Off-axis Integrated Cavity Output Spectroscopy Research Articles

Cavity-enhanced Faraday rotation spectroscopy for interference-free measurement of OH radical at 2.8 μm

An instrument based on cavity-enhanced Faraday rotation spectroscopy (CE-FRS) operating at 2.8 μm has been developed for interference-free measurement of OH radicals in the laboratory. By off-axis coupling of a continuous-wave laser into a high finesse optical cavity, FRS signal is obtained from balanced detection of time-integrated light intensity leaking out of the cavity in the presence of magnetic field. Radio-frequency white noise (5–520 MHz) was injected into laser current which reduced intensity fluctuations in cavity transmission, thus improved the signal-to-noise ratio of the spectroscopic signal by a factor of 2. The setup provides a simple and robust spectroscopic instrument for in-situ and highly-selective detection of paramagnetic species. We demonstrated the instrument’s capabilities using OH radical with concentration in the range of 1012 molecule.cm−3, generated by microwave discharge of water vapor at low pressure. The CE-FRS instrument exhibited a limit of detection of ∼ 1010 molecule.cm−3 in an integration time of 20 s, which is enhanced by a factor of 2.5 compared to cavity-enhanced wavelength modulation spectroscopy involving an off-axis integrated cavity output spectroscopy approach. A time-resolved FRS signal was recorded in a pulsed microwave discharge regime, giving a millisecond time resolution for the measurement of OH concentration profile. The developed instrument provides a potential analytical tool for the measurement of OH concentration for chemical kinetic study in reactor cells.

Near-Infrared Dual-Range Methane Sensor Using the OAIC-HC Mode Based on VMD-SG-Assisted Optical Noise Suppression.

Based on tunable diode laser absorption spectroscopy and off-axis integrated cavity output spectroscopy, a dual-range methane hybrid sensor was constructed utilizing the overtone absorption band of CH4 gas molecules at 1653.7 nm. By simultaneously utilizing an off-axis integrated cavity and Herriott cell with an effective absorption path of 11 and 405 m, respectively, the two received photoelectric signals are decomposed into different frequency components by VMD and then reconstructed after SG filtering. Applying the global optimization algorithm DA-ELM to CH4 concentration inversion, the correlation coefficient R2 is as high as 0.9995. Through long-term stability verification, the instrument's standard deviation at 1 ppm is 27 ppb. After Allan-Werle deviation analysis, the sensor's limit of detection is 2.298 ppb at an integration time of 138 s. Using the self-developed sensor, the detection of dual-range trace CH4 gas is achieved, enabling a dynamic detection range of trace CH4 gas ranging from 400 ppb to 1000 ppm. The sensor realizes dual-range methane trace detection and actively controls methane emissions to improve environmental quality while taking into account the safety benefits of reducing production accidents.

Foliar methane and nitrous oxide fluxes in tropical tree species

Methane (CH₄) and nitrous oxide (N₂O) are critical biogenic greenhouse gases (GHGs) with global warming potentials substantially greater than that of carbon dioxide (CO₂). The exchange of these gases in tropical forests, particularly via foliar processes, remains poorly understood. We quantified foliar CH₄ and N₂O fluxes among tropical tree species and examined their potential association with the leaf economics spectrum (LES) traits. Sampling within Lawachara National Park, Bangladesh, we used in-situ measurements of foliar CH₄ and N₂O fluxes employing off-axis integrated cavity output spectroscopy (CH₄, CO₂ and H₂O) and optical feedback–cavity enhanced absorption spectroscopy (N₂O) analyzers. Leaves were measured under dark, low, and high (0, 100, and 1000 μmol·m−2·s−1) light conditions. Surveyed tree species exhibited both net foliar uptake and efflux of CH₄, with a mean flux not different from zero, suggesting negligible net foliar emissions at the stand level. Plant families showed differences in CH₄, but not N₂O fluxes. Consistent efflux was observed for N₂O, with a mean of 0.562 ± 0.060 pmol·m−2·s−1. Pioneer species exhibited a higher mean N₂O flux (0.81 ± 0.17 pmol·m−2·s−1) compared to late-successional species (0.37 ± 0.05 pmol·m−2·s−1). Pioneer species also showed a trend toward a higher mean CH₄ flux (0.24 ± 0.21 nmol·m−2·s−1) compared to mid-successional (−0.01 ± 0.26 nmol·m−2·s−1) and late-successional species (−0.05 ± 0.28 nmol·m−2·s−1). Moreover, among all leaf traits within the leaf economic spectrum, a significant positive relationship was observed between leaf N₂O flux and total leaf nitrogen. Our results suggest that pioneer tree species significantly contribute to net CH₄ and N₂O emissions, potentially counteracting the carbon sequestration benefits in regenerating tropical forests. These findings indicate that accurate GHG budgeting should include direct measurements of foliar CH₄ and N₂O fluxes. Moreover, the results suggest that forest conservation and management strategies that prioritize late successional species will better mitigate GHG emissions.

Research on Off-Axis Integrated Cavity Output Spectrum Signal Denoising Based on CSGWO-SVMD-SVD method

To mitigate the susceptibility of absorption lines to noise interference during gas measurement using the off-axis integrated cavity output spectroscopy (OA-ICOS) technique, thereby enhancing measurement accuracy, this paper introduces a novel denoising method. This method synergistically integrates Grey Wolf Optimization (GWO) and Cuckoo Search (CS) with Singular Value Decomposition (SVD) and Sequential Variational Mode Decomposition (SVMD). Initially, the optimal solution for the quadratic penalty coefficient in SVMD is ascertained via iterative optimization using the CSGWO algorithm. Subsequently, a circulant matrix is established to extract the singular values of each modal component. A threshold is set to discriminate between noise and useful signals, and the singular values corresponding to noise are nullified. The useful signal components are then reconstructed, yielding the final processed signal. The proposed algorithm was applied to both simulated and experimental target signals and compared with common filtering algorithms such as WT, VMD-WTD, and S-G. In the experimental signal processing results, the signal-to-noise ratio (SNR) of the absorption spectrum signal improved from 21.4 to 39.95, and the correlation coefficient increased from 0.99715 to 0.99946. Results indicate that the proposed algorithm exhibits superior identification and noise suppression capabilities compared to other algorithms. After signal processing using the CSGWO-SVMD-SVD algorithm, the accuracy and stability of signal data detected based on off-axis integration cavity output spectroscopy technology have been greatly improved.

Large, sustained soil CO2 efflux but rapid recovery of CH4 oxidation in post-harvest and post-fire stands in a mixedwood boreal forest

The net effect of forest disturbances, such as fires and harvesting, on soil greenhouse gas fluxes is determined by their impacts on both biological and physical factors, as well as the temporal dynamics of these effects post-disturbance. Although harvesting and fire may have distinct effects on soil carbon (C) dynamics, the temporal patterns in soil CO2 and CH4 fluxes and the potential differences between types of disturbances, remain poorly characterized in boreal forests. In this study, we measured soil CO2 and CH4 fluxes using a off-axis integrated cavity output spectroscopy system in snow-free seasons over two years in post-harvest and post-fire chronosequence sites within a mixedwood boreal forest in northwestern Ontario, Canada. Soil CO2 efflux showed a post-disturbance peak, with differing dynamics depending on the disturbance type: post-harvest stands exhibited a nearly tenfold increase (from ∼1 to ∼11 μmol CO2.m-2.s−1) from 1 to 9–10 years post-disturbance, followed by a steep decline; post-fire stands showed a more gradual increase, peaking at ∼6–7.2 μmol CO2.m-2.s−1 after ∼12–15 years. The youngest post-harvest stands were net sources of CH4,whereas post-fire stands were never net CH4 sources. In both disturbance types, the strength of the CH4 sink increased with stand age, approaching ∼2.4 nmol.m-2.s−1 by 15 years post-disturbance. Volumetric water content, bulk density, litter depth, and pH were significant predictors of CO2 fluxes; for CH4 fluxes, litter depth, pH, and the interaction of VWC and soil temperature were significant predictors in both disturbance types, with EC also showing a relationship in post-harvest stands. Our findings indicate that while soil CH4 oxidation rapidly recovers following disturbance, both post-harvest and post-fire stands show a multi-decade release of soil CO2 that is too large to be offset by C gains over this period.

Simultaneous measurement of multi-component gases from pyrolysis of hydrocarbon fuels using absorption spectroscopy

Endothermic hydrocarbon fuel is cracked into small hydrocarbons at high temperatures. Quantitative analysis of the pyrolysis product’s component concentration is of great significance for the analysis of engine combustion regenerative cooling technology and combustion efficiency. Off-axis integrated cavity output spectroscopy technique was adopted in laser absorption spectroscopy. Using the near-infrared spectroscopic properties of methane and ethylene, the component concentrations of methane and ethylene at different temperatures and cracking times were measured. In addition, a high temperature kerosene pyrolysis spectrum measurement system was built, and a concentration measurement error of not more than 3.2 % was achieved. Pyrolysis experiment shows that at low temperatures and short heating times, the concentration of the components that crack methane and ethylene is low. Ethylene is the first to crack as the temperature rises, and it accounts for a relativity large proportion of the mixture. Other gases crack out with the increase of the cracking time. The component concentration of ethylene is basically stable at approximately 12 %.

Dual-path coupling V-shaped structure off-axis integrated cavity output spectroscopy (V-OA-ICOS) for water vapor stable isotope detection at 3.66 μm

Residual cavity mode noise and weak output signal are the main factors that restrict the detection performance of off-axis integrated cavity output spectroscopy (OA-ICOS). In order to overcome the above drawbacks, a novel V-shaped off-axis integrated cavity output spectroscopy (V-OA-ICOS) device was demonstrated, and the dual-path coupling method was further proposed. The dual-path coupling V-OA-ICOS device can not only suppress the residual cavity mode noise but also enhance the intensity of the output signal. The V-shaped resonator cavity ensures a more uniform and square-shaped spot pattern distribution on the surface of the highly reflective mirrors, significantly reducing spots overlap. The dual-path coupling method can re-couple the laser reflected by the front cavity mirror, thereby increasing the total energy and mode density of the cavity. The spectrum of water vapor stable isotopes HD16O and H216O at 3.66 μm was measured to illustrate the performance of the presented approach, the dual-path coupling V-OA-ICOS increases the signal-to-noise ratio (SNR) by a factor ∼ 2.1 compared to single path V-OA-ICOS and the signal intensity by a factor of ∼ 2. The δD variation was obtained by measuring the outdoor water vapor isotope using this device for three consecutive days. The variation range of δD is −220‰ ∼ −120‰. This work proposes a new cavity structure and optical re-coupling method, which offers a novel approach to enhance the performance of OA-ICOS devices.

Precipitation fuels dissolved greenhouse gas (CO2, CH4, N2O) dynamics in a peatland-dominated headwater stream: results from a continuous monitoring setup

Stream ecosystems are actively involved in the biogeochemical cycling of carbon (C) and nitrogen (N) from terrestrial and aquatic sources. Streams hydrologically connected to peatland soils are suggested to receive significant quantities of particulate, dissolved, and gaseous C and N species, which directly enhance losses of greenhouse gases (GHGs), i.e., carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), and fuel in-stream GHG production. However, riverine GHG concentrations and emissions are highly dynamic due to temporally and spatially variable hydrological, meteorological, and biogeochemical conditions. In this study, we present a complete GHG monitoring system in a peatland stream, which can continuously measure dissolved GHG concentrations and allows to infer gaseous fluxes between the stream and the atmosphere and discuss the results from March 31 to August 25 at variable hydrological conditions during a cool spring and warm summer period. Stream water was continuously pumped into a water-air equilibration chamber, with the equilibrated and actively dried gas phase being measured with two GHG analyzers for CO2 and N2O and CH4 based on Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) and Non-Dispersive Infra-Red (NDIR) spectroscopy, respectively. GHG measurements were performed continuously with only shorter measurement interruptions, mostly following a regular maintenance program. The results showed strong dynamics of GHGs with hourly mean concentrations up to 9959.1, 1478.6, and 9.9 parts per million (ppm) and emissions up to 313.89, 1.17, and 0.40 mg C or N m−2h−1 for CO2, CH4, and N2O, respectively. Significantly higher GHG concentrations and emissions were observed shortly after intense precipitation events at increasing stream water levels, contributing 59% to the total GHG budget of 762.2 g m−2 CO2-equivalents (CO2-eq). The GHG data indicated a constantly strong terrestrial signal from peatland pore waters, with high concentrations of dissolved GHGs being flushed into the stream water after precipitation. During drier periods, CO2 and CH4 dynamics were strongly influenced by in-stream metabolism. Continuous and high-frequency GHG data are needed to assess short- and long-term dynamics in stream ecosystems and for improved source partitioning between in-situ and ex-situ production.

Re-injection off-axis integrated cavity output spectroscopy for the simultaneous detection of N2O, H2O and CO with a mid-infrared QCL laser.

Off-axis integrated cavity output spectroscopy (OA-ICOS) has attracted much interest because it potentially allows highly sensitive field measurements with robust optical alignment. In this paper, a novel instrument that employs a high-finesse optical cavity as an absorption cell has been developed for sensitive measurements of multi-component gases N2O, H2O and CO in the atmosphere based on a mid-infrared quantum cascade laser (QCL) and OA-ICOS. In order to improve the energy utilization and increase the signal-to-noise ratio (SNR) of the signal, a new type of optical path structure of the laser re-injection method is adopted. Furthermore, the system performance can be effectively improved by using a new intervention method of injecting radio frequency (RF) white noise into a laser driver to suppress cavity mode noise and combining the wavelength modulation method (WMS). We compared the sensitivity of the second harmonic signal demodulation between the re-injection method and the standard OA-ICOS, and the SNR increased by 2.68 times compared to the latter. Analysis of the spectral measurements with Allan variance indicates that within an integration time of 1 s, the measurement accuracy of N2O, H2O, and CO is 6.71 ppb, 13.945 ppm, and 1.81 ppb, respectively, and within an integration time of 820 s, the measurement accuracy of N2O, H2O, and CO can be further improved to 1.26 ppb, 2.089 ppm, and 172 ppt, respectively. Our approach represents an underlying analytical method that provides guidelines for monitoring of representative gases in the atmosphere, industrial processes, emergency safety, etc.

High pressure ammonia/methanol oxidation up to 100 atm

High pressure ammonia/methanol oxidation and NOx formations were investigated using a recently developed supercritical pressure jet-stirred reactor (SP-JSR) at 20 and 100 atm with temperatures between 550 and 950 K and equivalence ratios of 0.138 and 1.15. The experimental results show that NH3 oxidation at high pressure is significantly accelerated by the active OH radicals produced from CH3OH oxidation. Furthermore, the kinetic interactions between NH3 and CH3OH are governed mainly by the reactions CH3OH + NH2 = CH2OH + NH3, CH3OH + NH2 = CH3O + NH3, and CH2O + NH2 = HCO + NH3. A HP-Mech model for high-pressure NH3/CH3OH oxidation was developed in this study. It consists of the most recent NH3 and CH3OH models including some new reactions and updated rate constants from the literature as well as NH3-CH3OH interactions where rate constants of CH3OH + NH2 = CH2OH + NH3, CH3OH + NH2 = CH3O + NH3, NH2 + CH2O = NH3 + HCO, and NH2 + CH2O = NH2CHO + H were theoretically calculated in this study. Our model with these updates improves the prediction for the measured N2O/NOx temperature dependence at 100 atm. In addition, the reaction pathway and sensitivity analysis show that N2O/NOx/HONO interactions with HO2 are very important, especially for a fuel-lean mixture at 100 atm. The HONO mole fraction for the fuel-lean mixture at 100 atm was then measured by off-axis integrated cavity output spectroscopy (ICOS) at wavenumber of 6638.26 cm−1. The experimental data show a significant HONO formation at intermediate temperature that is strongly underpredicted by numerical simulation at 100 atm. Therefore, the HONO related reactions with notable uncertainty at high pressure such as NO + OH (+M) = HONO (+M) and H2NO + NO2 = HONO + HNO need deeper exploration in the future.

Measurement of Carbon Isotopes in Methane Based on Off-Axis Integrated Cavity Output Spectroscopy Technology with Booster Optical Amplifier

Measurement of Carbon Isotopes in Methane Based on Off-Axis Integrated Cavity Output Spectroscopy Technology with Booster Optical Amplifier

Designing a Long Optical Path Direct-Injection-Integrated Cavity for Laser Absorption Spectroscopy

Trace gas measurement has a wide range of applications needed in industrial, medical, and environmental protection. With the evolution of time, the demand for real-time, sensitivity, and accuracy of gas detection has been increasingly heightened. Off-axis integrated cavity output spectroscopy (OA-ICOS) is an effective method for the high-sensitivity detection of trace gases. It uses an integrated cavity with two highly reflective mirrors to provide a long optical path, which guarantees its high sensitivity. However, as the reflectivity of the mirrors increases, the intensity of the output light decreases, and the signal-to-noise ratio decreases. This contradiction makes it difficult to achieve a long optical path and a high signal-to-noise ratio together. To combat this issue, this paper proposes a type of integrated cavity using a direct-injection method. This structure, under equivalent mirror conditions, can maintain an effective absorption optical path very close to the original off-axis integrated cavity while increasing the output light intensity hundreds of times. This enhancement increases the sensitivity of OA-ICOS.

Wavelength modulation enhanced off-axis integrated cavity output spectroscopy for OH radical measurement at 2.8 µm

Wavelength modulation enhanced off-axis integrated cavity output spectroscopy for OH radical measurement at 2.8 µm

Simultaneous detection of CO2/CH4 based on off-axis integrated cavity output spectroscopy and time-division-multiplexing-based wavelength modulation spectroscopy

A highly sensitive dual gases detection system was established for detecting the concentrations of carbon dioxide (CO2) and methane (CH4) simultaneously based on off-axis integrated cavity output spectroscopy (OA-ICOS) technology. Two Distributed Feedback (DFB) lasers with central wavelengths of 1580 nm and 1653 nm were used as the light sources of the system. A time-division-multiplexing-based wavelength modulation spectroscopy (WMS) method was used to scan the current of lasers for improving the signal-to-noise ratio of the system and getting the optimal modulation amplitudes of CO2 and CH4 simultaneously. In addition, the cavity mode noise was suppressed by radio frequency (RF) white noise loading and laser current to further improve the measurement accuracy. Allan deviation analysis indicates that measurement precision of 0.483 ppm for CO2 and 1.37 ppb for CH4 was achieved by averaging up to 993 s and 1093 s respectively. This compact and high-precision measurement system is suitable for measuring atmospheric greenhouse gases.

Simultaneous detection of CH4 and CO2 through dual modulation off-axis integrated cavity output spectroscopy

This study established a novel method for the simultaneous detection of two-component gases. Radio frequency (RF) white noise disturbance laser current and wavelength modulation were simultaneously used to improve the off-axis integrated cavity output spectroscopy technique, and a high-precision dual modulation OA-ICOS (RF-WM-OA-ICOS) system was established. The two laser beams were coupled into one laser beam that was applied incident to the cavity of RF-WM-OA-ICOS system. The second harmonic signals of CH4 and CO2 gas simultaneously appeared in the rising or falling edge of a triangular wave. This method was used to measure CH4 and CO2 with different concentrations. The results indicated that the proposed system has high stability and can accurately and simultaneously measure the concentrations of CH4 and CO2, with an optimal integration time of 220 s. The minimum detection limit was 10 ppb for CH4 and 1.5 ppm for CO2. The corresponding noise equivalent absorption sensitivity values were calculated as 2.67 × 10−13 cm−1⋅Hz−1/2 and 5.18 × 10−11 cm−1⋅Hz−1/2, respectively. The proposed dual-component gas simultaneous detection method can also be used for high-precision simultaneous detection of other gases. Therefore, this study may serve as a reference for developing portable multicomponent gas analyzers.

On-line measurement of atmospheric oxygen by an open-path OA-ICOS based sensor with high accuracy at ambient pressure

In this paper, a compact optical oxygen (O2) sensor system was demonstrated based on off-axis integrated cavity output spectroscopy (OA-ICOS). Light from a distributed feedback laser near 761 nm was coupled into an open-path F-P cavity by off-axis configuration. To address the challenges of changing field environments for open-path configuration, the influence of pressure on the sensor system was studied. A linewidth enhancement factor (δ) was proposed to measure the ambient pressure and a pressure correction method was developed to improve the measurement accuracy. A minimum detection limit of 0.0038% was obtained for an optimum averaging time of 20 s under 760 Torr. Indoor and outdoor O2 concentration measurements were conducted and the sensing applicability of the sensors system at ambient pressure was verified. The demonstrated sensor system allows operation without a vacuum pump and a pressure controller combine with the self-developed laser driver and digital lock-in amplifier, which has advantages of reduced size, low power consumption, low cost and has wide application prospect for O2 sensing in field environments.

Measurement of 24-h continuous human CH4 release in a whole room indirect calorimeter.

We describe the technology and validation of a new whole room indirect calorimeter (WRIC) methodology to quantify volume of methane (VCH4) released from the human body over 24 h concurrently with the assessment of energy expenditure and substrate utilization. The new system extends the assessment of energy metabolism by adding CH4, a downstream product of microbiome fermentation that could contribute to energy balance. Our new system consists of an established WRIC combined with the addition of off-axis integrated-cavity output spectroscopy (OA-ICOS) to measure CH4 concentration ([CH4]). Development, validation, and reliability of the system included environmental experiments to measure the stability of the atmospheric [CH4], infusing CH4 into the WRIC and human cross-validation studies comparing [CH4] quantified by OA-ICOS and mid-infrared dual-comb spectroscopy (MIR DCS).Our infusion data indicated that the system measured 24-h [CH4] and VCH4 with high sensitivity, reliability, and validity. Cross-validation studies showed good agreement between OA-ICOS and MIR DCS technologies (r = 0.979, P < 0.0001). Human data revealed 24-h VCH4 was highly variable between subjects and within/between days. Finally, our method to quantify VCH4 released by breath or colon suggested that over 50% of the CH4 was eliminated through the breath. The method allows, for the first time, measurement of 24-h VCH4 (in kcal) and therefore the measurement of the proportion of human energy intake fermented to CH4 by the gut microbiome and released via breath or from the intestine; also, it allows us to track the effects of dietary, probiotic, bacterial, and fecal microbiota transplantation on VCH4.NEW & NOTEWORTHY This is the first time that continuous assessment of CH4 is reported in parallel with measurements of O2 consumption and CO2 production inside a whole room indirect calorimeter in humans and over 24 h. We provide a detailed description of the whole system and its parts. We carried out studies of reliability and validity of the whole system and its parts. CH4 is released in humans during daily activities.

Wavelength Modulation Off-axis Integrated Cavity Output Spectroscopy for High-sensitivity Detection of OH Radicals in Mid-infrared

Wavelength Modulation Off-axis Integrated Cavity Output Spectroscopy for High-sensitivity Detection of OH Radicals in Mid-infrared

Near-infrared detection and line intensity measurements of H2S with ECDL source based off-axis integrated cavity output spectroscopy

Hydrogen sulphide (H2S) is an extremely hazardous gas which leads to poisoning at higher concentration. Here, we demonstrate the trace detection and line intensity measurement of H2S by means of efficient and indigenously developed off–axis integrated cavity output spectroscopy (OA-ICOS) technique coupled with an external cavity diode laser operating in near-infrared spectral region. Trace detection of H2S was made with OA-ICOS absorbance spectra of ro-vibrational line (51, 5 → 61, 6 & 50, 5 → 60, 6) centered at ∼ 6344 cm−1 of ν1 + ν2 + ν3 band. A ∼ 600 ppbv limit of detection was achieved for an averaging time of 350 s with Allan deviation analysis. Additionally, line intensities of several degenerate R branch transitions of ν1 + ν2 + ν3 band including 51, 5 → 61, 6 & 50, 5 → 60, 6 transition was experimentally measured with OA-ICOS. The study will be valuable in aiding the spectroscopic databases of H2S and have potential application in extracting molecular concentrations in the planet’s atmospheres.

Methane emissions from forested closed landfill sites: Variations between tree species and landfill management practices

Trees in natural and managed environments can act as conduits for the transportation of methane (CH4) from below ground to the atmosphere, bypassing oxidation in aerobic surface soils. Tree stem emissions from landfill sites exhibit large temporal and spatial variability in temperate environments and can account for approximately 40% of the total surface CH4 flux. Emission variability was further investigated in this study by measuring CH4 and CO2 fluxes from landfill sites with different management strategies and varying tree species over a 7-month period. Stem and soil measurements were obtained using flux chambers and an off-axis integrated cavity output spectroscopy analyser. Analysis showed average stem and soil CH4 emissions varied significantly (p < 0.01) between landfills with different management practices. On average, tree stem CH4 fluxes from sites with no clay cap but gas extraction, clay cap and gas extraction, and no clay cap and no gas extraction were 1.4 ± 0.4 μg m−2 h−1, 47.2 ± 19.0 μg m−2 h−1, and 111.9 ± 165.1 μg m−2 h−1, respectively. There was no difference in stem CH4 fluxes between species at each site, suggesting environmental conditions (waterlogging) and site age had a greater influence on both stem and soil fluxes. These results highlight the importance of management practices, and the resultant environmental conditions, in determining CH4 emissions from historic landfill sites.