Detailed Chemical Processes Research Articles

Modeling the Air Pollution and Aerosol‐PBL Interactions Over China Using a Variable‐Resolution Global Model

AbstractModeling air quality has always been a challenge in global models constrained by coarse grids. Here, the variable‐resolution Community Atmosphere Model with full chemistry based on the scalable spectral element (SE) dynamical core (MUSICAv0) is applied in simulating air pollution with a finer grid resolution of ∼0.25° over East Asia (SE_VR), in contrast to the same model with a uniform resolution of ∼1.0° (SE_UR). Two nudging experiments and four free‐running experiments are conducted to investigate the capabilities of SE_VR in modeling the air pollution and aerosol‐planetary boundary layer (PBL) interactions over China. Results show the regional refinement in SE_VR is essential for simulating haze events over complex terrain areas attributed to its better performance in representing the local vertical and horizontal dispersion conditions. SE_VR shows prominent advantages over SE_UR in simulating surface ozone because of better resolving spatial segregation of NOx and volatile organic compounds (VOC) chemical regimes and subsequently representing more detailed chemical processes related to ozone formation, although the model generally overestimates surface ozone over China. Further analysis of SE_VR shows the daytime radiative effect of black carbon (BC) aerosols lowers PBL height by 12.0% (17.9%), and leads to an increase of PM2.5 by 14.5% (10.8%) under the moderate (severe) air pollution conditions over Sichuan Basin. However, SE_UR has deficiencies in simulating BC‐PBL interactions due to its inability to reproduce the strong inverse temperature structure caused by BC aerosols in the lower atmosphere layer. Our results highlight the value of variable‐resolution global models for simulating air pollution and its interactions with climate.

Complex Organic Molecules Detected in 12 High-mass Star-forming Regions with Atacama Large Millimeter/submillimeter Array

Recent astrochemical models and experiments have explained that complex organic molecules (COMs; molecules composed of six or more atoms) are produced on the dust grain mantles in cold and dense gas in prestellar cores. However, the detailed chemical processes and the roles of physical conditions on chemistry are still far from understood. To address these questions, we investigated 12 high-mass star-forming regions using Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 observations. They are associated with 44/95 GHz class I and 6.7 GHz class II CH3OH masers, indicative of undergoing active accretion. We found 28 hot cores with COM emission among 68 continuum peaks at 1.3 mm and specified 10 hot cores associated with 6.7 GHz class II CH3OH masers. Up to 19 COMs are identified including oxygen- and nitrogen-bearing molecules and their isotopologues in cores. The derived abundances show a good agreement with those from other low- and high-mass star-forming regions, implying that the COM chemistry is predominantly set by the ice chemistry in the prestellar core stage. One clear trend is that the COM detection rate steeply grows with the gas column density, which can be attributed to the efficient formation of COMs in dense cores. In addition, cores associated with a 6.7 GHz class II CH3OH maser tend to be enriched with COMs. Finally, our results suggest that the enhanced abundances of several molecules in our hot cores could be originated by the active accretion as well as different physical conditions of cores.

A decoupling strategy for protecting sensitive process information in cooperative optimization of power flow

AbstractOptimal power flow (OPF) with close cooperation between the power grid and flexible electricity‐intensive chemical processes can reduce costs for the grid and for electricity users. However, this would require sharing detailed chemical process models, which may reveal confidential information and potentially jeopardize competitive advantages for a chemical process operator. We propose an algorithm that enables economically advantageous cooperation without the need to exchange sensitive information. Low‐order linear models are used to represent the dynamic behavior of electricity‐intensive processes. We integrate these models into the OPF problem and solve the problem using a decoupling strategy based on Benders‐type cuts. The cuts introduce limited communication between the chemical processes and the grid without exchanging explicit information pertaining to the process dynamics and performance. Our results reproduce solutions obtained by sharing detailed process models up to a user‐defined optimality gap for several test cases that reflect both normal and congested grid states. We also investigate the trade‐off between the value of the optimality gap and computational effort. Finally, we study the scaling behavior of the iterative procedure with respect to flexible loads at multiple grid locations.

Rapid mass growth and enhanced light extinction of atmospheric aerosols during the heating season haze episodes in Beijing revealed by aerosol–chemistry–radiation–boundary layer interaction

Abstract. Despite the numerous studies investigating haze formation mechanism in China, it is still puzzling that intensive haze episodes could form within hours directly following relatively clean periods. Haze has been suggested to be initiated by the variation of meteorological parameters and then to be substantially enhanced by aerosol–radiation–boundary layer feedback. However, knowledge on the detailed chemical processes and the driving factors for extensive aerosol mass accumulation during the feedback is still scarce. Here, the dependency of the aerosol number size distribution, mass concentration and chemical composition on the daytime mixing layer height (MLH) in urban Beijing is investigated. The size distribution and chemical composition-resolved dry aerosol light extinction is also explored. The results indicate that the aerosol mass concentration and fraction of nitrate increased dramatically when the MLH decreased from high to low conditions, corresponding to relatively clean and polluted conditions, respectively. Particles having their dry diameters in the size of ∼400–700 nm, and especially particle-phase ammonium nitrate and liquid water, contributed greatly to visibility degradation during the winter haze periods. The dependency of aerosol composition on the MLH revealed that ammonium nitrate and aerosol water content increased the most during low MLH conditions, which may have further triggered enhanced formation of sulfate and organic aerosol via heterogeneous reactions. As a result, more sulfate, nitrate and water-soluble organics were formed, leading to an enhanced water uptake ability and increased light extinction by the aerosols. The results of this study contribute towards a more detailed understanding of the aerosol–chemistry–radiation–boundary layer feedback that is likely to be responsible for explosive aerosol mass growth events in urban Beijing.

Novel instrumentation for tracking molecular products in fast pyrolysis of carbohydrates with sub-second temporal resolution

Despite recent advances, little has been experimentally established regarding the detailed chemical processes that occur during biomass pyrolysis reactions. We developed a new technique that allows for the monitoring of each molecular product from fast pyrolysis with ∼0.2 s temporal resolution. This was achieved by directly coupling a micropyrolyzer with a time-of-flight mass spectrometer via a soft ionization. Molecular products studied were produced in thin-film pyrolysis of a series of glucose-based carbohydrates. Unprecedented details of the pyrolysis reaction process were revealed, including the timescale of molecular product formation and the existence of metastable intermediates. Small carbohydrates are completely pyrolyzed within one second and as short as one-half second for glucose pyrolysis. Individual time profiles could be extracted and examined for each molecular product. Additionally, the effect of sample dimensions on the pyrolysis of cellulose and α-cyclodextrin, as both thin films and particles, was studied. A surprising time delay of one second is observed for the thin-film pyrolysis of cellulose and α-cyclodextrin, which is attributed to the transition to the molten phase. When a large amount of cellulose or α-cyclodextrin is pyrolyzed, random fluctuations of temporal profiles are observed and explained as coming from aerosol ejections. This is well correlated with the high abundance of non-volatile products such as cellobiosan that cannot be detected in typical GC–MS or pyrolysis GC–MS analysis.

A mathematical model to analyze solid oxide electrolyzer cells (SOECs) for hydrogen production

In this analysis, we report an in-house model to describe the complex fundamental and functional interactions between various internal physico-chemical phenomena of a SOEC. Electrochemistry at the three-phase boundary is modeled using a modified Butler–Volmer approach that considers H2 as the electrochemically active species. Also, a multi-step elementary heterogeneous reaction mechanism for the thermo-catalytic H2 electrode chemistry, dusty-gas model to account for multi-component diffusion through porous media, and plug flow model for flow through the channels are used. Results pertaining to detailed chemical processes within the cathode, electrochemical behavior and irreversible losses during SOEC operation are demonstrated. Furthermore, efficiency analysis is performed and limiting current behavior of the SOEC system is investigated.

Modeling of Solid-Oxide Electrolyser Cells: From H2, CO Electrolysis to Co-Electrolysis

In this analysis, we report an in-house model to describe the complex fundamental and functional interactions between various internal physico-chemical phenomena of a SOEC. Electrochemistry at the three-phase boundary is modeled using a modified Butler-Volmer approach that considers H2 and CO, individually, as electrochemically active species. Also, a multi-step elementary heterogeneous reaction mechanism for the thermo-catalytic H2/CO2 electrode chemistry, along with the dusty gas model (DGM) to account for multi-component diffusion of ideal gases through porous media, are used. The model is geometry independent. Results pertaining to detailed chemical processes within the cathode, electrochemical behavior and irreversible losses during SOEC operation are demonstrated.

Stratospheric ozone chemistry in the Antarctic: what determines the lowest ozone values reached and their recovery?

Abstract. Balloon-borne observations of ozone from the South Pole Station have been reported to reach ozone mixing ratios below the detection limit of about 10 ppbv at the 70 hPa level by late September. After reaching a minimum, ozone mixing ratios increase to above 1 ppmv on the 70 hPa level by late December. While the basic mechanisms causing the ozone hole have been known for more than 20 yr, the detailed chemical processes determining how low the local concentration can fall, and how it recovers from the minimum have not been explored so far. Both of these aspects are investigated here by analysing results from the Chemical Lagrangian Model of the Stratosphere (CLaMS). As ozone falls below about 0.5 ppmv, a balance is maintained by gas phase production of both HCl and HOCl followed by heterogeneous reaction between these two compounds in these simulations. Thereafter, a very rapid, irreversible chlorine deactivation into HCl can occur, either when ozone drops to values low enough for gas phase HCl production to exceed chlorine activation processes or when temperatures increase above the polar stratospheric cloud (PSC) threshold. As a consequence, the timing and mixing ratio of the minimum ozone depends sensitively on model parameters, including the ozone initialisation. The subsequent ozone increase between October and December is linked mainly to photochemical ozone production, caused by oxygen photolysis and by the oxidation of carbon monoxide and methane.

Glyoxal processing by aerosol multiphase chemistry: towards a kinetic modeling framework of secondary organic aerosol formation in aqueous particles

Abstract. This study presents a modeling framework based on laboratory data to describe the kinetics of glyoxal reactions that form secondary organic aerosol (SOA) in aqueous aerosol particles. Recent laboratory results on glyoxal reactions are reviewed and a consistent set of empirical reaction rate constants is derived that captures the kinetics of glyoxal hydration and subsequent reversible and irreversible reactions in aqueous inorganic and water-soluble organic aerosol seeds. Products of these processes include (a) oligomers, (b) nitrogen-containing products, (c) photochemical oxidation products with high molecular weight. These additional aqueous phase processes enhance the SOA formation rate in particles and yield two to three orders of magnitude more SOA than predicted based on reaction schemes for dilute aqueous phase (cloud) chemistry for the same conditions (liquid water content, particle size). The application of the new module including detailed chemical processes in a box model demonstrates that both the time scale to reach aqueous phase equilibria and the choice of rate constants of irreversible reactions have a pronounced effect on the predicted atmospheric relevance of SOA formation from glyoxal. During day time, a photochemical (most likely radical-initiated) process is the major SOA formation pathway forming ∼5 μg m−3 SOA over 12 h (assuming a constant glyoxal mixing ratio of 300 ppt). During night time, reactions of nitrogen-containing compounds (ammonium, amines, amino acids) contribute most to the predicted SOA mass; however, the absolute predicted SOA masses are reduced by an order of magnitude as compared to day time production. The contribution of the ammonium reaction significantly increases in moderately acidic or neutral particles (5 < pH < 7). Glyoxal uptake into ammonium sulfate seed under dark conditions can be represented with a single reaction parameter keffupt that does not depend on aerosol loading or water content, which indicates a possibly catalytic role of aerosol water in SOA formation. However, the reversible nature of uptake under dark conditions is not captured by keffupt, and can be parameterized by an effective Henry's law constant including an equilibrium constant Kolig = 1000 (in ammonium sulfate solution). Such reversible glyoxal oligomerization contributes <1% to total predicted SOA masses at any time. Sensitivity tests reveal five parameters that strongly affect the predicted SOA mass from glyoxal: (1) time scales to reach equilibrium states (as opposed to assuming instantaneous equilibrium), (2) particle pH, (3) chemical composition of the bulk aerosol, (4) particle surface composition, and (5) particle liquid water content that is mostly determined by the amount and hygroscopicity of aerosol mass and to a lesser extent by the ambient relative humidity. Glyoxal serves as an example molecule, and the conclusions about SOA formation in aqueous particles can serve for comparative studies of other molecules that form SOA as the result of multiphase chemical processing in aerosol water. This SOA source is currently underrepresented in atmospheric models; if included it is likely to bring SOA predictions (mass and O/C ratio) into better agreement with field observations.

Progress-variable approach for large-eddy simulation of non-premixed turbulent combustion

A new approach to chemistry modelling for large-eddy simulation of turbulent reacting flows is developed. Instead of solving transport equations for all of the numerous species in a typical chemical mechanism and modelling the unclosed chemical source terms, the present study adopts an indirect mapping approach, whereby all of the detailed chemical processes are mapped to a reduced system of tracking scalars. Here, only two such scalars are considered: a mixture fraction variable, which tracks the mixing of fuel and oxidizer, and a progress variable, which tracks the global extent of reaction of the local mixture. The mapping functions, which describe all of the detailed chemical processes with respect to the tracking variables, are determined by solving quasi-steady diffusion-reaction equations with complex chemical kinetics and multicomponent mass diffusion. The performance of the new model is compared to fast-chemistry and steady-flamelet models for predicting velocity, species concentration, and temperature fields in a methane-fuelled coaxial jet combustor for which experimental data are available. The progress-variable approach is able to capture the unsteady, lifted flame dynamics observed in the experiment, and to obtain good agreement with the experimental data, while the fast-chemistry and steady-flamelet models both predict an attached flame.

Steady and transient flamelet modelling of turbulent non-premixed combustion

Laminar flamelet models have shown to be useful for the prediction of detailed chemical processes in turbulent combustion. Recently the existence of laminar flamelets in turbulent flow has been questioned. This is because there are several different ways to implement the flamelet concept and the implication of some modelling assumptions are not yet fully understood. Also, most existing flamelet models are based on a 1D and quasi-steady description of the local flame structure. These steady flamelet models are limited in predicting transient or unsteady effects as for example local extinction due to high strain rate with subsequent re-ignition of partially premixed fluid elements. Here the flamelet concept is discussed in view of these questions. It is shown that unsteady effects can be included by solving the transient Peters equations with an initial guess that represents a partially premixed state. Partially premixing is considered by a simple model that is based on a single reaction progress variable and that is similar to the IEM model. The use of the Peters equations is of advantage in comparison to the commonly used counterflow diffusion flame formulation. The results of the steady flamelet model applied to the ETH-Sandia hydrogen jet flame demonstrate that the scalar dissipation rate is superior to the strain rate. The steady and transient flamelet models are applied to the piloted Masri-Bilger Methane-air jet flame. The results show that the behaviour of this flame is only captured by the transient flamelet model and that, therefore, the flamelet concept can be extended if transient effects are considered.

The role of the geometry and dust environment in stationary models of translucent interstellar clouds

A full radiative transfer model is presented for the ultraviolet (UV) radiation impinging on an interstellar cloud of spherical or finite plane-parallel slab geometry containing gas and dust. The penetration of the UV photons is coupled to detailed chemical processes. Photodestruction rates of atomic and molecular species are calculated from the corresponding cross-sections. We show that CO line intensities are quite sensitive to geometrical effects and to the extinction curve in the far-UV.