CO2 Mixing Research Articles

Molecular Insights into CO2 Diffusion Behavior in Crude Oil

CO2 flooding plays a significant part in enhancing oil recovery and is essential to achieving CCUS (Carbon Capture, Utilization, and Storage). This study aims to understand the fundamental theory of CO2 dissolving and diffusing into crude oil and how these processes vary under reasonable reservoir conditions. In this paper, we primarily use molecular dynamics simulation to construct a multi-component crude oil model with 17 hydrocarbons, which is on the basis of a component analysis of oil samples through laboratory experiments. Then, the CO2 dissolving capacity of the multi-component crude was quantitatively characterized and the impacts of external conditions—including temperature and pressure—on the motion of the CO2 dissolution and diffusion coefficients were systematically investigated. Finally, the swelling behavior of mixed CO2–crude oil was analyzed and the diffusion coefficients were predicted; furthermore, the levels of CO2 impacting the oil’s mobility were analyzed. Results showed that temperature stimulation intensified molecular thermal motion and increased the voids between the alkane molecules, promoting the rapid dissolution and diffusion of CO2. This caused the crude oil to swell and reduced its viscosity, further improving the mobility of the crude oil. As the pressure increased, the voids between the internal and external potential energy of the crude oil models became wider, facilitating the dissolution of CO2. However, when subjected to external compression, the CO2 molecules’ diffusing progress within the oil samples was significantly limited, even diverging to zero, which inhabited the improvement in oil mobility. This study provides some meaningful insights into the effect of CO2 on improving molecular-scale mobility, providing theoretical guidance for subsequent investigations into CO2–crude oil mixtures’ complicated and detailed behavior.

The role of phase saturation in depressurization and CO2 storage in natural gas hydrate reservoirs: Insights from a pilot-scale study

The natural gas recovery and CO2 storage in natural gas hydrate (NGH) reservoirs is a promising integrated strategy for implementing clean energy production and CCUS. However, the fundamental characteristics (phase saturation) of NGH reservoirs on the feasibility and efficiency of the above-integrated processes remain unclear. In this study, we synthesized methane hydrate-bearing sediments (MHBS) with varying hydrate saturation (SH = 18–66 %) and aqueous saturation (SA = 7–92 %) at marine NGH conditions (T = 283.8 K, P = 11.0 MPa). The effect of phase saturation on depressurization-induced hydrate dissociation, and subsequent CO2 storage and hydrate restoration by CO2/N2 injection was investigated. The water-saturated MHBS with high SA can promote faster MH dissociation during the depressurization stage and prevent secondary hydrate formation, but a slower MH dissociation during the later stage due to slow heat transfer. A relatively high SH (> 63 %) results in sustained low temperatures due to insufficient heat supply slowing down MH dissociation. The CH4/CO2/N2 mixed hydrates (Mix-H) formation and CO2 storage in depleted MHBS after CO2/N2 injection initially increase and then decrease with post-mining SA (peak at SA = 54.5 %). The rate of Mix-H formation decreases as the post-mining SA increases. Higher pre-mining SH and CO2/N2 injection rate facilitates rapid gas phase mixing within the sediments, thereby improving the kinetics and homogeneity of Mix-H formation and hydrate-based CO2 storage efficiency. MHBS with relatively low SH and SA exhibit enhanced safety for the integrated process, offering reduced depressurization-induced sediment subsidence and increased hydrate restoration ratios (∼85 %) by Mix-H formation.

Effect of CO2 mixing on the rheological and electrochemical properties of fresh mortar at the early age

The study aimed to elucidate the mechanism behind rheological modification due to CO2 mixing at the mixing and post-mixing stages from an electrochemical perspective. The results indicated that CO2 mixing reduced the flowability while increasing penetration resistance and static yield stress. The electrostatic attraction between particles with opposite surface charges and the bridging effect of calcium carbonate constitute the primary factors for influencing the rheological properties of mortar at an early age. The altered surface charge of carbonized cement particles, primarily resulting from CO2 injection lowering the pH and ion concentration, reversed the zeta potential of particles from the traditionally negative charge (-3.59 mV) to a positive value (+13.3 mV). Furthermore, CO2 mixing further enhanced the dissolution of cement particles and accelerated the hydration process, thereby increasing the rate of structural build-up. CO2 mixing was demonstrated to be a potential rheological modifier for 3D-printed concrete applications.

Synergistic effects of CO2 mixing and CO2 curing in CO2 uptake capacity of Portland cement

This study investigates the synergistic effects of CO2 mixing and CO2 curing on the CO2 uptake capacity of Portland cement. After a normal mixing or ten minutes of CO2 mixing, samples underwent 0.5, 1, or 3 days of CO2 curing at a 10 % CO2 concentration. Hydration and carbonation characteristics were assessed by means of a thermogravimetry analysis, reacted water and CO2 uptake measurements, pore solution analysis, X-ray diffractometry, and Rietveld-refinement-based quantification. The CO2-mixed samples displayed an escalating trend in terms of CO2 uptake with longer curing times and greater reaction degrees compared to normally mixed samples. In addition, extended CO2 curing durations resulted in decreased levels of portlandite and increased amounts of crystalline carbonates, alongside a reduction in the ratio of amorphous to crystalline carbonates. Subsequently, improvements in the compressive strength were noted with longer CO2 curing durations.

The impact of different batter mixing atmospheres on the quality of reduced sucrose sponge and cream cakes

Cakes are chemically-leavened sugar-rich bakery products. Depending on the cake type, air is incorporated differently during mixing. Sponge cake batter is a gas bubble rich foam, while cream cake batter is an emulsion containing less gas cells. Given the consumer interest in reduced sucrose food systems, we here examined the potential of using altered mixing atmospheres [pure nitrogen (N2) or carbon dioxide (CO2)] to improve reduced sucrose cake quality. The use of poorly soluble N2 resulted in voluminous, stable batter. This was especially the case for foam-type sponge cake batters. During baking of sponge and cream cake batters, leavening was similar for batters prepared either under air or N2, resulting in similar cake qualities. Use of the highly soluble CO2 as mixing atmosphere negatively affected batter density, but positively leavening during baking due to temperature-induced release of previously solubilized CO2. In foam-type sponge cake making, the significant negative effect of CO2 on batter density was not overruled by the positive effect of CO2 release during leavening/baking. In contrast, the CO2 mixing atmosphere during batter-type cream cake making had only a limited negative effect on batter density while greatly affecting leavening, and resulting in high-quality, voluminous (reduced sucrose) cakes.

Insights on influence of polymer molecular weight on gas sorption, transport and plasticization in glassy 6FDA-DAM polyimide membranes

It is well-known that spinning of defect-free asymmetric polyimide hollow fibers is promoted by using high polymer molecular weight. On the other hand, effects of polymer molecular weight on microstructure and separation performance of dense film 6FDA-based polyimide membranes relevant to sheath layers of composite membranes are less well-explored. In this work, gas sorption, diffusion and permeation of CO2, N2 and CH4 for 6FDA-DAM dense films are considered for 45 kDa and 176 kDa weight average molecular weights. Such molecular weights are relevant for practical hollow fiber spinning of membranes. Earlier results for polystyrene samples show similar trends in dual mode sorption model results like those noted here for the 6FDA-DAM polyimide case; however, effects on permeation like those considered here were not addressed for the polystyrene case. Here we also address actual pure and mixed gas CO2 and CH4 permeation and show higher free volumes, higher gas permeability but lower 50:50 CO2/CH4 selectivity associated with the higher molecular weight sample. Analysis using the self-consistent dual-mode sorption and transport models help understand the observed trends. Our results suggest denser packing of polymer segments exists in the Henry's law environment of the low molecular weight sample. Moreover, the higher polymer segmental packing in the Henry's law environment suppresses CO2 plasticization with a dual-mode microstructure controlling separation performance and plasticization behavior for this important member of the 6FDA polyimide family. We show evidence of effects of charge transfer complexes as possible main features in these trends. These dense film results help guide future work on dense sheath layers on composite hollow fiber membranes.

Enhanced oil recovery in tight reservoirs by ultrasonic-assisted CO2 flooding: Experimental study and molecular dynamics simulation

CO2-enhanced oil recovery (CO2-EOR) has attracted great attention due to its dual effects of geological carbon storage and economic benefits, but the clay swelling, asphaltene deposition and gas channeling induced by CO2 still restrict its field application. The employment of chemical agents for the purpose of improving CO2-EOR performance is often limited by the heterogeneity of reservoirs and the potential for formation damage. Physical methods, such as ultrasonic-assisted CO2 flooding, appear to offer a promising alternative for CO2-EOR and geological sequestration. This study has conducted oil displacement experiments to investigate the EOR potential of ultrasonic-assisted CO2 flooding in tight reservoirs and clarify the effects of different ultrasound parameters. The ultrasonic waves with 20 kHz of acoustic frequency, 30 W/cm2 of sound intensity, 30 min of sonication time and moderate intermittent working mode can yield violent cavitation effects and avoid undesirable energy dissipation. Nearly a quarter of the saturated crude oil can be recovered from the tight core through ultrasonic-assisted CO2 flooding. It can be attributed to the composition alterations of crude oil and property changes of core matrix. Coupling with ultrasonic waves, CO2 would extract and vaporize crude oil more easily, which promotes oil swelling, lowers oil density and viscosity, decreases oil–gas interfacial tension and thus improves oil recovery. Besides, the ultrasound can help to mitigate the impact of CO2 adsorption-induced clay swelling on pore sizes through fluctuating pressure. And the strong hydrodynamic shear forces generated by mechanical vibration of ultrasound may also help to connect individual small pores and extend micro-fractures. These synergistic effects result in the transformation of micropores into mesopores and macropores. Furthermore, the wettability alteration of core samples implies that the ultrasonic-assisted CO2 flooding can induce the detachment of oil films from the rock surface. Finally, the results of molecular dynamics simulations indicate that the improvement in pressure inside the core sample resulting from ultrasonic cavitation facilitates the mixing of CO2 and oil, thereby weakening gas channeling and improving the displacement efficiency.

Zinc-cluster-based MOF with super selective separation adsorption catalyzes direct chemical conversion of CO2 from quasi-polluted air

The CO2 in the atmosphere has a serious impact on the human living environment. Direct utilization of the discharged CO2 to produce useful chemicals is challenging due to the practical presence of mixed compositions and low concentrations for CO2. Metal-organic frameworks (MOFs) with high internal surface areas, porosity and open metal site potentially possess superior adsorption separation capacity for CO2 gas, and on this basis, it can be used as a catalyst to achieve efficient chemical conversion of CO2. We demonstrate the synthesis and characterization of MOF ([Zn4O(ntba)2]n, MOF 1, H3ntba=4,4',4''-nitrilotribenzoic acid) with excellent selective adsorption properties from quasi- polluted air (O2, N2, CO2, CH4) and as heterogeneous catalysts for direct efficient CO2 chemical conversion. Detailed structural analysis reveals show that zinc ions behave multiple open metal sites and two coordination modes in the polyhedral zinc clusters building unit forming a 3D porous network by polyhedral zinc clusters building unit. Thanks to its structural features, at the 273 K and 100kPa, due to the ideal specific surface area and porosity, MOF 1 have ideal adsorption-separation performance for CO2, its values of adsorption and separation ratio are 75.7 cm3g−1, 373(CO2/N2), 79(CO2/O2), 13(CO2/CH4), respectively. The cycloaddition catalytic reaction investigations show that MOF 1 can be employed as a highly efficient catalyst for converting CO2 with a yield of 78 %. The result of this experiment offers the potential to directly utilize low-concentration and mixed CO2 while avoiding the economic and environmental costs of obtaining purified CO2 feeds tocks.

Spatial mapping of greenhouse gases using a UAV monitoring platform over a megacity in China

Urban environments are recognized as main anthropogenic contributors to greenhouse gas (GHG) emissions, characterized by unevenly distributed emission sources over the urban environments. However, spatial GHG distributions in urban regions are typically obtained through monitoring at only a limited number of locations, or through model studies, which can lead to incomplete insights into the heterogeneity in the spatial distribution of GHGs. To address such information gap and to evaluate the spatial representation of a planned GHG monitoring network, a custom-developed atmospheric sampler was deployed on a UAV platform in this study to map the CO2 and CH4 mixing ratios in the atmosphere over Zhengzhou in central China, a megacity of nearly 13 million people. The aerial survey was conducted along the main roads at an altitude of 150 m above ground, covering a total distance of 170 km from the city center to the suburbs. The spatial distributions of CO2 and CH4 mixing ratios in Zhengzhou exhibited distinct heterogeneities, with average mixing ratios of CO2 and CH4 at 439.2 ± 10.8 ppm and 2.12 ± 0.04 ppm, respectively. A spatial autocorrelation analysis was performed on the measured GHG mixing ratios across the city, revealing a spatial correlation range of approximately 2 km for both CO2 and CH4 in the urban area. Such a spatial autocorrelation distance suggests that the urban GHG monitoring network designed for emission inversion purposes need to have a spatial resolution of 4 km to characterize the spatial heterogeneities in the GHGs. This UAV-based measurement approach demonstrates its capability to monitor GHG mixing ratios across urban landscapes, providing valuable insights for GHG monitoring network design.

Effect of in situ CO2 mixing of cement paste on the leachability of hexavalent chromium (Cr(VI))

In situ CO2 mixing technology is a potential technology for permanently sequestering CO2 during concrete manufacturing processes. Although it has been approved as a promising carbon capture and utilisation (CCU) method, its effect on the leachability of heavy metals from cementitious compounds has not yet been studied. This study focuses on the effect of in situ CO2 mixing of cement paste on the leaching of hexavalent chromium (Cr(VI)). The tank leaching test of the CO2 mixing cement specimen resulted in a Cr(VI) cumulative leaching of 0.614 mg/m2 in 28 d, which is ten times lower than that of the control mixing specimens. The results in thermogravimetric analysis indicated that a relatively significant amount of CrO42− is immobilised as CaCrO4 during the CO2-mixing, and a higher Cr–O extension is observed in the Fourier transform infrared spectra. Furthermore, a portion of the monocarboaluminate is inferred from microstructural analyses to incorporate CrO42− ions. These results demonstrate that in situ CO2 mixing is beneficial not only in reducing CO2 emissions, but also in controlling the leaching of toxic substances.

A Review of the Utilization of CO2 as a Cushion Gas in Underground Natural Gas Storage

A cushion gas is an indispensable and the most expensive part of underground natural gas storage. Using CO2 injection to provide a cushion gas, not only can the investment in natural gas storage construction be reduced but the greenhouse effect can also be reduced. Currently, the related research about the mechanism and laws of CO2 as a cushion gas in gas storage is not sufficient. Consequently, the difference in the physical properties of CO2 and CH4, and the mixing factors between CO2 and natural gas, including the geological conditions and injection–production parameters, are comprehensively discussed. Additionally, the impact of CO2 as a cushion gas on the reservoir stability and gas storage capacity is also analyzed by comparing the current research findings. The difference in the viscosity, density, and compressibility factor between CO2 and CH4 ensures a low degree of mixing between CO2 and natural gas underground, thereby improving the recovery of CH4 in the operation process of gas storage. In the pressure range of 5 MPa–13 MPa and temperature range of 303.15 K–323.15 K, the density of CO2 increases five to eight times, while the density of natural gas only increases two to three times, and the viscosity of CO2 is more than 10 times that of CH4. The operation temperature and pressure in gas storage should be higher than the temperature and pressure in the supercritical conditions of CO2 because the diffusion ability between the gas molecules is increased in these conditions. However, the temperature and pressure have little effect on the mixing degree of CO2 and CH4 when the pressure is over the limited pressure of supercritical CO2. The CO2, with higher compressibility, can quickly replenish the energy of the gas storage facility and provide sufficient elastic energy during the natural gas production process. In addition, the physical properties of the reservoir also have a significant impact on the mixing and production of gases in gas storage facilities. The higher porosity reduces the migration speed of CO2 and CH4. However, the higher permeability promotes diffusion between gases, resulting in a higher degree of gas mixing. For a large inclination angle or thick reservoir structure, the mixed zone width of CO2 and CH4 is small under the action of gravity. An increase in the injection–production rate intensifies the mixing of CO2 and CH4. The injection of CO2 into reservoirs also induces the CO2–water–rock reactions, which improves the porosity and is beneficial in increasing the storage capacity of natural gas.

Exploring how the heterogeneous urban landscape influences CO2 concentrations: The case study of the Metropolitan Area of Barcelona

Monitoring CO2 concentrations in urban areas is crucial for determining the efficacy of climate change mitigation policies. However, highly heterogeneous land use, local geography, and local convection patterns, which vary throughout the urban landscape, complicate this task. To establish continuous monitoring programs, it is important to first determine the heterogeneity of urban landscapes on the ground. To understand the role these factors play in the distribution of CO2 over an urban area, we conducted a CO2 measurement campaign over the Metropolitan Area of Barcelona (AMB) over four urban land uses: impervious, green, forest, and agricultural. There is a clear tendency for CO2 mixing ratios to decrease as the degree of urban vegetation increases, even in the midst of a developed boundary layer. For example, CO2 concentrations were 429 and 427 ppm at forest and agricultural sites, respectively, while 485 ppm was reported at urban sites. A decrease in atmospheric CO2 was observed from 458 to 428 ppm in the gradient from urban to suburban areas, in which the biosphere component increased. The biosphere component of the CO2 signal was significant and was observed in the gradient from urban to suburban areas, which averaged a reduction from 458 to 428 ppm. Our findings show that the large spatial variability in CO2 concentrations (ranging from 410 to 495 ppm) is best explained by anthropogenic activity. We propose increasing the spatiotemporal resolution of CO2 monitoring in the AMB to determine these trends more precisely over longer periods of time.

Characteristics of methane and carbon dioxide in ice caves at a high-mountain glacier of China

The exploration of the spatiotemporal distribution of greenhouse gas (GHG) exchange in the cryosphere (including ice sheet, glaciers, and permafrost) is important for understanding its future feedback to the atmosphere. Mountain glaciers and ice sheets may be potential sources of GHG emissions, but the magnitude and distribution of GHG emissions from glaciers and ice sheets remain unclear because observation data are lacking. In this study, in situ CH4 and CO2 and the mixing ratios of their carbon isotope signatures in the air inside an ice cave were measured, and CH4 and CO2 exchange in the meltwater of Laohugou glacier No. 12, a high-mountain glacier in an arid region of western China, was also analyzed and compared with the exchange in downstream rivers and a reservoir. The results indicated elevated CH4 mixing ratios (up to 5.7 ppm) and depleted CO2 (down to 168 ppm) in the ice cave, compared to ambient levels during field observations. The CH4 and CO2 fluxes in surface meltwater of the glacier were extremely low compared with their fluxes in rivers from the Tibetan Plateau (TP). CH4 and CO2 mixing ratios in the air inside the ice cave were mainly controlled by local meteorological conditions (air temperature, wind speed and direction) and meltwater runoff. The carbon isotopic compositions of CH4 and CO2 in the ice cave and terminus meltwater indicated δ13C-CH4 depletion compared to ambient air, suggesting an acetate fermentation pathway. The abundances of key genes for methanogenic archaea/genes encoding methyl coenzyme M reductase further indicated the production of CH4 by methanogenic archaea from the subglacial meltwater of high-mountain glaciers. The discovery of CH4 emissions from even small high-mountain glaciers indicates a more prevalent characteristic of glaciers to produce and release CH4 from the subglacial environment than previously believed. Nevertheless, further research is required to understand the relationship between this phenomenon and glacial dynamics in the third pole.

Mineral carbonation performance of wollastonite-blended cementitious composites through in-situ CO2 mixing

This study assessed the feasibility of in-situ CO2 mixing in a wollastonite-blended cementitious composite by evaluating its mineral carbonation performance. Wollastonite (CS) polymorphs were synthesized and used to replace ordinary Portland cement with up to 30 wt%. The results showed that amorphous CaCO3, calcite, and monocarboaluminate were formed as carbonation products. The wollastonite-blended cementitious composite with a 20 % replacement of β-CS exhibited the highest CaCO3 production of 22.5 wt%, which was mainly attributed to amorphous CaCO3. β-CS showed a superior mineral carbonation ability to α-CS because the Ca-depleted silicate layer formed by the dissolution of β-CS accelerated the formation of amorphous CaCO3. Moreover, CS retarded the early-stage formation of ettringite and monocarboaluminate by delaying gypsum dissolution. The wollastonite-blended cementitious composites exhibited superior compressive strength exceeding 40 MPa. These results demonstrate that the mineral carbonation of wollastonite-blended cementitious composite can be accomplished through in-situ CO2 mixing, especially with β-CS replacement, which has significant potential as a CO2 storable binder.

An innovative strategy for maximizing CO2 reduction in concrete through preparing carbon sequestration precursors by accelerated carbonation

Concrete possesses significant potential as a medium for CO2 sequestration. Nevertheless, conventional approaches like CO2 mixing and carbonation curing have shown limited efficacy in this regard. The present work introduces a groundbreaking method for enhancing CO2 reduction in concrete production. It involves preparing carbon sequestration precursors (CSP) through carbonation of cement reinforced by mechanochemical effects. The study investigated the kinetics and phase assemblage during the CSP production process, along with examining the impact of CSP on the reaction kinetics and strength development of cement composites. Due to the mechanochemical effects, CSP was characterized by the prevalence of metastable calcium carbonate (Cc) with a small crystalline size. Moreover, CSP not only accelerated the hydration of cement composites but also led to increased compressive strength at all ages, even with the replacement of 30 wt% of cement by CSP. The multifunctional attributes of CSP, including nucleation, reactant, and inert filler roles, contributed to its exceptional performance. Furthermore, utilizing the CSP method demonstrated a significant capacity for CO2 reduction, exceeding 36.5 %, which is more than seven times that of the traditional CO2 mixing method. The positive outcomes from this study underscore the high efficacy of the CSP method for efficient CO2 sequestration in concrete.

Carbon sequestration in cementitious systems through CO2-rich hydration and chemically enforced CO2 mineralization

Cementitious materials as a medium of carbon capture, and utilization (CCU) have recently attracted considerable attentions. Atmospheric CO2 can be absorbed in hardened concrete, which can be also accelerated by early age CO2 curing. Compared to the CO2 curing of concrete materials, in-situ CO2 mixing technology can be widely applied because it can be used in a batch plant without an additional curing facility. In this study, the CO2 mixing time was set as the primary variable to elucidate the precipitation of the carbonate phases in the early stages and its effect on cement hydration. The dissociated CO2 is directly mineralized into calcium carbonate (CaCO3) in calcite phase. In addition, the longer the CO2 mixing time, the greater the precipitation of calcite (i.e., CCU capacity), thereby densifying the internal microstructure and improving early strength development. Interestingly, a certain amount of calcite converted to monocarboaluminate—an important factor for quantitatively assessing the degree of mineral carbonation in cementitious materials.

Empirical modelling of a semihermetic reciprocating compressor for different CO2-based mixtures

Mixing CO2 with other refrigerants gives carbon dioxide new thermophysical properties, such as higher critical temperature or lower working pressures.However, most of these new mixtures havenotyetbeenevaluatedexperimentally, introducing a lack of knowledge about how the main components of a refrigerating plant will operate with these blends.One clear example is the compressor, whose operationdirectly affects the performance and energy consumption of theplant. Accordingly, this work evaluates experimentally the behaviour of a CO2 semi-hermetic reciprocating compressor working with four different CO2-based mixtures of CO2/R32, CO2/R290, CO2/R1270 and CO2/R1234yf. The analysis covers the parameters of volumetric and global efficiency, discharge temperature, mass flow rate, and electrical power consumption, where noticeable differences are shown in terms of efficiencies, affecting the mass flow rate and the electrical power consumption significantly, and, to a lesser extent, the discharge temperature. The experimental results are compared with the standardized models from UNE-12900 and AHRI-540, proposing a simplified empirical model with a prediction error below 4% for all analysed mixtures and an extensive model for other CO2-based mixtures.

Three-dimensional detection of CO2 and wind using a 1.57 µm coherent differential absorption lidar

A 1.57-µm coherent differential absorption lidar is demonstrated for measuring three-dimensional CO2 and wind fields simultaneously. The maximum detection range of CO2 is up to 6 km with a range resolution of 120 m and a time resolution of 1 min. A preliminary assessment of instrument performance is made with a 1-week continuous observation. The CO2 concentration over a column from 1920 to 2040 m is compared with the one measured by an optical cavity ring-down spectrometer placed on a 2 km-away meteorological tower. The concentration is strongly correlated with the in-situ spectrometer with a correlation coefficient and RMSE of 0.91 and 5.24 ppm. The measurement accuracy of CO2 is specified with a mean and standard deviation of 2.05 ppm and 7.18 ppm, respectively. The regional CO2 concentration and the three-dimensional wind fields are obtained through different scanning modes. Further analysis is conducted on vertical mixing and horizontal transport of CO2 by combining with the measured wind fields.

Mixing of endogenous CO2 and meteoric H2O causes extremely efficient carbonate dissolution

Karst corrosion of carbonate rocks by water with dissolved gases proceeds in most cases along two major scenarios: (i) meteoric water absorbs CO2 from soil and atmosphere, or (ii) ascending water of deep circulation carries with it dissolved endogenous gases, mainly CO2 and H2S. We have observed a peculiar variant where meteoric water absorbs ascending endogenous gases at a natural gas vent on a travertine mound in Slovakia. Carbonate dissolution's extreme effectiveness is demonstrated by mineralization of rainwater ponded at a gas vent, rising to 3.2 g/L of dissolved solids shortly after the rainfall. One liter of water ponded at the vent and mixing with the venting gas, dissolved up to 800 mg of calcium at a rate exceeding 5.8 mg/L·min. Limestone tablets placed at the vent show signs of significant corrosion, at rates up to 126 mm/ka. The rate is comparable to those in coastal karst, where freshwater is mixing with seawater and to those in sulfuric acid speleogenesis (SAS), both the highest hitherto known rates of karst corrosion in carbonates. The geomorphic effects of the process described are depressions on the surface of travertine near the vents of endogenous CO2. This type of corrosion seems to be universal and probably occurs everywhere where endogenous CO2 is exhaled to the surface from carbonate rocks.

Distribution of ozone, carbon monoxide and oxides of nitrogen over an urban location in the foothills of the North-Western Himalayas

Increasing anthropogenic activities and biomass burning events over the Northern Indian region severely affect the pristine environment of North-Western Himalayas. To investigate their influence in this data sparse region, continuous monitoring of O3, CO and NOx are made during January 2018 to December 2019 over Dehradun, an urban location in the foothills of North-western Himalayas. Ozone shows daytime broad peak and CO and NOx show two peaks during morning and evening hours. The maximum daytime ozone is observed during spring with 58.2 ± 2.0 ppbv (56.2 ± 2.7 ppbv) during 2018 (2019). CO and NOx show maximum value during winter and minimum during summer-monsoon. Their concentrations are found to be influenced from stubble burning and forest fire emissions during late spring. WRF-Chem simulations are made with and without biomass burning emissions to quantify the contribution of these emissions on the variability of these gases over Dehradun. Sensitivity simulations reveal that biomass burning over northern Indian region contributed 43 ppbv, 79 ppbv and 1.2 ppbv increase in O3, CO and NOx mixing ratios over Dehradun. In addition, dust storm event in June 2018 contributed for lower ozone levels in 2018 (16.1 ± 9.3 ppbv) and stratosphere troposphere exchange process enhanced surface ozone in June 2019 (53.6 ± 22.3 ppbv).