Adsorption studies of aqueous Pb(II) onto a sugarcane bagasse/multi-walled carbon nanotube composite

Adsorption of Pb²⁺ from aqueous solution onto a sugarcane bagasse/multi-walled carbon nanotube (MWCNT) composite was investigated by using a series of batch adsorption experiments and compared with the metal uptake ability of sugarcane bagasse. The efficiency of the adsorption processes was studied experimentally at various pH values, contact times, adsorbent masses, temperatures and initial Pb²⁺ concentrations. A pH of 4.5 was found to be the optimum pH to obtain a maximum adsorption percentage in 120 min of equilibration time. The composite showed a much enhanced adsorption capacity for Pb²⁺ of 56.6 mg g⁻¹ compared with 23.8 mg g⁻¹ for bagasse at 28 °C. The Langmuir adsorption isotherm provided the best fit to the equilibrium adsorption data. The pseudo first-order, pseudo second-order, intraparticle diffusion and Elovich kinetics models were used to analyse the rate of lead adsorption and the results show that the Elovich model is more suitable. The thermodynamic parameters of adsorption, namely ΔG°, ΔH° and ΔS°, were determined over the temperature range of 20–45 °C. The adsorption of Pb²⁺ onto both bagasse and the sugarcane bagasse/MWCNT composite was found to be spontaneous but for the former adsorbent it was enthalpy-driven whereas for the latter it was entropy-driven. Desorption of the lead-loaded adsorbents was fairly efficient with 0.1 mol dm⁻³ HCl. Overall this composite has the potential to be a good adsorbent for the removal of Pb²⁺ from wastewaters.     

Volatile emissions and gas geochemistry of Hot Spring Basin,Yellowstone National Park,USA

We characterize and quantify volatile emissions at Hot Spring Basin (HSB), a large acid-sulfate region that lies just outside the northeastern edge of the 640 ka Yellowstone Caldera. Relative to other thermal areas in Yellowstone, HSB gases are rich in He and H₂, and mildly enriched in CH₄ and H₂S. Gas compositions are consistent with boiling directly off a deep geothermal liquid at depth as it migrates toward the surface. This fluid, and the gases evolved from it, carries geochemical signatures of magmatic volatiles and water–rock reactions with multiple crustal sources, including limestones or quartz-rich sediments with low K/U (or ^40?Ar/^4?He). Variations in gas chemistry across the region reflect reservoir heterogeneity and variable degrees of boiling. Gas-geothermometer temperatures approach 300 °C and suggest that the reservoir feeding HSB is one of the hottest at Yellowstone. Diffuse CO₂ flux in the western basin of HSB, as measured by accumulation-chamber methods, is similar in magnitude to other acid-sulfate areas of Yellowstone and is well correlated to shallow soil temperatures. The extrapolation of diffuse CO₂ fluxes across all the thermal/altered area suggests that 410 ± 140 t d^− 1 CO₂ are emitted at HSB (vent emissions not included). Diffuse fluxes of H₂S were measured in Yellowstone for the first time and likely exceed 2.4 t d^− 1 at HSB. Comparing estimates of the total estimated diffuse H₂S emission to the amount of sulfur as SO₄²⁻ in streams indicates ~ 50% of the original H₂S in the gas emission is lost into shallow groundwater, precipitated as native sulfur, or vented through fumaroles. We estimate the heat output of HSB as ~ 140–370 MW using CO₂ as a tracer for steam condensate, but not including the contribution from fumaroles and hydrothermal vents. Overall, the diffuse heat and volatile fluxes of HSB are as great as some active volcanoes, but they are a small fraction (1–3% for CO₂, 2–8% for heat) of that estimated for the entire Yellowstone system.     

Effects of soil compaction,rain exposure and their interaction on soil carbon dioxide emission

Soils release more carbon, primarily as carbon dioxide (CO₂), per annum than current global anthropogenic emissions. Soils emit CO₂ through mineralization and decomposition of organic matter and respiration of roots and soil organisms. Given this, the evaluation of the effects of abiotic factors on microbial activity is of major importance when considering the mitigation of greenhouse gases emissions. Previous studies demonstrate that soil CO₂ emission is significantly affected by temperature and soil water content. A limited number of studies have illustrated the importance of bulk density and soil surface characteristics as a result of exposure to rain on CO₂ emission, however, none examine their relative importance. Therefore, this study investigated the effects of soil compaction and exposure of the soil surface to rainfall and their interaction on CO₂ release. We conducted a factorial laboratory experiment with three soil types after sieving (clay, silt and sand soil), three different bulk densities (1·1 g cm^–3, 1·3 g cm^–3, 1·5 g cm^–3) and three different exposures to rainfall (no rain, 30 minutes and 90 minutes of rainfall). The results demonstrated CO₂ release varied significantly with bulk density, exposure to rain and time. The relationship between rain exposure and CO₂ is positive: CO₂ emission was 53% and 42% greater for the 90 minutes and 30 minutes rainfall exposure, respectively, compared to those not exposed to rain. Bulk density exhibited a negative relationship with CO₂ emission: soil compacted to a bulk density of 1·1 g cm^–3 emitted 32% more CO₂ than soil compacted to 1·5 g cm^–3. Furthermore we found that the magnitude of CO₂ effluxes depended on the interaction of these two abiotic factors. Given these results, understanding the influence of soil compaction and raindrop impact on CO₂ emission could lead to modified soil management practices which promote carbon sequestration. Copyright © 2012 John Wiley & Sons, Ltd.     

Ecosystem scale evapotranspiration and net CO2 exchange from a restored peatland

An understanding of the symbiotic water and gas exchange processes at the ecosystem scale is essential to the development of appropriate restoration plans of extracted peatlands. This paper presents ecosystem scale measurements of the atmospheric exchange of water and carbon dioxide (CO₂) from a restored vacuum extracted peatland in eastern Québec, utilizing full‐scale micrometeorological measurements of both evaporation and CO₂. The results indicate that the adopted restoration practices reduce the loss of water from the peat, but CO₂ emissions are ～25% greater than an adjacent nonrestored comparison site. The blockage of drainage ditches and the existence of a mulch cover at the site keep the moisture conditions more or less constant. Consequently, the CO₂ flux, which is predominantly soil respiration, is strongly controlled by peat temperature fluctuations. Copyright © 2001 John Wiley & Sons, Ltd.     

Chemical composition and photochemical reactivity of exhaust from aircraft turbine engines

Assessment of the environmental impact of aircraft emissions is required by planners and policy makers. Seveal areas of concern are: 1. exposure of airport workers and urban residents to toxic chemicals emitted when the engines operate at low power (idle and taxi) on the ground; 2. contributions to urban photochemical air pollution of aircraft volatile organic and nitrogen oxides emissions from operations around airports; and 3. emissions of nitrogen oxides and particles during high-altitude operation. The environmental impact of chemicals emitted from jet aircraft turbine engines has not been firmly established due to lack of data regarding emission rates and identities of the compounds emitted. This paper describes an experimental study of two different aircraft turbine engines designed to determine detailed organic emissions, as well as emissions of inorganic gases. Emissions were measured at several engine power settings. Measurements were made of detailed organic composition from C₁ through C₁₇, CO, CO₂, NO, NO _x , and polycyclic aromatic hydrocarbons. Measurements were made using a multi-port sampling pro be positioned directly behind the engine in the exhaust exit plane. The emission measurements have been used to determine the organic distribution by carbon number and the distribution by compound class at each engine power level. The sum of the organic species was compared with an independent measurement of total organic carbon to assess the carbon mass balance. A portion of the exhaust was captured and irradiated in outdoor smog chambers to assess the photochemical reactivity of the emissions with respect to ozone formation. The reactivity of emissions from the two engines was apportioned by chemical compound class.     

Sources and variability of CO2 in a prealpine stream gravel bar

Gravel bars (GBs) contribute to carbon dioxide (CO₂) emissions from stream corridors, with CO₂ concentrations and emissions dependent on prevailing hydraulic, biochemical, and physicochemical conditions. We investigated CO₂ concentrations and fluxes across a GB in a prealpine stream over three different discharge‐temperature conditions. By combining field data with a reactive transport groundwater model, we were able to differentiate the most relevant hydrological and biogeochemical processes contributing to CO₂ dynamics. GB CO₂ concentrations showed significant spatial and temporal variability and were highest under the lowest flow and highest temperature conditions. Further, observed GB surface CO₂ evasion fluxes, measured CO₂ concentrations, and modelled aerobic respiration were highest at the tail of the GB over all conditions. Modelled CO₂ transport via streamwater downwelling contributed the largest fraction of the measured GB CO₂ concentrations (31% to 48%). This contribution increased its relative share at higher discharges as a result of a decrease in other sources. Also, it decreased from the GB head to tail across all discharge‐temperature conditions. Aerobic respiration accounted for 17% to 36% of measured surface CO₂ concentrations. Zoobenthic respiration was estimated to contribute between 4% and 8%, and direct groundwater CO₂ inputs 1% to 23%. Unexplained residuals accounted for 6% to 37% of the observed CO₂ concentrations at the GB surface. Overall, we highlight the dynamic role of subsurface aerobic respiration as a driver of spatial and temporal variability of CO₂ concentrations and evasion fluxes from a GB. As hydrological regimes in prealpine streams are predicted to change following climatic change, we propose that warming temperatures combined with extended periods of low flow will lead to increased CO₂ release via enhanced aerobic respiration in newly exposed GBs in prealpine stream corridors.     

Acoustic emission and volumetric strain induced in coal by the displacement sorption of methane and carbone dioxide

The aim of this study was to assess whether acoustic emission (AE) could carry information on preferential sorption/desorption of CH₄ or CO₂ in coal. AE and expansion/contraction of two nearly identical cylindrical coal samples were continuously monitored during displacement sorption experiments. One sample was subjected to presorption of CH₄, followed by sorption of CH₄/CO₂ mixture. With the other one, presorption of CO₂ preceded sorption of the mixture. The results obtained are the following: first, AE and stain kinetics show that the affinity of the coal tested is higher for CO₂ than for CH₄; second, methane is preferentially desorbed after presorption of CH₄ — sorption of mixture of CH₄ and CO₂; third, during displacement sorption, kinetics of AE and sample swelling/shrinkage bring out the importance of presorption and the sorbate used. It matters whether the coal is first exposed to CH₄ or to CO₂. The present study has demonstrated that injection of CO₂ into the coal previously exposed to CH₄ causes considerable swelling of the coal. On desorption after CH₄/CO₂ exchange sorption, initial shrinkage is followed by swelling of the coal. These results could have implications for the sequestration of CO₂ in coal seams and CH₄ recovery from coalbeds (ECBM). Swelling/shrinkage of the coal matrix should be included in models used to predict coal permeability and gas flow rates. They also show that the AE technique can give more insights into coal matrix-gas interactions.     

Diffuse Emission of CO2 from Showa-Shinzan, Hokkaido, Japan: A Sign of Volcanic Dome Degassing

Two soil CO₂ efflux surveys were carried out in September 1999 and June 2002 to study the spatial distribution of diffuse CO₂ degassing and estimate the total CO₂ output from Showa-Shinzan volcanic dome, Japan. Seventy-six and 81 measurements of CO₂ efflux were performed in 1999 and 2002, respectively, covering most of Showa-Shinzan volcano. Soil CO₂ efflux data showed a wide range of values up to 552 g m^-2 d^-1. Carbon isotope signatures of the soil CO₂ ranged from -0.9‰ to -30.9‰, suggesting a mixing between different carbon reservoirs. Most of the study area showed CO₂ efflux background values during the 1999 and 2002 surveys (B = 8.2 and 4.4 g m^-2 d^-1, respectively). The spatial distribution of CO₂ efflux anomalies for both surveys showed a good correlation with the soil temperature, indicating a similar origin for the extensive soil degassing generated by condensation processes and fluids discharged by the fumarolic system of Showa-Shinzan. The total diffuse CO₂ output of Showa-Shinzan was estimated to be about 14.0–15.6 t d^-1 of CO₂ for an area of 0.53 km².     

Reduction in Exhaust Gas Temperature of Biodiesel Fueled Engine by Exhaust Gas Recirculation

Biodiesel is a renewable alternative fuel that can be used as substitute for mineral diesel. The use of biodiesel in diesel engines reduces all of the exhaust emissions expect NO_x. The higher temperature in the combustion chamber increases the NO_x emissions. The recirculation of a small percentage of cooled exhaust gases into the combustion chamber through the intake manifold reduces the in‐cylinder temperature. Moreover, the cooling of exhaust gases before mixing with fresh air introduced to the engine does not affect its volumetric efficiency but it absorbs the heat of combustion. This leads to a reduction in NO_x emissions and improves the engine brake thermal efficiency. This paper describes the investigation of the performance and emission characteristics of a biodiesel‐fueled, naturally aspirated, single cylinder diesel engine with cooled exhaust gas recirculation.     

Transfer of carbon dioxide and methane through the soil‐water‐atmosphere system at Mer Bleue peatland,Canada

Surface waters associated with peatlands, supersaturated with CO₂ and CH₄ with respect to the atmosphere, act as important pathways linking a large and potentially unstable global repository of C to the atmosphere. Understanding the drivers and mechanisms which control C release from peatland systems to the atmosphere will contribute to better management and modelling of terrestrial C pools. We used non‐dispersive infra‐red (NDIR) CO₂ sensors to continuously measure gas concentrations in a beaver pond at Mer Bleue peatland (Canada); measurements were made between July and August 2007. Concentrations of CO₂ in the surface water (10 cm) reached 13 mg C l^?1 (epCO₂ 72), and 26 mg C l^?1 (epCO₂ 133) at depth (60 cm). The study also showed large diurnal fluctuations in dissolved CO₂ which ranged in amplitude from ～1·6 mg C l^?1 at 10 cm to ～0·2 mg C l^?1 at 60 cm depth. CH₄ concentration and supersaturation (epCH₄) measured using headspace analysis averaged 1·47 mg C l^?1 and 3252, respectively; diurnal cycling was also evident in CH₄ concentrations. Mean estimated evasion rates of CO₂ and CH₄ over the summer period were 44·92 ± 7·86 and 0·44 ± 0·25 µg C m^?2s^?1, respectively. Open water at Mer Bleue is a significant summer hotspot for greenhouse gas emissions within the catchment. Our results suggest that CO₂ concentrations during the summer in beaver ponds at Mer Bleue are strongly influenced by biological processes within the water column involving aquatic plants and algae (in situ photosynthesis and respiration). In terms of carbon cycling, soil‐stream connectivity at this time of year is therefore relatively weak. Copyright © 2008 John Wiley & Sons, Ltd.     

Potential application of activated carbon from maize tassel for the removal of heavy metals in water

Water-pollution problems worldwide have led to an acute shortage of clean and pure water for both domestic and human consumption. Various technologies and techniques are available for water treatment which includes the use of activated carbon. In this study activated carbons used for the removal of lead (II) ions from water samples were prepared from maize tassels (an agricultural waste residue) which were modified using physical and chemical activation. In the physical activation CO₂ was used as the activating agent, while in chemical activation H₃PO₄ with an impregnation ratio ranging from 1 to 4 was employed. The maize tassel was pyrolysed at different temperatures ranging from 300 °C to 700 °C in an inert atmosphere for a period of 60 min and activated at 700 °C for 30 min. The effects of activation temperature, impregnation ratio and duration were examined. The resultant modified tassels were characterised by measuring their particle-size distribution, porosities, pore volume, and pore-size distribution using scanning electron microscopy (SEM). The activated carbon produced by chemical activation had the highest BET surface area ranging from 623 m² g⁻¹ to 1 262 m² g⁻¹. The surface chemistry characteristics of the modified tassels were determined by FT–IR spectroscopy and Boehm’s titration method. The experimental data proved that properties of activated carbon depend on final temperature of the process, impregnation ratio and duration of the treatment at final temperature. The adsorption studies showed that chemically prepared activated carbon performed better than physically prepared activated carbon.     

A CO2-rich gas trigger of explosive paroxysms at Stromboli basaltic volcano,Italy

In addition to rhythmic slug-driven Strombolian activity, Stromboli volcano occasionally produces discrete explosive paroxysms (2 per year on average for the most frequent ones) that constitute a major hazard and whose origin remains poorly elucidated. Partial extrusion of the volatile-rich feeding basalt as aphyric pumice during these events has led to consider their triggering by the fast ascent of primitive magma blobs from possibly great depth. Here I examine and discuss the alternative hypothesis that most of the paroxysms could be triggered and driven by the fast upraise of CO₂-rich gas pockets generated by bubble foam growth and collapse in the sub-volcano plumbing system. Data for the SO₂ and CO₂ crater plume emissions are used to show that Stromboli's feeding magma may originally contain as much as 2 wt.% of carbon dioxide and early coexists with an abundant CO₂-rich gas phase with high CO₂/SO₂ molar ratio (≥ 60 at 10 km depth below the vents, compared to ～ 7 in time-averaged crater emissions). Pressure-related modelling indicates that the time-averaged crater gas composition and output are well accounted for by closed system decompression of the basalt–gas mixture until the volcano–crust interface (～ 3 km depth), followed by open degassing and crystallization in the volcano conduits. However, both the low viscosity and high vesicularity of the basaltic magma permit bubble segregation and bubble foam growth at deep sill-like feeder discontinuities and at shallower physical boundaries (such as the volcano–crust interface) where the gas-rich aphyric basalt interacts with the unerupted crystal-rich and viscous magma drained back from the volcano conduits. Gas pressure build-up and bubble foam collapse at these boundaries will intermittently trigger the sudden upraise of CO₂-rich gas blobs that constitute the main driving force of the paroxysms. Deeper-sourced gas blobs, driving the most powerful explosions, will be the richest in CO₂ and have highest CO₂/SO₂ ratios. This mechanism is shown to account well for the dynamic, seismic and petrologic features of Stromboli's paroxysms and, hence, to provide a potential alternative interpretation for their genesis and their forecasting. Enhanced bubble foam leakage prior to a paroxysm, or foam emptying in several steps, should lead indeed to precursory upstream of CO₂-rich gas and increasing CO₂/SO₂ ratio in crater plume emissions. The recent detection of such signals prior to two explosions in December 2006 and March 2007 strongly supports this expectation and the model proposed in this study.     

Very high-frequency and seasonal cave atmosphere PCO2 variability: Implications for stalagmite growth and oxygen isotope-based paleoclimate records

Cave air P_CO2 at two Irish sites varied dramatically on daily to seasonal timescales, potentially affecting the timing of calcite deposition and consequently climate proxy records derived from stalagmites collected at the same sites. Temperature-dependent biochemical processes in the soil control CO₂ production, resulting in high summer P_CO2 values and low winter values at both sites. Large Large-amplitude, high-frequency variations superimposed on this seasonal cycle reflect cave air circulation. Here we model stalagmite growth rates, which are controlled partly by CO₂ degassing rates from drip water, by considering both the seasonal and high-frequency cave air P_CO2 variations. Modeled hourly growth rates for stalagmite CC-Bil from Crag Cave in SW Ireland reach maxima in late December (0.063 μm h^− 1) and minima in late June/early July (0.033 μm h^− 1). For well-mixed ‘diffuse flow’ cave drips such as those that feed CC-Bil, high summer cave air P_CO2 depresses summer calcite deposition, while low winter P_CO2 promotes degassing and enhances deposition rates. In stalagmites fed by well-mixed drips lacking seasonal variations in δ¹⁸O, integrated annual stalagmite calcite δ¹⁸O is unaffected; however, seasonality in cave air P_CO2 may influence non-conservative geochemical climate proxies (e.g., δ¹³C, Sr/Ca). Stalagmites fed by ‘seasonal’ drips whose hydrochemical properties vary in response to seasonality may have higher growth rates in summer because soil air P_CO2 may increase relative to cave air P_CO2 due to higher soil temperatures. This in turn may bias stalagmite calcite δ¹⁸O records towards isotopically heavier summer drip water δ¹⁸O values, resulting in elevated calcite δ¹⁸O values compared to the ‘equilibrium’ values predicted by calcite–water isotope fractionation equations. Interpretations of stalagmite-based paleoclimate proxies should therefore consider the consequences of cave air P_CO2 variability and the resulting intra-annual variability in calcite deposition rates.     

Carbon Dioxide Discharged through the Las Cañadas Aquifer, Tenerife, Canary Islands

Carbon dioxide is one of the first gases to escape the magmatic environment due to its low solubility in basaltic magmas at low pressures. The exsolved CO₂ gas migrates towards the surface through rock fractures and high permeability paths. If an aquifer is located between the magmatic environment and the surface, a fraction of the CO₂ emitted is dissolved in the aquifer. In this paper, an estimation of the water mass balance and the CO₂ budget in Las Cañadas aquifer, Tenerife, Canary Islands, is presented. Magmatic CO₂ is transported by groundwater and discharged through man-made sub-horizontal drains or galleries that exist in this island, and by the flow of groundwater discharged laterally towards other aquifers or to the ocean. In addition, the pCO₂ at the gallery mouth (or entrance) and at the gallery bottom (internal and deepest discharge point where the gallery starts) are calculated and mapped. The total CO₂ advectively transported by groundwater is estimated to range from 143 to 211 t CO₂ d^?1. Considering that the diffuse soil emission of CO₂ for the same area is 437 t d^?1, the diffuse/dissolved CO₂ flux ratio varies between 2 and 3. The high dissolved inorganic carbon content of groundwater explains the ability of this low temperature hydrothermal water to dissolve and transfer magmatic CO₂ at volcanoes, even during quiescence periods.     

Eddy covariance imaging of diffuse volcanic CO<Subscript>2</Subscript> emissions at Mammoth Mountain,CA, USA

Use of eddy covariance (EC) techniques to map the spatial distribution of diffuse volcanic CO₂ fluxes and quantify CO₂ emission rate was tested at the Horseshoe Lake tree-kill area on Mammoth Mountain, California, USA. EC measurements of CO₂ flux were made during September–October 2010 and ranged from 85 to 1,766 g m⁻² day⁻¹. Comparative maps of soil CO₂ flux were simulated and CO₂ emission rates estimated from three accumulation chamber (AC) CO₂ flux surveys. Least-squares inversion of measured eddy covariance CO₂ fluxes and corresponding modeled source weight functions recovered 58–77% of the CO₂ emission rates estimated based on simulated AC soil CO₂ fluxes. Spatial distributions of modeled surface CO₂ fluxes based on EC and AC observations showed moderate to good correspondence (R ² = 0.36 to 0.70). Results provide a framework for automated monitoring of volcanic CO₂ emissions over relatively large areas.     

Limestone assimilation and the origin of CO2 emissions at the Alban Hills (Central Italy): Constraints from experimental petrology

The Alban Hills volcanic region (20 km south of Rome, in the Roman Province) emitted a large volume of potassic magmas (> 280 km³) during the Quaternary. Chemical interactions between ascending magmas and the ∼ 7000–8000-m-thick sedimentary carbonate basement are documented by abundant high temperature skarn xenoliths in the eruptive products and have been frequently corroborated by geochemical surveys. In this paper we characterize the effect of carbonate assimilation on phase relationships at 200 MPa and 1150–1050 °C by experimental petrology. Calcite and dolomite addition promotes the crystallization of Ca-rich pyroxene and Mg-rich olivine respectively, and addition of both carbonates results in the desilication of the melt. Furthermore, carbonate assimilation liberates a large quantity of CO₂-rich fluid. A comparison of experimental versus natural mineral, glass and bulk rock compositions suggests large variations in the degree of carbonate assimilation for the different Alban Hills eruptions. A maximum of 15 wt.% assimilation is suggested by some melt inclusion and clinopyroxene compositions; however, most of the natural data indicate assimilation of between 3 and 12 wt.% carbonate. Current high CO₂ emissions in this area most likely indicate that such an assimilation process still occurs at depth. We calculate that a magma intruding into the carbonate basement with a rate of ∼ 1 – 2 · 10⁶ m³/year, estimated by geophysical studies, and assimilating 3–12 wt.% of host rocks would release an amount of CO₂ matching the current yearly emissions at the Alban Hills. Our results strongly suggest that current CO₂ emissions in this region are the shallow manifestation of hot mafic magma intrusion in the carbonate-hosted reservoir at 5–6 km depth, with important consequences for the present-day volcanic hazard evaluation in this densely populated and historical area.     

Simulating CO<Subscript>2</Subscript> transport into the ocean from a CO<Subscript>2</Subscript> lake at the seafloor using a <Emphasis Type="Italic">z</Emphasis>- and a <Emphasis Type="Italic">σ</Emphasis>-coordinate model

The ocean takes up approximately 2 GT carbon per year due to the enhanced CO₂ concentrations in the atmosphere. Several options have been suggested in order to reduce the emissions of CO₂ into the atmosphere, and among these are CO₂ storage in the deep ocean. Topographic effects of dissolution and transport from a CO₂ lake located at 3,000-m depth have been studied using the z-coordinate model Massachusetts Institute of Technology general circulation model (MITgcm) and the σ-coordinate model Bergen ocean model (BOM). Both models have been coupled with the general ocean turbulence model (GOTM) in order to account for vertical subgrid processes. The chosen vertical turbulence mixing scheme includes the damping effect from stable stratification on the turbulence intensity. Three different topographic scenarios are presented: a flat bottom and the CO₂ lake placed within a trench with depths of 10 and 20 m. The flat case scenario gives good correlation with previous numerical studies of dissolution from a CO₂ lake. When topography is introduced, it is shown that the z-coordinate model and the σ-coordinate model give different circulation patterns in the trench. This leads to different dissolution rates, 0.1 μmol cm^− 2 s^− 1 for the scenario of a 20-m-deep trench using BOM and 0.005–0.02 μmol cm^− 2 s^− 1 for the same scenario using the MITgcm. The study is also relevant for leakages of CO₂ stored in geological formations and to the ocean.     

Seasonality of epCO2 at different scales along an integrated river continuum within the Dee basin,NE Scotland

Spatial and temporal variations of free CO₂ concentrations in surface waters are mainly controlled by dynamic processes encompassing terrestrial inputs and in‐stream biotic cycling. Free CO₂ can be expressed as ‘excess partial pressure of CO₂’ (epCO₂), indicating supersaturation or under‐saturation with respect to atmospheric CO₂. Seasonal patterns of epCO₂ at sites draining nested upland catchments between 3·40 and 1837 km² were assessed within the River Dee basin in NE Scotland. EpCO₂ values ranged from 0·14 at the lowermost site on the mainstem in autumn to 12·7 on a major tributary during spring. A seasonality index was derived to describe contrasting winter/spring maxima and summer/autumn minima as annual mean epCO₂ values could not clearly distinguish between different sites. Seasonal differences tended to increase downstream as progressive changes in physicochemical conditions enhanced the influence of autotrophic communities. However, perturbations to this continuum occurred as CO₂ inputs from high DOC, heterotrophic tributaries and land‐use changes between open moorland and forest affected downstream continuity. Major tributaries showed reduced differences between seasons compared to the mainstem. Smaller headwaters indicated a lack of seasonality as high connectivity of responsive, peaty soils enabled continual inputs of terrestrially derived free CO₂ to streams concomitant with limited autotrophic CO₂ removal, maintaining epCO₂ > 1 throughout. Seasonality of epCO₂ was mainly driven by the ability of in‐stream biota to consume CO₂ during optimal conditions in summer/autumn. This was confirmed by multiple linear regression analysis, which indicated that, apart from catchment area, baseflow and biotic activity indicators were the best predictors of epCO₂ seasonality characteristics at any particular stage of the river system. Copyright © 2009 John Wiley & Sons, Ltd.     

Supersaturation and evasion of CO2 and CH4 in surface waters at Mer Bleue peatland,Canada

Carbon dioxide (CO₂) and methane (CH₄) concentrations and evasion rates were measured in surface waters draining Mer Bleue peatland (Ontario, Canada) between spring and autumn 2005. All sites exhibit a consistent pattern of supersaturation throughout the year, which is broadly related to hydrological and temperature changes between spring snowmelt and autumn freezing. Both measurements and estimates of CO₂ and CH₄ evasion from open water to the atmosphere suggest that parts of the catchment (including beaver dams) are significant degassing hot spots. We present data showing how vertical gaseous carbon fluxes compare with lateral carbon fluxes and make an initial estimate of the importance to the overall carbon budget of CO₂ and CH₄ evasion to the atmosphere from water surfaces at Mer Bleue. Copyright © 2007 John Wiley & Sons, Ltd.