves, 35 exhibited subatmospheric CH 4 concentrations in at least one location compared to their local atmospheric backgrounds. CH 4 concentrations, δ 13 C CH 4 , and δ 2 H CH 4 values suggest that microbial methanotrophy within caves is the primary CH 4 consumption mechanism. Only 5 locations from 3 caves showed elevated CH 4 concentrations compared to the atmospheric background and could be ascribed to local CH 4 sources from sewage and outgassing swamp water. Several associated δ 13 C CH 4 and δ 2 H CH 4 values point to carbonate reduction and acetate fermentation as biochemical pathways of limited methanogenesis in karst environments and suggest that these pathways occur in the environment over large spatial scales. Our data show that karst environments function as a global CH 4 sink. [ABSTRACT FROM AUTHOR]

To explore microbial mechanisms of straw-induced changes in CO 2 , CH 4 , and N 2 O emissions from paddy field, wheat straw was amended to two paddy soils from Taizhou (TZ) and Yixing (YX), China for 60 d under flooded condition. Illumia sequencing was used to characterize shift in bacterial community compositions. Compared to control, 1–5% straw amendment significantly elevated CO 2 and CH 4 emissions with higher increase at higher application rates, mainly due to increased soil DOC concentrations. In contrast, straw amendment decreased N 2 O emission. Considering CO 2 , CH 4 , and N 2 O emissions as a whole, an overall increase in global warming potential was observed with straw amendment. Total CO 2 and CH 4 emissions from straw-amended soils were significantly higher for YX than TZ soil, suggesting that straw-induced greenhouse gas emissions depended on soil characteristics. The abundance of C-turnover bacteria Firmicutes increased from 28–41% to 54–77% with straw amendment, thereby increasing CO 2 and CH 4 emissions. However, straw amendment reduced the abundance of denitrifying bacteria Proteobacteria from 18% to 7.2–13% or increased the abundance of N 2 O reducing bacteria Clostridium from 7.6–11% to 13–30%, thereby decreasing N 2 O emission. The results suggested straw amendment strongly influenced greenhouse gas emissions via alerting soil properties and bacterial community compositions. Future field application is needed to ascertain the effects of straw return on greenhouse gas emissions. [ABSTRACT FROM AUTHOR]

Straw incorporation and domestic sewage irrigation have been recommended as an environmentally friendly agricultural practice and are widely used not only in China but also in other countries. The individual effects on yield and environmental impacts have been studied extensively, but the comprehensive effect when straw returning and domestic sewage irrigation are combined together has seldom been reported. This study was conducted to examine the effects of straw returning and domestic sewage irrigation on rice yields, greenhouse gas emissions (GHGs) and ammonia (NH 3 ) volatilization from paddy fields from 2015 to 2016. The results showed that the rice yield was not affected by the irrigation water sources and straw returning under the same total N input, which was similar in both years. Due to the rich N in the domestic sewage, domestic sewage irrigation could reduce approximately 45.2% of chemical nitrogen fertilizer input without yield loss. Compared to straw removal treatments, straw returning significantly increased the CH 4 emissions by approximately 7–9-fold under domestic sewage irrigation and 13–14-fold under tap water irrigation. Straw returning also increased the N 2 O emissions under the two irrigation water types. In addition, the seasonal NH 3 volatilization loss was significantly increased by 88.8% and 61.2% under straw returning compared to straw removal in 2015 and 2016, respectively. However, domestic sewage irrigation could decrease CH 4 emissions by 24.5–26.6%, N 2 O emissions by 37.0–39.0% and seasonal NH 3 volatilization loss by 27.2–28.3% under straw returning compared to tap water irrigation treatments. Global warming potentials (GWP) and greenhouse gas intensities (GHGI) were significantly increased with straw returning compared with those of straw removal, while they were decreased by domestic sewage irrigation under straw returning compared to tap water irrigation. Significant interactions between straw returning and domestic sewage irrigation on NH 3 volatilization loss, CH 4 and N 2 O emissions were observed. The results indicate that domestic sewage irrigation combined with straw returning could be an environmentally friendly and resource-saving agricultural management measure for paddy fields with which to reduce the chemical N input, GHG emissions, and NH 3 volatilization loss while maintaining high rice productivity. [ABSTRACT FROM AUTHOR]

We have conducted systematic observations of the CH4 mole fraction and its carbon isotope ratio (δ13C) at Ny-Ålesund, Svalbard (78°55′N, 11°56′E) using air samples collected weekly since 1991 and 1996, respectively. The CH4 mole fraction showed long-term increase until 1999, stagnation between 2000 and 2006, followed by an increase after 2006. On the other hand, δ13C showed monotonous increase until 2006 and decrease after 2006. By comparing the rates of change in the CH4 mole fraction and δ13C under the assumption that the atmospheric CH4 lifetime is constant, it is suggested that the temporal pause of the CH4 mole fraction observed at Ny-Ålesund is attributed to reductions of CH4 release from the microbial and fossil fuel sectors. On the other hand, the increase in CH4 after 2006 could be ascribed to an increase in microbial CH4 release. The CH4 and δ13C data presented in this paper would be useful for clarifying their temporal variations in the Arctic atmosphere, as well as providing additional constraints on the global CH4 budget.

Subterranean methanotrophy is a potentially important but overlooked sink for the atmospheric greenhouse gas methane (CH 4 ). This study documents a microbial CH 4 sink in tropical subterranean karst cavities in Vietnam's northern karst province where porosity, steep topography, and scarce soil and vegetation cover foster the exchange of subterranean air with the atmosphere. Our data are based on (i) surveys of CH 4 , carbon dioxide, and radon concentrations in the air of 11 caves, (ii) in situ mesocosm experiments in caves, as well as (iii) laboratory mesocosm measurements using sediment and rock from caves. The extent of CH 4 depletion in cave air depends on the ventilation rate and the availability of moisture to provide a habitat for CH 4 -oxidizing bacteria, both of which are seasonally variable in northern Vietnam and in part depend on monsoonal activity. Mesocosm experiments using fresh versus sterilized rock and sediment confirmed the role of microbial methanotrophy towards uptake of CH 4 from cave air. Our results also suggest that within-cave heterogeneity of environmental variables like salinity may affect rates of CH 4 oxidation. We conservatively estimate that 150,000 metric tons of atmospheric CH 4 are microbially oxidized annually in the ~ 29,000 km 2 of Vietnamese tropical karst, which would compensate for ~ 7% of Vietnam's agricultural CH 4 emissions from rice farming and livestock. Future studies estimating the global fluxes of the atmospheric greenhouse gas CH 4 should consider subterranean karst as a potentially important CH 4 sink. [ABSTRACT FROM AUTHOR]

In order to expand current atmospheric methane monitoring capabilities, we investigated the performance of an off-the-shelf, low-cost, metal oxide based methane (CH 4 ) gas sensor. A sensor assembly was designed that powered the gas sensor and its resistive heater, provided ancillary temperature ( T ) and relative humidity ( rH ) measurements, and controlled sensor read-out and data storage. After calibrating the gas sensor with respect to methane concentration ( [CH 4 ] ), T , and rH , we were able to estimate the [CH 4 ] of lab air over a period of 31 days for a large range of T and rH conditions with a systematic error of −1.0 ppm and a variable error within ±1.7 ppm. The sensor showed no significant drift in [CH 4 ] estimate. We show that sensor accuracy can likely be improved by optimizing the voltage regulator that powers the gas sensor’s heater, and by measuring and compensating for the difference in partial oxygen pressure of the air that was sampled during calibration and validation experiments. Such improvements are expected to allow the use of the sensor assembly for fence-line methane monitoring, for example near fossil fuel extraction sites. In its current form, the sensor assembly is suitable for detecting fugitive methane from leak-prone equipment. [ABSTRACT FROM AUTHOR]