Convective Gas Transport Research Articles

Investigating the convective transport possibilities of lower-atmospheric pollutants to the UTLS region using rainwater and aerosol chemical characterization

Various attempts using different platforms to understand the chemical composition of the Asian Tropopause Aerosol Layer (ATAL) have concluded that it primarily consists of nitrate (NO3−) aerosols. Recent in-situ measurements from aircraft flying through ATAL have suggested that ammonia (NH3) pollution from Asia could serve as the precursor for nitrate aerosols in ATAL through gas-to-particle formation after undergoing convective uplift to the Upper Troposphere and Lower Stratosphere (UTLS) regions. In this study, we aimed to investigate the potential pathways of convective transport of lower-atmospheric pollutants by analysing the chemical composition of rainwater and aerosols in a highly convective region, Kolkata (Head Bay of Bengal), India. We utilized the PILS-IC system established at Kolkata Camp Observatory of the National Atmospheric Research Laboratory (KCON). The analysis revealed a significant dominance of NH4+ and NO3− ions in both rainwater and aerosol samples during the monsoon and post-monsoon seasons, with the dominance increasing during the post-monsoon season. Air mass trajectories indicated a clear influence of the Indo-Gangetic Plain (IGP) region during the post-monsoon season, which is well-known for its dense agricultural activities and industries. High concentrations of NH3 even at higher altitudes, have been observed in Atmospheric Infrared Sounder (AIRS) measurements. Moreover, using Outgoing Longwave Radiation (OLR) and vertical wind as proxies for tropical deep convection and updrafts/downdrafts, respectively, we found evidence supporting the possibility of convective transport of these lower-atmospheric pollutants to the UTLS regions during the monsoon season. In conclusion, this study provides evidence for the potential convective transport of lower-atmospheric pollutants, including NH3 and other precursor gases, to higher levels. These pollutants could partially serve as the source of nitrate aerosols in the ATAL through gas-to-particle formation processes.

Effect of warming on the carbon flux of the alpine wetland on the Qinghai–Tibet Plateau

Under the scenario of global warming, the response of greenhouse gas emissions from alpine wetlands remains unclear. In this study, fluxes of CO2 and CH4 were measured during daytime for the microtopographic features of hollows and hummocks in a wetland in the Tibetan Plateau under two elevated temperatures, increments of ∼1°C (T1 treatment) and ∼2°C (T2 treatment), during the growing season in 2019. The results showed that warming significantly increased the cumulative net ecosystem CO2 exchanges (NEE) for both microtopographic features in the wetland compared to the control due to a combination of the increased gross primary production (GPP) with an increase in ecosystem respiration (ER). Similarly, warming also increased cumulative CH4 emission significantly. The effect was stronger for T2 than that for T1 for all component fluxes (GPP, ER, NEE, and CH4). Generally, NEE and CH4 fluxes both rose at first and then decreased. NEE peaked at the end of July for both hollows and hummocks, while CH4 emissions peaked in the middle of August. The cumulative CH4 emissions from the hummocks were significantly higher than those of the hollows, and CH4 emissions under illumination were significantly higher than those in darkness, which may be caused by the irradiation-sensitive vegetable internal convective gas transport system which diffuses CH4 from the pedosphere. This study revealed that warming strengthened the function of the CO2 sink but also increased CH4 emissions from the alpine wetlands on the Qinghai–Tibet Plateau.

Combustion of n-butyl acetate synthesized by a new and sustainable biological process and comparisons with an ultrapure commercial n-butyl acetate produced by conventional Fischer esterification

This paper reports a study of the combustion dynamics of n-butyl acetate (BA) using the configuration of a burning droplet. Two grades of BA were examined: one (SBA) synthesized by a new process described in the paper that uses a metabolically engineered solventogenic Clostridiumstrain through an extractive fermentation process using n-hexadecane as the extractant; and one commercially available as a high-purity (99.9%) ‘neat’ BA grade produced by conventional Fischer esterification (NBA). The initial droplet diameter was primarily 0.6 mm with some limited experiments carried out for 0.4 mm droplets to show the influence of convection. Experiments were performed in the standard atmosphere and ignition was by spark discharge. The results showed the presence of impurities in the SBA at mass concentrations totaling about 6% which included n-butanol, n-hexadecane, iso-propyl alcohol and ethyl acetate. Droplet burning rates and flame structures were not influenced by these impurities at this concentration level. In the presence of convection created by buoyancy, droplets burned faster with stretched flames and a luminosity revealing the presence of soot by incandescence at the flame tips. Reducing the initial droplet diameter to 0.4 mm eliminated the convective effect and resulted in near spherical flames. The results presented show that the new synthesis process is a sustainable alternative for BA production with burning characteristics identical to NBA in both convective and stagnant gas transport fields.

1D Finite Volume Scheme for Simulating Gas–Solid Reactions in Porous Spherical Particles with Application to Biomass Pyrolysis

Models for gas–solid reactions in porous particles typically consist of a set of mass and energy balances in the form of conservation equations. For spherical or close to spherical particles, these equations are formulated in 1D spherical coordinates. In the case where accumulation of gas inside the particle is significant, the balance equations contain convective terms. The present work presents a simple numerical scheme based on flux limited finite volume methods for discretizing conservation equations for convective and diffusive transport of mass and energy in radial direction in a porous sphere. The velocity is governed by Darcy’s law coupled to an equation of state. The proposed scheme is applied to a series of test problems that admit full or partial analytical solutions. For the cases where only partial analytical solutions are available, a Comsol model is adopted for comparison. It is found that the scheme is able to resolve step gradients without generating oscillations and that it properly handles changes in the sign of the convective velocity. Applying the scheme for solving a common model for biomass pyrolysis reveals the importance of convective gas transport in the pyrolysis of thermally thick biomass particles.

Capture efficiency of four chamber designs for measuring ammonia emissions

AbstractAmmonia (NH3) emissions are an economically and environmentally significant loss pathway of fertilizer and soil‐derived N. Chambers are a commonly used method to quantify NH3 emissions in plot‐scale agricultural research. Although this method is widely used, its accuracy may be influenced by the overall design of the chamber, its components, and its interaction with the environment. Four NH3 chamber designs, including open, open + polytetrafluoroethylene (PTFE), semi‐open, and closed, were deployed over a dilute NH3 solution for 6 h on four dates to determine the effect of chamber design on NH3 capture efficiency. The solution volume and concentration were measured before and after acid trap deployment, and total volatile NH3 emission was assumed to be equal to the mass N loss. The NH3 capture efficiency relative to the estimated total emissions was greatest for the open design (12.9%), whereas the semi‐open chamber was the least efficient (3.5%). The closed chamber reduced NH3 emissions relative to the open and semi‐open designs by inhibiting convective gas transport beneath the chamber footprint.

Development and validation of a comprehensive model for biotrickling filters upgrading biogas

This paper details the development, validation, and analysis of a conceptually correct mathematical model for biogas upgrading biological trickling filter (BTF) reactors. The model considers convective transport and dispersion of gases through the fixed bed and mass transfer and reaction of absorbed gases in immobilized biofilms. Wetted and non-wetted biofilms, gas volume contraction through the bed as hydrogen and CO2 are converted to methane, and axial dispersion were all included in the model. The model successfully predicted the performance of a laboratory scale BTF upgrading various raw biogas compositions. A parametric sensitivity analysis revealed that the influent gas flow rate, biofilm specific surface area, and maximum rate of reaction (Rmax) were the most sensitive parameters. Furthermore, reducing the wetted biofilm ratio by 54% (12% wetted) was the easiest optimization measure for achieving renewable natural gas standards (>97% effluent methane). Practically, this could be achieved e.g., by applying an intermittent trickling regime. The liquid-film mass transfer coefficient (kLa) was not a sensitive parameter. This was because the model predicted that a majority of substrate conversion was occurring in the non-wetted biofilm, a finding that further supports the importance of reducing biofilm wetting in these BTFs. Overall, the model provides a strong conceptual framework for future studies modeling biogas upgrading processes. Additionally, it has the potential to be a useful tool in scaling and optimizing BTF bioreactors for biogas upgrading applications.

Multi-wafer batch synthesis of graphene on Cu films by quasi-static flow chemical vapor deposition

The chemical vapor deposition (CVD) of graphene on thin Cu film wafers is highly desirable for the development of technological applications as it offers superior flatness, rigidity, high purity, and compatibility with conventional thin film techniques. Here, we report the high-throughput synthesis of uniform single-layer highly crystalline graphene on a batch of 3-inch wafers. The production throughput is optimized by using closely-packed vertically-standing wafers instead of placing them flat on a horizontal support. Significantly reducing convective gas transport is found to be essential to minimize the variation of graphene single crystal seeding density and growth rate across individual wafers. Given the very small amount of carbon required to grow an atomically-thin layer, we show that graphene can be grown under static gas flow conditions and that the growth rate does not significantly vary from one wafer to another, even within a large batch (∼25 wafers). These findings constitute an important technological step toward the manufacturing of graphene on an industrial scale and its integration into mainstream electronics or micro-electromechanical devices.

Development of 1-D multiphysics PEMFC model with dry limiting current experimental validation

The limiting current method is an effective in-situ diagnostic tool for studying oxygen transport resistance in a PEM fuel cell. In this study, we present both experimental and modeling results of a PEMFC operating from open circuit voltage to limiting current conditions. Operating conditions are 80∘ C and 64% relative humidity. Pressure is varied from 118 kPa, 151 kPa, 201 kPa and 301 kPa. For each pressure the dry oxygen mole fraction is also varied from 1%, 2%, 3% and 4%. The ratio of porosity to tortuosity and the pressure independent O2 transport resistance used in the newly developed 1-D steady state model are obtained from the in-situ limiting current experiment. The model incorporates non-isothermal heat transfer, convective and diffusive gas transport, electrochemical reaction kinetics, shorting and hydrogen cross over current, effects of land and channel geometry on transport resistance, membrane water balance and proton resistance based on non-linear water content in the membrane. The simulation results are validated by polarization curves and limiting current tests at various operation conditions. This steady state 1-D dry model successfully integrate and implement interactive multiphysics to predict fuel cell performance under dry operating conditions.

Evaluation of Parameterized Convective Transport of Trace Gases in Simulation of Storms Observed During the DC3 Field Campaign.

Deep convective transport of surface moisture and pollution from the planetary boundary layer to the upper troposphere and lower stratosphere affects the radiation budget and climate. This study uses cloud-parameterized Weather Research and Forecasting model coupled with Chemistry simulations to analyze the subgrid deep convective transport of CO at 12- and 36-km horizontal resolution in supercell and mesoscale convective systems observed during the 2012 Deep Convective Clouds and Chemistry field campaign and compares the simulation results with aircraft measurements and cloud-resolved simulations. The best Weather Research and Forecasting simulation of these storms was obtained with the use of the Grell-Freitas convective scheme. The default Weather Research and Forecasting model coupled with Chemistry subgrid convective transport scheme was replaced with a scheme to compute convective transport within the Grell-Freitas subgrid cumulus parameterization, which resulted in improved transport simulations. We examined the CO tendencies due to subgrid- and grid-scale convective transport. Results showed that the subgrid convective transport started earlier than the grid-scale convective transport. The subgrid-scale convective transport reached its maximum during the hour prior to the formation of the grid-scale constant-altitude detrainment layer. After that, both the subgrid- and grid-scale convective transport began to decrease. The subgrid-scale convective transport played a more significant role in the supercell case than the mesoscale convective system case. Subgrid contribution reached ~90% at the beginning of the storm and decreased to ~30% (17%) for the 36-km (12-km) domain 4 hr later.

Thermogravimetric measurement of the equilibrium vapour pressure: Application to water and triethanolamine

A model that describes the pure evaporation kinetics is introduced. This model takes into account gas diffusion and convective gas transport; it describes the evaporation kinetics under the general conditions of thermogravimetric measurements. The model is used to determine the equilibrium vapour pressure in a relatively wide temperature range. The validity of the model is checked against experimental data of triethanolamine (TEA) and water evaporation. The applicability of isoconversional methods to the evaporation kinetics is also studied. Besides, it is shown that the degree of TEA decomposition depends on the surrounding atmosphere and on the conditions for gas evaporation; the easiest the gas evaporation, the smallest the degree of decomposition. In view of the volatiles formed, a reaction pathway is proposed for the thermal decomposition of TEA.

Soil water retention, air flow and pore structure characteristics after corn cob biochar application to a tropical sandy loam

Soil structure is a key soil physical property that affects soil water balance, gas transport, plant growth and development, and ultimately plant yield. Biochar has received global recognition as a soil amendment with the potential to ameliorate the structure of degraded soils. We investigated how corn cob biochar contributed to changes in soil water retention, air flow by convection and diffusion, and derived soil structure indices in a tropical sandy loam. Intact soil cores were taken from a field experiment that had plots without biochar (CT), and plots each with 10tha−1 (BC-10), 20tha−1 without or with phosphate fertilizer (BC-20 and BC-20+P respectively). Soil water retention was measured within a pF range of 1 to 6.8. Gas transport parameters (air permeability, ka, and relative gas diffusivity, Dp/D0) were measured between pF 1.5 and 3.0. Application of 20tha−1 led to significant increase in soil water retention compared to the CT and BC-10 as a result of increased microporosity (pores <3μm) whereas for soil specific surface area, biochar had minimal impact. No significant influence of biochar was observed for ka and Dp/D0 for the BC treatments compared to the CT despite the larger values for the two properties in the 20tha−1 treatments. Although not significant, the diffusion percolation threshold reduced by 34% and 18% in the BC-20 and BC-20+P treatments, respectively, compared to the CT. Similarly, biochar application reduced the convection percolation threshold by 15 to 85% in the BC-amended soils. The moderate impact of corn cob biochar on soil water retention, and minimal improvements in convective and diffusive gas transport provides an avenue for an environmentally friendly disposal of crop residues, particularly for corn cobs, and structural improvement in tropical sandy loams.

Role of the intake generated thermal stratification on the temperature distribution at top dead center of the compression stroke

This work investigates the role of the intake generated thermal stratification in the temperature field evolution during the compression stroke using direct numerical simulation. The analysis compares two direct numerical simulations during the compression stroke, from which one is initialized with a homogeneous temperature distribution and the other one with a stratified temperature field resulting from a precursor direct numerical simulation of the intake stroke. All other initial and boundary conditions are identically imposed. The results show that the thermal situation at bottom dead center has nearly no impact on the evolution of the temperature field and the wall heat transfer during compression. Dominating mechanism for the temperature field evolution is found to be the convective transport of cold gases from the boundary layers toward the cylinder center. As a consequence, the wall temperature and the flow field are the main influencing parameters controlling the evolution of the temperature distribution during compression. This finding can assist practical engine experiments, since it points out which mechanisms are promising to affect the temperature field during the compression stroke. In addition, it explains why the stratification of the temperature field during intake by varying the gas temperatures in the two intake channels showed only a minor impact on the thermal situation at top dead center.

Effects of Confining Pressure on Unsteady‐State Air Permeability Measurement Method in an Aggregated Andisol

Core Ideas We propose an unsteady‐state air permeability method with confined sleeve pressure. Interaggregate permeability increased slightly compared with the intraaggregate region. Air permeability is affected by air leakage through the sample–column wall space. Air permeability is also affected by soil structure and water state in an Andisol. Soil air permeability is an important parameter to characterize convective gas transport through the soil gas phases. However, current air permeability measurements have limitations, specifically in Andisols, which are highly aggregated and often shrink during drying processes. Determining the air content at different tension gradient points by drying is also time consuming because experiments have to reach several steady‐state conditions. Thus, the objective of this research was to demonstrate an unsteady‐state air permeability measurement method with confined sleeve pressure. Furthermore, the effects of confined pressure and air leakage on the measurement of air permeability were determined using an Andisol across a wide range of soil water and air contents. Under confined pressure, the air flow rates at air contents of 0.2, 0.4, and 0.6 m3 m−3 and soil bulk density of 0.75 Mg m−3 were 5.8, 2.3, and 0.8 times higher, respectively, than those under unconfined pressure. Using the unsteady‐state air permeability measurements method on the Andisol, our results show that the interaggregate region had air contents in the range from 0.13 to 0.25 m3 m−3, with air permeability ranging from 12.6 to 54.8 μm2. Conversely, the intraaggregate region had air contents ranging from 0.25 to 0.65 m3 m−3, with air permeability ranging from 54.8 to 81.5 μm2. The intraaggregate air permeability increased only slightly due to less space in the aggregate.

Chapter 15 Diffuse soil emanations of radon and hazard implications at Furnas Volcano, São Miguel Island (Azores)

Abstract Furnas Caldera Volcanic Complex, São Miguel Island (Azores), last erupted in 1630 and is famous for its intense hydrothermal activity (i.e. fumarolic fields, thermal springs, cold CO 2 -rich mineral waters and diffuse CO 2 soil emanations), which directly affect the villages of Furnas and Ribeira Quente. Here we report the first systematic investigations and mapping of soil radon ( 222 Rn) emanations in the Furnas Volcanic Complex, and we examine their potential health risks for local inhabitants. 222 Rn in volcanic soils (60 cm depth) was repeatedly measured between 2005 and 2010 using a portable solid-state alpha detector (RAD7, Durridge Company Inc.). Results reveal a local background of 8000 Bq m −3 and several areas with anomalously high 222 Rn activity (up to c. 400 000 Bq m −3 ), which coincide with advective or convective gas transport through volcanic structures and active fault zones. High 222 Rn radioactivity in Furnas and Ribeira Quente villages represents a risk to the population. Continuous monitoring performed during November and December 2005 in a house in Furnas shows indoor 222 Rn reaching 13 273 Bq m −3 , two orders of magnitude greater than the reference level (150 Bq m −3 ), when the ventilation efficiency is reduced.

Effects of Soil Bulk Density on Gas Transport Parameters and Pore‐Network Properties across a Sandy Field Site

The gas diffusion coefficient, air permeability, and their interrelations with air‐filled porosity are essential for characterization of diffusive and convective transport of gases in soils. Variations in soil bulk density can affect water retention, air‐filled pore space, and pore‐network connectivity and tortuosity and, thereby, control gas diffusion and air permeability. Considering 86 undisturbed core samples with variable bulk densities that were extracted on a 15 by 15 m grid from the top layer of a sandy field, the effects of soil bulk density on gas transport parameters and the soil water characteristic were investigated. Interactions with soil organic matter, sand, and clay fractions were also examined. To evaluate bulk density effects, two constitutive parameters were derived from each of the three measured relationships. The Campbell pore‐size distribution index (b) and the air‐entry matric potential (ψae) were derived from the soil water characteristic; the diffusive percolation threshold (εDPT), the air‐filled porosity where gas diffusivity ceases to almost zero because of interconnected water films creating isolated–inactive air content, and a pore‐network connectivity index (A2) were derived from the gas diffusivity curve, and the analogous parameters convective percolation threshold (εCPT) and convective pore‐network connectivity index (B2) from the air permeability curve. All six parameters showed significant negative correlations with bulk density. To further account for the effects of both bulk density and macroporosity in parametric gas transport models, a diffusive‐analog macroporosity–dependent model (DAMP) for gas diffusivity and a generalized Kawamoto et al. model (GK) for air permeability, which yielded improved predictive capabilities when compared with previous models, were developed. Both new models apply a reference point of prediction at −100 cm H2O matric potential (macroporosity drained), corresponding to the point where analysis of pore‐network tortuosity (T) and equivalent pore diameter for gas transport (dg) showed diminishing effects of water blockage on gas transport in the sandy soil.

A dedicated compression device for high resolution X-ray tomography of compressed gas diffusion layers

We present an experimental approach to study the three-dimensional microstructure of gas diffusion layer (GDL) materials under realistic compression conditions. A dedicated compression device was designed that allows for synchrotron-tomographic investigation of circular samples under well-defined compression conditions. The tomographic data provide the experimental basis for stochastic modeling of nonwoven GDL materials. A plain compression tool is used to study the fiber courses in the material at different compression stages. Transport relevant geometrical parameters, such as porosity, pore size, and tortuosity distributions, are exemplarily evaluated for a GDL sample in the uncompressed state and for a compression of 30 vol.%. To mimic the geometry of the flow-field, we employed a compression punch with an integrated channel-rib-profile. It turned out that the GDL material is homogeneously compressed under the ribs, however, much less compressed underneath the channel. GDL fibers extend far into the channel volume where they might interfere with the convective gas transport and the removal of liquid water from the cell.

Opaque closed chambers underestimate methane fluxes of Phragmites australis (Cav.) Trin. ex Steud

Closed chamber measurements for methane emission estimation are often carried out with opaque chambers to avoid heating of the headspace. However, mainly in wetlands, some plants possess an internal convective gas transport which quickly responds to changes in irradiation. These plants have also been found to often channel a large part of the released methane in temperate fens. We compare methane fluxes derived from transparent versus opaque chambers on Carex-, Phragmites-, and Typha-dominated stands of a temperate fen. Transparent chamber fluxes almost doubled opaque chamber fluxes in the convective transporting Phragmites stand. In Typha, a trend of higher fluxes determined with the transparent chambers was detectable, whereas in Carex, transparent and opaque chamber fluxes did not differ significantly. Thus, opaque chambers bias the outcome of methane measurements, depending on dominant vegetation. We recommend the use of transparent chambers when determining emissions of convective plants or extrapolating fluxes to larger scales.

Response of ammonium removal to growth and transpiration of Juncus effusus during the treatment of artificial sewage in laboratory-scale wetlands

The correlation between nitrogen removal and the role of the plants in the rhizosphere of constructed wetlands are the subject of continuous discussion, but knowledge is still insufficient. Since the influence of plant growth and physiological activity on ammonium removal has not been well characterized in constructed wetlands so far, this aspect is investigated in more detail in model wetlands under defined laboratory conditions using Juncus effusus for treating an artificial sewage. Growth and physiological activity, such as plant transpiration, have been found to correlate with both the efficiency of ammonium removal within the rhizosphere of J. effusus and the methane formation. The uptake of ammonium by growing plant stocks is within in a range of 45.5%, but under conditions of plant growth stagnation, a further nearly complete removal of the ammonium load points to the likely existence of additional nitrogen removal processes. In this way, a linear correlation between the ammonium concentration inside the rhizosphere and the transpiration of the plant stocks implies that an influence of plant physiological activity on the efficiency of N-removal exists. Furthermore, a linear correlation between methane concentration and plant transpiration has been estimated. The findings indicate a fast response of redox processes to plant activities. Accordingly, not only the influence of plant transpiration activity on the plant-internal convective gas transport, the radial oxygen loss by the plant roots and the efficiency of nitrification within the rhizosphere, but also the nitrogen gas released by phytovolatilization are discussed. The results achieved by using an unplanted control system are different in principle and characterized by a low efficiency of ammonium removal and a high methane enrichment of up to a maximum of 72.7% saturation.

Aerosol and gas re‐distribution by shallow cumulus clouds: An investigation using airborne measurements

Aircraft measurements during the 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS) are used to examine the influence of shallow cumulus clouds on vertical profiles of aerosol chemical composition, size distributions, and secondary aerosol precursor gases. The data show signatures of convective transport of particles, gases and moisture from near the surface to higher altitudes, and of aqueous‐phase production of aerosol mass (sulfate and organics) in cloud droplets and aerosol water. In cloudy conditions, the average aerosol volume concentration at an altitude of 2850 m, above typical cloud top levels, was found to be 34% of that at 450 m; for clear conditions, the same ratio was 13%. Both organic and sulfate mass fractions were on average constant with altitude (around 50%); however, the ratio of oxalate to organic mass increased with altitude (from 1% at 450 m to almost 9% at 3450 m), indicative of the influence of in‐cloud production on the vertical abundance and characteristics of secondary organic aerosol (SOA) mass. A new metric termed “residual cloud fraction” is introduced as a way of quantifying the “cloud processing history” of an air parcel. Results of a parcel model simulating aqueous phase production of sulfate and organics reproduce observed trends and point at a potentially important role of SOA production, especially oligomers, in deliquesced aerosols. The observations emphasize the importance of shallow cumulus clouds in altering the vertical distribution of aerosol properties that influence both their direct and indirect effect on climate.

Ventilation heterogeneity: small length scales, big challenges

the distribution of regional ventilation is heterogeneous in normal lung and increases dramatically under pathological conditions. This has been demonstrated both in humans and in animal models using a number of different imaging techniques. General anesthesia and mechanical ventilation can