Inorganic Gases Research Articles

Heterogeneous Reactions between Toluene and NO2 on Mineral Particles under Simulated Atmospheric Conditions.

Heterogeneous reactions between organic and inorganic gases with aerosols are important for the study of smog occurrence and development. In this study, heterogeneous reactions between toluene and NO2 with three atmospheric mineral particles in the presence or absence of UV light were investigated. The three mineral particles were SiO2, α-Fe2O3, and BS (butlerite and szmolnokite). In a dark environment, benzaldehyde was produced on α-Fe2O3. For BS, nitrotoluene and benzaldehyde were obtained. No aromatic products were produced in the absence of NO2 in the system. In the presence of UV irradiation, benzaldehyde was detected on the SiO2 surface. Identical products were produced in the presence and absence of UV light over α-Fe2O3 and BS. UV light promoted nitrite to nitrate on mineral particles surface. On the basisi of the X-ray photoelectron spectroscopy (XPS) results, a portion of BS was reduced from Fe3+ to Fe2+ with the adsorption of toluene or the reaction with toluene and NO2. Sulfate may play a key role in the generation of nitrotoluene on BS particles. From this research, the heterogeneous reactions between organic and inorganic gases with aerosols that occur during smog events will be better understood.

Updated Abraham model correlations for correlating solute transfer into dry butanone and dry cyclohexanone solvents

ABSTRACTUpdated Abraham solvation parameter model correlations are derived for describing the partitioning and solubility behaviour of organic nonelectrolyte solutes and inorganic gases in both anhydrous butanone and cyclohexanone solvents. The butanone and cyclohexanone correlations are based on data sets that contain 103 and 72 experimental data points, respectively. The derived correlations for butanone describe the experimental partitioning and solubility behaviour to within a standard deviation of 0.145 log units. A slightly smaller standard deviation of 0.135 was observed between the experimental and back-calculated values for the cyclohexanone solvent correlation equations.

Development of a thoracic personal sampler system for co-sampling of sulfuric acid mist and sulfur dioxide gas

ABSTRACTA novel personal sampler was designed to measure inorganic acid mists and gases for determining human exposure levels to these acids in workplaces. This sampler consists of (1) a parallel impactor for classifying aerosol by size following the ISO/CEN/ACGIH defined human thoracic fraction, (2) a cellulose filter to collect the residual acid mist but allowing penetration of sulfur dioxide gas, and (3) an accordion-shaped porous membrane denuder (aPMD) for adsorbing the penetrating sulfur dioxide gas. Acid-resistant PTFE was chosen as the housing material to minimize sampling interference.To test the performance of the parallel impactor, monodisperse aerosol was created by a vibrating orifice aerosol generator. The results showed that the penetration curve of the impactor run at 2 LPM flow rate agreed well with the defined thoracic fraction. Almost all sampling biases were within 10% for particle size distributions with MMAD between 1–25 µm and GSD between 1.75–4, which meets the criteria of the EN 13205 standard. To evaluate the performance of the aPMDs, sulfur dioxide gas was sourced directly from a cylinder. The aPMDs maintained a gas collection efficiency greater than 95% for 4 hr when sampling 8.6 ppm of sulfur dioxide gas. While the aPMD had similar performance to the commonly adopted annular or honeycomb denuders made of glass, this shatterproof aPMD is only half of the volume and 1/25th the weight of the honeycomb denuder. Testing of the entire sampler with a mixture of sulfuric acid mist and sulfur dioxide gas showed the system could sample both with negligible interference. All the test results illustrate that the new sampler, which is flat, lightweight, and portable, is suitable for personal use and is capable of a more accurate assessment of human exposure to inorganic acid mist and SO2 gas.

Highly sensitive and selective fiber-optic Fabry-Perot volatile organic compounds sensor based on a PMMA film

A compact fiber-optic Fabry-Perot interferometric (FFPI) volatile organic compounds (VOCs) sensor is fabricated based on a single mode fiber (SMF) and a polymethyl methacrylate (PMMA) film. The VOCs induce the swelling effect and refractive index changes of PMMA film. The swelling of PMMA film affects the cavity length of Fabry-Perot interferometer resulting in shift of resonant dip, while the change in refractive index influences reflection resulting in change of extinction ratio (ER). The sensitivities of 2.7 pm/ppm for ethanol and 2.17 pm/ppm for acetone are achieved. Moreover, this sensor is insensitive to the inorganic gases. In addition, it has the advantages of ease of fabrication and high sensitivity to VOCs.

Role identification of NH3 in atmospheric secondary new particle formation in haze occurrence of China

Haze pollution in China, dominated by the aerosol particulate matters smaller than 2.5 μm (PM2.5) has raised much concern over the past decade. PM2.5 is principally derived from secondary new particulate formation (NPF) with main precursors SO2, NO2, NH3 and organic compounds (OC), where NH3 is the only alkaline inorganic gas. However, due to the lack understanding of NH3's role in NPF, few attention has been paid to NH3, which hinders the haze mitigation in China. In this work, the role of NH3 in NPF is investigated theoretically with a new kinetic model. Combining the oxidation of SO2/NO2 in SO2/NO2/NH3/H2O/air system and the aggregation of clusters in H2SO4/HNO3/NH3/OC system, this model has been established based on gas-kinetic theory, and tested upon uncertainty analysis with Monte Carlo method. The results of NPF calculations at the atmospheric concentration of each precursor show that NH3 is able to enhance NPF indirectly by facilitating conversions of SO2 and NO2, and directly by promoting aggregations of H2SO4, HNO3, NH3 and OC. The major effect of NH3 on NPF is found to be the enhancement of conversion fractions for SO2 and NO2 during oxidation processes. In addition, the promotion in contribution of HNO3 to NPF due to NH3 has been particularly observed. Therefore, controlling the emission of NH3 strictly as current restrictions on SO2 and NOx is reasonable to mitigating the haze pollution in China which has much higher atmospheric NH3 concentration over the global average level.

Enhanced hydrophobicity and volatility of submicron aerosols under severe emission control conditions in Beijing

Abstract. A series of strict emission control measures was implemented in Beijing and the surrounding seven provinces to ensure good air quality during the 2015 China Victory Day parade, rendering a unique opportunity to investigate the anthropogenic impact of aerosol properties. Submicron aerosol hygroscopicity and volatility were measured during and after the control period using a hygroscopic and volatile tandem differential mobility analyzer (H/V-TDMA) system. Three periods, namely the control clean period (Clean1), the non-control clean period (Clean2), and the non-control pollution period (Pollution), were selected to study the effect of the emission control measures on aerosol hygroscopicity and volatility. Aerosol particles became more hydrophobic and volatile due to the emission control measures. The hygroscopicity parameter (κ) of 40–200 nm particles decreased by 32.0–8.5 % during the Clean1 period relative to the Clean2 period, while the volatile shrink factor (SF) of 40–300 nm particles decreased by 7.5–10.5 %. The emission controls also changed the diurnal variation patterns of both the probability density function of κ (κ-PDF) and the probability density function of SF (SF-PDF). During Clean1 the κ-PDF showed one nearly hydrophobic (NH) mode for particles in the nucleation mode, which was likely due to the dramatic reduction in industrial emissions of inorganic trace gases. Compared to the Pollution period, particles observed during the Clean1 and Clean2 periods exhibited a more significant nonvolatile (NV) mode throughout the day, suggesting a more externally mixed state particularly for the 150 nm particles. Aerosol hygroscopicities increased as particle sizes increased, with the greatest increases seen during the Pollution period. Accordingly, the aerosol volatility became weaker (i.e., SF increased) as particle sizes increased during the Clean1 and Clean2 periods, but no apparent trend was observed during the Pollution period. Based on a correlation analysis of the number fractions of NH and NV particles, we found that a higher number fraction of hydrophobic and volatile particles during the emission control period.

Emerging applications driving innovations in gas sensing

PurposeThis paper aims to show how a range of new and emerging applications are driving technological innovations in gas sensing.Design/methodology/approachFollowing a short introduction, this paper first considers developments relating to the needs of the military and security sectors. Wearable gas sensors, energy harvesting and self-powered gas sensors are then discussed. The role of gas sensors in mobile phones is then considered, together with details of new developments in sensors for carbon-dioxide, particulates and formaldehyde. Finally, brief conclusions are drawn.FindingsThis paper shows that a technologically diverse range of gas sensors is being investigated and developed in response to a number of new and emerging requirements and applications. The gas sensors respond to numerous inorganic and organic gases and vapours over a wide range of application-specific concentrations and are based on a multitude of often innovative sensing techniques, technologies and materials.Originality/valueThis paper provides technical details of a selection of gas sensor research activities and product developments that reflect the needs of a range of new and emerging applications.

Hydrogen gas of organic origin in shales and metapelites

The changes in inorganic and organic gases retained in shales and metapelites with increasing heating temperature are not fully understood. The compositional and isotopic changes of residual gases such as H2, CH4, and CO2 in marine shales and metapelites during burial diagenesis and metamorphism were investigated in the present study. Shale rocks and metapelites which experienced paleo-temperatures in the range of 100–600°C were collected from the borehole cores in Niigata sedimentary basin and outcrops exposed in Kochi district, Japan. Gases released from shale or metapelite fragments during pulverization in the laboratory were analyzed as the residual gas.The major residual gas in shales and metapelites changes from CO2, CH4, to H2 in this order with increasing temperature. Drastic decrease of CO2 derived from decarboxylation of sedimentary organic matter is due to the expulsion of pore fluids with dissolved CO2. The CH4 concentration in residual gas subsequently increases with organic maturation and reaches the maximum at paleo-temperature of ca. 250°C. The CH4 concentration decreases during late metagenesis to metamorphism probably due to the formation of H2 gas and graphite. The H2 starts to increase at paleo-temperature of ca. 200°C. The H2 gas is the most abundant gas in residual gas of metapelite. The δ2Η value of H2 in shales and metapelites is quite low to be in the range of ca. –850 to −650‰. The change of major residual gas from CH4 to H2 with increasing paleo-temperature and the significantly low δ2H values of H2 suggest that H2 gas in shales and metapelites is largely derived from liberation of hydrogen in organic matter.

Indoor air pollutants, ventilation rate determinants and potential control strategies in Chinese dwellings: A literature review

After nearly twenty years of rapid modernization and urbanization in China, huge achievements have transformed the daily lives of the Chinese people. However, unprecedented environmental consequences in both indoor and outdoor environments have accompanied this progress and have triggered public awareness and demands for improved living standards, especially in residential environments. Indoor pollution data measured for >7000 dwellings (approximately 1/3 were newly decorated and were tested for volatile organic compound (VOC) measurements, while the rest were tested for particles, phthalates and other semi-volatile organic compounds (SVOCs), moisture/mold, inorganic gases and radon) in China within the last ten years were reviewed, summarized and compared with indoor concentration recommendations based on sensory or health end-points. Ubiquitous pollutants that exceed the concentration recommendations, including particulate matter, formaldehyde, benzene and other VOCs, moisture/mold, inorganic gases and radon, were found, indicating a common indoor air quality (IAQ) issue in Chinese dwellings. With very little prevention, oral, inhalation and dermal exposure to those pollutants at unhealthy concentration levels is almost inevitable. CO2, VOCs, humidity and radon can serve as ventilation determinants, each with different ventilation demands and strategies, at typical occupant densities in China; and particle reduction should be a prerequisite for determining ventilation requirements. Two directional ventilation modes would have profound impacts on improving IAQ for Chinese residences are: 1) natural (or window) ventilation with an air cleaner and 2) mechanical ventilation with an air filtration unit, these two modes were reviewed and compared for their applicability and advantages and disadvantages for reducing human exposure to indoor air pollutants. In general, mode 2 can more reliably ensure good IAQ for occupants; while mode 1 is more applicable due to its low cost and low energy consumption. However, besides a roadmap, substantial efforts are still needed to develop affordable, applicable and general ventilation solutions to improve the IAQ of residential buildings in China.

Theoretical study on the gas adsorption capacity and selectivity of CPM-200-In/Mg and CPM-200-In/Mg–X (–X = –NH2, –OH, –N, –F)

The adsorption capacities of a heterometallic metal-organic framework (CPM-200-In/Mg) to VOCs (HCHO, C2H4, CH4, C2H2, C3H8, C2H6, C2H3Cl, C2H2Cl2, CH2Cl2 and CHCl3) and some inorganic gas molecules (HCN, SO2, NO, CO2, CO, H2S and NH3), as well as its selectivity in ternary mixture systems of natural gas and post-combustion flue gas are theoretically explored at the grand canonical Monte Carlo (GCMC) and density functional theory (DFT) levels. It is shown that CPM-200-In/Mg is suitable for the adsorption of VOCs, particularly for HCHO (up to 0.39 g g-1 at 298 K and 1 bar), and the adsorption capacities of some inorganic gas molecules such as SO2, H2S and CO2 match well with the sequence of their polarizability (SO2 > H2S > CO2). The large adsorption capacities of HCN and HCHO in the framework result from the strong interaction between adsorbates and metal centers, based on analyzing the radial distribution functions (RDF). Comparing C2H4 and CH4 molecules interacting with CPM-200-In/Mg by VDW interaction, we speculate that the high adsorption capacities of their chlorine derivatives in the framework could be due to the existence of halogen bonding or strong electrostatic and VDW interactions. It is found that the basic groups, including -NH2, -N and -OH, can effectively improve both the adsorption capacities and selectivity of CPM-200-In/Mg for harmful gases. Note that the adsorption capacity of CPM-200-In/Mg-NH2 (site 2) (245 cm3 g-1) for CO2 exceeded that of MOF-74-Mg (228 cm3 g-1) at 273 K and 1 bar and that for HCHO can reach 0.41 g g-1, which is almost twice that of 438-MOF and nearly 45 times of that in active carbon. Moreover, for natural gas mixtures, the decarburization and desulfurization abilities of CPM-200-In/Mg-NH2 (site 2) have exceeded those of the MOF-74 series, while for post-combustion flue gas mixtures, the desulfurization ability of CPM-200-In/Mg-NH2 (site 2) is still comparable to those of the MOF-74 series at 303 K and 4 MPa. We hope that the current theoretical study could guide experimental research in the future.

Hourly composition of gas and particle phase pollutants at a central urban background site in Milan, Italy

A comprehensive range of gas and particle phase pollutants were sampled at 1-hour time resolution in urban background Milan during summer 2012. Measurements include several soluble inorganic aerosols (Cl−,NO2−,NO3−,SO42−,Ca2+,K+,Mg2+,Na+,NH4+) and gases (HCl, HNO2,HNO3, NH3, NO, NO2,O3, SO2), organic, elemental and black carbon and meteorological parameters. Analysis methods used include mean diurnal pattern on weekdays and Sundays, pollution roses, bivariate polar plots and statistical models using backtrajectories. Results show how nitrous acid (HONO) was mainly formed heterogeneously at nighttime, with a dependence of its formation rate on NO2 consistent with observations during the last HONO campaign in Milan in summer 1998, although since 1998 a drop in HONO levels occurred following to the decrease of its precursors. Nitrate showed two main formation mechanisms: one occurring through N2O5 at nighttime and leading to nitrate formation onto existing particles; another occurring both daytime and nighttime following the homogeneous reaction of ammonia gas with nitric acid gas. Air masses reaching Milan influenced nitrate formation depending on their content in ammonia and the timing of arrival. Notwithstanding the low level of SO2 in Milan, its peaks were associated to point source emissions in the Po valley or shipping and power plant emissions SW of Milan, beyond the Apennines. A distinctive pattern for HCl was observed, featured by an afternoon peak and a morning minimum, and best correlated to atmospheric temperature, although it was not possible to identify any specific source. The ratio of primary-dominated organic carbon and elemental carbon on hourly PM2.5 resulted 1.7. Black carbon was highly correlated to elemental carbon and the average mass absorption coefficient resulted MAC=13.8 ± 0.2 m2 g−1. It is noteworthy how air quality for a large metropolitan area, in a confined valley and under enduring atmospheric stability, is nonetheless influenced by sources within and outside the valley.

Size-resolved simulations of the aerosol inorganic composition with the new hybrid dissolution solver HyDiS-1.0: description, evaluation and first global modelling results

Abstract. The dissolution of semi-volatile inorganic gases such as ammonia and nitric acid into the aerosol aqueous phase has an important influence on the composition, hygroscopic properties, and size distribution of atmospheric aerosol particles. The representation of dissolution in global models is challenging due to inherent issues of numerical stability and computational expense. For this reason, simplified approaches are often taken, with many models treating dissolution as an equilibrium process. In this paper we describe the new dissolution solver HyDiS-1.0, which was developed for the global size-resolved simulation of aerosol inorganic composition. The solver applies a hybrid approach, which allows for some particle size classes to establish instantaneous gas-particle equilibrium, whereas others are treated time dependently (or dynamically). Numerical accuracy at a competitive computational expense is achieved by using several tailored numerical formalisms and decision criteria, such as for the time- and size-dependent choice between the equilibrium and dynamic approaches. The new hybrid solver is shown to have numerical stability across a wide range of numerical stiffness conditions encountered within the atmosphere. For ammonia and nitric acid, HyDiS-1.0 is found to be in excellent agreement with a fully dynamic benchmark solver. In the presence of sea salt aerosol, a somewhat larger bias is found under highly polluted conditions if hydrochloric acid is represented as a third semi-volatile species. We present first results of the solver's implementation into a global aerosol microphysics and chemistry transport model. We find that (1) the new solver predicts surface concentrations of nitrate and ammonium in reasonable agreement with observations over Europe, the USA, and East Asia, (2) models that assume gas-particle equilibrium will not capture the partitioning of nitric acid and ammonia into Aitken-mode-sized particles, and thus may be missing an important pathway through which secondary particles may grow to radiation- and cloud-interacting size, and (3) the new hybrid solver's computational expense is modest, at around 10 % of total computation time in these simulations.

Integrated Strategy toward Self-Powering and Selectivity Tuning of Semiconductor Gas Sensors

Inorganic conductometric gas sensors struggle to overcome limitations in high power consumption and poor selectivity. Herein, recent advances in developing self-powered gas sensors with tunable selectivity are introduced. Alternative general approaches for powering gas sensors were realized via proper integration of complementary functionalities (namely, powering and sensing) in a singular heterostructure. These solar light driven gas sensors operating at room temperature without applying any additional external powering sources are comparatively discussed. The TYPE-1 gas sensor based on integration of pure inorganic interfaces (e.g., CdS/n-ZnO/p-Si) is capable of delivering a self-sustained sensing response, while it shows a nonselective interaction toward oxidizing and reducing gases. The structural and the optical merits of TYPE-1 sensor are investigated giving more insight into the role of light activation on the modulation of the self-powered sensing response. In the TYPE-2 sensor, the selectivity of...

Seasonal variability and source apportionment of volatile organic compounds (VOCs) in the Paris megacity (France)

Abstract. Within the framework of air quality studies at the megacity scale, highly time-resolved volatile organic compound (C2–C8) measurements were performed in downtown Paris (urban background sites) from January to November 2010. This unique dataset included non-methane hydrocarbons (NMHCs) and aromatic/oxygenated species (OVOCs) measured by a GC-FID (gas chromatograph with a flame ionization detector) and a PTR-MS (proton transfer reaction – mass spectrometer), respectively. This study presents the seasonal variability of atmospheric VOCs being monitored in the French megacity and their various associated emission sources. Clear seasonal and diurnal patterns differed from one VOC to another as the result of their different origins and the influence of environmental parameters (solar radiation, temperature). Source apportionment (SA) was comprehensively conducted using a multivariate mathematical receptor modeling. The United States Environmental Protection Agency's positive matrix factorization tool (US EPA, PMF) was used to apportion and quantify ambient VOC concentrations into six different sources. The modeled source profiles were identified from near-field observations (measurements from three distinct emission sources: inside a highway tunnel, at a fireplace and from a domestic gas flue, hence with a specific focus on road traffic, wood-burning activities and natural gas emissions) and hydrocarbon profiles reported in the literature. The reconstructed VOC sources were cross validated using independent tracers such as inorganic gases (NO, NO2, CO), black carbon (BC) and meteorological data (temperature). The largest contributors to the predicted VOC concentrations were traffic-related activities (including motor vehicle exhaust, 15 % of the total mass on the annual average, and evaporative sources, 10 %), with the remaining emissions from natural gas and background (23 %), solvent use (20 %), wood-burning (18 %) and a biogenic source (15 %). An important finding of this work is the significant contribution from wood-burning, especially in winter, where it could represent up to ∼ 50 % of the total mass of VOCs. Biogenic emissions also surprisingly contributed up to ∼ 30 % in summer (due to the dominating weight of OVOCs in this source). Finally, the mixed natural gas and background source exhibited a high contribution in spring (35 %, when continental air influences were observed) and in autumn (23 %, for home heating consumption).

Fine particle pH and the partitioning of nitric acid during winter in the northeastern United States

Particle pH is a critical but poorly constrained quantity that affects many aerosol processes and properties, including aerosol composition, concentrations, and toxicity. We assess PM1 pH as a function of geographical location and altitude, focusing on the northeastern US, based on aircraft measurements from the Wintertime Investigation of Transport, Emissions, and Reactivity (WINTER) campaign (01 Feb to 15 Mar 2015). Particle pH and water were predicted with the ISORROPIA-II thermodynamic model and validated by comparing predicted to observed partitioning of inorganic nitrate between the gas and particle phases. Good agreement was found for relative humidity (RH) above 40%; at lower RH observed particle nitrate was higher than predicted, possibly due to organic-inorganic phase separations or nitrate measurement uncertainties associated with low concentrations (nitrate < 1 µgm-3). Including refractory ions in the pH calculations did not improve model predictions, suggesting they were externally mixed with PM1 sulfate, nitrate, and ammonium. Sample line volatilization artifacts were found to be minimal. Overall, particle pH for altitudes up to 5000 m ranged between -0.51 and 1.9 (10th and 90th percentiles) with a study mean of 0.77 ± 0.96, similar to those reported for the southeastern US and eastern Mediterranean. This expansive aircraft data set is used to investigate causes in variability in pH and pH-dependent aerosol components, such as PM1 nitrate, over a wide range of temperatures (-21 to 19 °C), RH (20 to 95%), inorganic gas and particle concentrations and also provides further evidence that particles with low pH are ubiquitous.

Field performance of a semi-continuous monitor for ambient PM2.5 water-soluble inorganic ions and gases at a suburban site

To reduce sampling artifacts and to improve time-resolved measurements of inorganic aerosol system, a recently commercialized semi-continuous In-situ Gas and Aerosol Composition (IGAC) monitoring system was evaluated against a reference annular denuder system (ADS; denuder/two-stage filter pack) at a suburban site over a year, during which the average PM2.5 was 37.0 ± 24.8 μg/m3. A suite of eight ions SO42−, NO3−, Cl−, NH4+, Na+, K+, Ca2+ and Mg2+ and two gases SO2 and NH3 were the target species. In comparison to the reference ADS method, the IGAC performed well in measuring the major ions SO42−, NO3− and NH4+, and the SO2. For those species, the linear slopes, intercepts and R2 values between the two methods all passed the performance evaluation criteria outlined by earlier similar studies. The performance of IGAC on Cl−, Na+, K+ and NH3 was marginally acceptable, whereas Ca2+ and Mg2+ could not be properly evaluated due to the low concentrations (<0.2 μg/m3) and hence inadequate amount of sample size. The ionic balance of the hourly IGAC samples averaged very close to unity, as did the daily ADS samples, though the former was considerably more variable than the latter. The overall performance of the IGAC has been shown to be comparable to other similar monitors and its improvements are discussed.

Arsine gas sensor based on gold-modified reduced graphene oxide

A gas sensor was developed for arsine (AsH3) which is the most toxic inorganic gas. The sensor was prepared from reduced graphene oxide (rGO), modified with a thin gold film, on an interdigitated array electrode. The conductance of the Au/rGO sensor was monitored in the presence of flowing AsH3 that was prepared by reduction of aqueous arsenite with borohydride and vaporization of the hydride. Gas sensors fabricated with only rGO or Au did not exhibit AsH3 sensitivity. However, rGO sensors combined with Au exhibited reversible conductivity enhancement with AsH3. The increase in conductivity probably occurred because the AsH3 depleted adsorbed oxygen on the Au islands, resulting in the enhancement of hole conduction in the rGO film. Responses were observed for sub-ppmv levels of AsH3. The amounts of gold and rGO, the GO reduction, and the operating temperature were optimized to obtain a detection limit of 0.01ppmv (determined from a signal level three times the baseline noise). Interference from other gases and vapors was examined. The sensor responded to NO2, but this is not expected to be present in air-quality-controlled clean rooms. AsH3 is one of the most toxic chemicals used in the semiconductor industry, and the Au/rGO-based AsH3 sensor is expected to have widespread applications.

Metabolic potential of microbial mats and microbialites: Autotrophic capabilities described by an in silico stoichiometric approach from shared genomic resources

Microbialites and microbial mats are complex communities with high phylogenetic diversity. These communities are mostly composed of bacteria and archaea, which are the earliest living forms on Earth and relevant to biogeochemical evolution. In this study, we identified the shared metabolic pathways for uptake of inorganic C and N in microbial mats and microbialites based on metagenomic data sets. An in silico analysis for autotrophic pathways was used to trace the paths of C and N to the system, following an elementary flux modes (EFM) approach, resulting in a stoichiometric model. The fragility was analyzed by the minimal cut sets method. We found four relevant pathways for the incorporation of CO2 (Calvin cycle, reverse tricarboxylic acid cycle, reductive acetyl-CoA pathway, and dicarboxylate/4-hydroxybutyrate cycle), some of them present only in archaea, while nitrogen fixation was the most important source of N to the system. The metabolic potential to incorporate nitrate to biomass was also relevant. The fragility of the network was low, suggesting a high redundancy of the autotrophic pathways due to their broad metabolic diversity, and highlighting the relevance of reducing power source. This analysis suggests that microbial mats and microbialites are "metabolic pumps" for the incorporation of inorganic gases and formation of organic matter.

Numerical study of a stress dependent triple porosity model for shale gas reservoirs accommodating gas diffusion in kerogen

A model accommodating multi-scale pores containing kerogen within an inorganic matrix is used to explore the complex multi-mechanistic transport mechanisms of shale gas reservoirs. These include the complex evolution of pressure, diffusion and flow within both kerogen and inorganic components and their interaction with effective stresses. A general poromechanical model is proposed considering desorption and molecular diffusion in the kerogen, viscous flow in the inorganic matrix and fracture system, and composite deformation of the triple porosity assemblage. The model is verified by history matching against field data for gas production rate. The simulation results indicate that the pattern of gas flow is sequential during gas depletion – pressure first declines in the fracture, followed by the inorganic phase and then in the kerogen. The evolution of permeability is pressure dependent and the evolution of pressure is closely related to the intrinsic gas diffusion coefficient in the kerogen, inorganic matrix intrinsic permeability and fracture intrinsic permeability. A series of sensitivity analyses are completed to define key parameters affecting gas production. The study shows that dominant influence of the fracture network in acting as the main permeable conduit. The intrinsic permeability and porosity of the fracture have a positive correlation with gas production, while fracture spacing has a negative correlation to gas production. Kerogen also plays a critical role in gas production for shale reservoirs with higher total organic carbon. The enhancement of inorganic matrix permeability and gas diffusion coefficient in kerogen could efficiently guarantee a long-term gas production with a higher rate.

Pulsed laser deposition of metal oxide nanostructures for highly sensitive gas sensor applications

Nanosecond and picosecond pulsed laser deposition (PLD) was used to prepare metal oxide nanostructures with different morphologies as gas sensing materials on top of oxidized silicon substrates and commercial SGX Sensortech SA MEMS microheater platforms. The layers were formed of different types of nanostructures including nanoparticles, agglomerates, and nanotrees with fractal-like growth. Clear dependencies between the deposition parameters, structural morphology, and gas sensing performance were found. Also, some differences in the morphologies of the layers were seen when picosecond PLD was used instead of nanosecond PLD. Many of the sensing materials were found to be highly sensitive to different types of gaseous species. We investigated inorganic gases in the ppm range (2–400ppm) including NO, CO, and NH3, and the selectivity and sensitivity were shown to be dependent, not only on layer morphology, but also on the measurement temperature. Moreover, an investigation with volatile organic compound gases in the ppb range demonstrated that WO3 layers are highly sensitive and selective towards naphthalene at least down to 2.5ppb.