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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Self-Assembly and Critical Aggregation Concentration Measurements of ABA Triblock Copolymers with Varying B Block Types: Model Development, Prediction, and Validation

The dissipative particle dynamics (DPD) simulation technique is a coarse-grained (CG) molecular dynamics-based approach that can effectively capture the hydrodynamics of complex systems while retaining essential information about the structural properties of the molecular species. An advantageous feature of DPD is that it utilizes soft repulsive interactions between the beads, which are CG representation of groups of atoms or molecules. In this study, we used the DPD simulation technique to study the aggregation characteristics of ABA triblock copolymers in aqueous medium. Pluronic polymers (PEG-PPO-PEG) were modeled as two segments of hydrophilic beads and one segment of hydrophobic beads. Tyrosine-derived PEG 5K -b-oligo(desaminotyrosyl tyrosine octyl ester-suberate)-b-PEG 5K (PEG5K-oligo(DTO-SA)-PEG 5K ) block copolymers possess alternate rigid and flexible components along the hydrophobic oligo(DTO-SA) chain, and were modeled as two segments of hydrophilic beads and one segment of hydrophobic, alternate soft and hard beads. The formation, structure, and morphology of the initial aggregation of the polymer molecules in aqueous medium were investigated by following the aggregation dynamics. The dimensions of the aggregates predicted by the computational approach were in good agreement with corresponding results from experiments, for the Pluronic and PEG 5K -oligo(DTO-SA)-PEG 5K block copolymers. In addition, DPD simulations were utilized to determine the critical aggregation concentration (CAC), which was compared with corresponding results from an experimental approach. For Pluronic polymers F68, F88, F108, and F127, the computational results agreed well with experimental measurements of the CAC measurements. For PEG 5K -b-oligo(DTO-SA)-b-PEG 5K block polymers, the complexity in polymer structure made it difficult to directly determine their CAC values via the CG scheme. Therefore, we determined CAC values of a series of triblock copolymers with 3–8 DTO-SA units using DPD simulations, and used these results to predict the CAC values of triblock copolymers with higher molecular weights by extrapolation. In parallel, a PEG 5K -b-oligo(DTO-SA)-b-PEG 5K block copolymer was synthesized, and the CAC value was determined experimentally using the pyrene method. The experimental CAC value agreed well with the CAC value predicted by simulation. Furthermore, these results validate our CG models, and demonstrate an avenue to simulate and predict aggregation characteristics of ABA amphiphilic triblock copolymers with complex structures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Molecular Dynamics Study of the Bulk and Interface Properties of Frother and Oil with Saltwater and Air

For water treatment purposes, the separation processes involving surfactants and crude oil at seawater-air interfaces are of importance for chemical and energy industries. Little progress has been made in understanding the nanoscale phenomena of surfactants on oily saltwater-air interfaces. This work focuses on using molecular dynamics with a united-atom force field to simulate the interface of linear alkane oil, saltwater, and air with three surfactant frothers: methyl isobutyl carbinol (MIBC), terpineol, and ethyl glycol butyl ether (EGBE). For each frother, although the calculated diffusivities and viscosities are lower than the expected experimental values, our results showed that diffusivity trends between each frother agree with experiments but was not suitable for viscosity. Binary combinations of liquid (frother or saltwater)-air and liquid-liquid interfaces are equilibrated to study the density profiles and interfacial tensions. The calculated surface tensions of the frothers-air interfaces are like that of oil-air, but lower than that of saltwater-air. Only MIBC-air and terpineol-air interfaces agreed with our experimental measurements. For frother-saltwater interfaces, the calculated results showed that terpineol has interfacial tensions higher than those of the MIBC-saltwater. Here, the simulated results indicated that the frother-oil systems underwent mixing such that the density profiles depicted large interfacial thicknesses.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Structure, Polarization, and Sum Frequency Generation Spectrum of Interfacial Water on Anatase TiO 2

The photocatalytic activity of TiO 2 for water splitting has been known for decades, yet the adsorption structure and hydrogen bonding of water at the interface with TiO 2 have remained controversial. We investigate the prototypical aqueous interface with anatase TiO 2 (101) using ab initio molecular dynamics (AIMD) with the strongly constrained and appropriately normed (SCAN) density functional, recently shown to provide an excellent description of the properties of bulk liquid water. We find that water forms a stable bilayer of intact molecules with ice-like dynamics and enhanced dipole moment and polarizability on the anatase surface. Here, the orientational order and H-bond environment of interfacial water are reflected in the computed sum frequency generation (SFG) spectrum, which agrees well with recent measurements in the OH stretching frequency range (3000–3600 cm –1 ). Additional AIMD simulations for a model interface with 66% of dissociated water in the contact layer show that surface hydroxyls disrupt the order in the bilayer and lead to a much faster orientational dynamics of interfacial water. Nonetheless, the computed SFG spectrum for the hydroxylated surface also agrees with experiment, suggesting that SFG measurements in a wider frequency range would be necessary to unambiguously identify the character of interfacial water on anatase.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Direct Measurement of the Tunable Electronic Structure of Bilayer MoS 2 by Interlayer Twist

Using angle-resolved photoemission on micrometer-scale sample areas, we directly measure the interlayer twist angle-dependent electronic band structure of bilayer molybdenum-disulfide (MoS 2 ). Our measurements, performed on arbitrarily stacked bilayer MoS 2 flakes prepared by chemical vapor deposition, provide direct evidence for a downshift of the quasiparticle energy of the valence band at the Brillouin zone center ($\bar{Γ}$ point) with the interlayer twist angle, up to a maximum of 120 meV at a twist angle of ~40°. Our direct measurements of the valence band structure enable the extraction of the hole effective mass as a function of the interlayer twist angle. While our results at $\bar{Γ}$agree with recently published photoluminescence data, our measurements of the quasiparticle spectrum over the full 2D Brillouin zone reveal a richer and more complicated change in the electronic structure than previously theoretically predicted. The electronic structure measurements reported here, including the evolution of the effective mass with twist-angle, provide new insight into the physics of twisted transition-metal dichalcogenide bilayers and serve as a guide for the practical design of MoS 2 optoelectronic and spin-/valley-tronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Sol-gel Synthesis and Characterization of Gels with Compositions Relevant to Hydrated Glass Alteration Layers

During the processes associated with glass corrosion, porous alteration gel layers typically form upon exposure to aqueous conditions for extended time periods. The impacts of the alteration gel layer on glass durability have not been agreed upon in the glass science community. In particular, the formation mechanisms of the alteration gel layers are still largely unknown and require further investigation. However, alteration gel layers often require months to years to develop and are often too thin to adequately characterize. Meanwhile, silicate sol-gels are relatively easy to synthesize in bulk with custom compositions relevant to alteration gel layers. If glass alteration gel layers and synthetic sol-gel silicate gels demonstrate physical and chemical properties that are sufficiently similar, synthetic silicate gels could be used as analogues for glass alteration gel layers in future studies. In this study, sol-gels were made with custom compositions at various pH values to evaluate the effect of pH on gel structure and morphology. Several other variables were examined as well, such as composition, drying, and aging. Sol-gels were created by sequential additions of organometallic precursors in a single container. Synthesis procedural steps are provided in further detail within the text. Sol-gels were analyzed with several techniques including Small Angle X-ray Scattering (SAXS), gas adsorption, and He pynchometry to determine the effects of the variables on physical properties. Results show that sol-gels prepared at pH 3 consistently contained fewer primary particles with diameters larger than 72 Å and fewer pores with diameters larger than 300 Å compared to sol-gels synthesized at pH 7 and 9. Composition was shown to have no discernable effect on primary particle and pore sizes at any pH.

Reiser, Joelle TG↗
DOE OSTI2012

Effects of Measurement Materials and Oxygen Partial Pressure on the Viscosity of synthetic Eastern and Western United States Coal Slags

The viscosity of the molten ash (slag) resulting from the mineral constituents in carbon feedstock used in slagging gasifiers is critical for controlling the gasification process. The viscosity of two synthetic slags with compositions resembling the mineral impurities in average eastern and western coal feedstock was examined at temperatures from 1300–1500 °C using a rotating bob viscometer. A few combinations of atmospheres and experimental materials were investigated with respect to one another to determine slag viscosity. A CO/CO{sub 2} atmosphere (CO/CO{sub 2} = 1.8, corresponding to a P{sub O{sub 2}} = 10–8 atm) is required to sustain ferrous ions in FeO-containing slags, an environment that is oxidizing to most metals. Iron oxide in the slag prevents usage of Fe parts. In unpurified Ar, the Fe metal surface oxidizes. Using purified argon prevents iron measurement components from oxidation; however, the metallic surfaces act as nucleation sites for the reduction of the Fe oxide in the slag into metallic Fe. Dissolution of ceramic materials into the slag, including Al{sub 2}O{sub 3} and ZrO{sub 2}, occurs in both atmospheres. Therefore, evaluating slag properties in the laboratory is challenging. The measured viscosities of two synthetic slags in this study diverged depending upon material selection. This difference is likely attributable to container/spindle-slag interactions. Viscosity measurements of the eastern coal slag using all ceramic parts agreed best with FactSage prediction above 1350 °C, with an average activation energy of 271.2 kJ. For western coal slag, the dissolution of container/spindle materials was substantial during the measurement, with precipitation of crystalline phase noted. The experimental viscosity data of the western coal slag agreed best with Kalmanovitch prediction above 1350 °C. The activation energy changed dramatically for both data sets of western coal slag, likely indicating the Newtonian-to-non-Newtonian transition.

01 COAL, LIGNITE, AND PEAT↗
DOE OSTI2014

Magnetic Properties and Electronic Structures of Ar 3 U IV –L Complexes with Ar = C 5 (CH 3 ) 4 H – or C 5 H 5 – and L = CH 3 , NO, and Cl

Electronic structures and magnetic properties of the U 4+ complexes (C 5 Me 4 H) 3 UNO, (C 5 Me 4 H) 3 UCl, (C 5 H 5 ) 3 UCH 3 , and (C 5 H 5 ) 3 UCl are investigated by quantum chemical calculations. On the basis of wave function calculations including spin–orbit (SO) interactions, all complexes have nondegenerate nonmagnetic ground states. However, for L = CH 3 and Cl magnetic doublet excited states are very low in energy, rendering the magnetic susceptibility strongly temperature dependent above ca. 50–100 K. In contrast, (C 5 Me 4 H) 3 UNO exhibits temperature-independent paramagnetism even at room temperature. The calculated susceptibilities agree well with available experimental data. An analysis of the ground states and the magnetic behavior is performed using crystal-field (CF) models with parameters extracted from the ab initio calculations, and with the help of natural orbitals contributing to the electron density, generated from scalar relativistic and SO wave functions for the ground states and selected excited states. Electronic g-factors calculated from the CF models agree well with ab initio data. The U–NO bond order in (C 5 Me 4 H) 3 UNO decreases somewhat due to SO coupling, because U–NO bonding π orbitals with strong U 5f π character mix with nonbonding 5f δ orbitals under the SO interaction. This complex also exhibits pronounced multireference character. All complexes afford U-ligand 5f covalent character.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Rigorous Kinetic Modeling and Optimization Study of a Modified Claus Unit for an Integrated Gasification Combined Cycle (IGCC) Power Plant with CO{sub 2} Capture

The modified Claus process is one of the most common technologies for sulfur recovery from acid gas streams. Important design criteria for the Claus unit, when part of an Integrated Gasification Combined Cycle (IGCC) power plant, are the ability to destroy ammonia completely and the ability to recover sulfur thoroughly from a relatively low purity acid gas stream without sacrificing flame stability. Because of these criteria, modifications to the conventional process are often required, resulting in a modified Claus process. For the studies discussed here, these modifications include the use of a 95% pure oxygen stream as the oxidant, a split flow configuration, and the preheating of the feeds with the intermediate pressure steam generated in the waste heat boiler (WHB). In the future, for IGCC plants with CO{sub 2} capture, the Claus unit must satisfy emission standards without sacrificing the plant efficiency in the face of typical disturbances of an IGCC plant, such as rapid change in the feed flow rates due to load-following and wide changes in the feed composition because of changes in the coal feed to the gasifier. The Claus unit should be adequately designed and efficiently operated to satisfy these objectives. Even though the Claus process has been commercialized for decades, most papers concerned with the modeling of the Claus process treat the key reactions as equilibrium reactions. Such models are validated by manipulating the temperature approach to equilibrium for a set of steady-state operating data, but they are of limited use for dynamic studies. One of the objectives of this study is to develop a model that can be used for dynamic studies. In a Claus process, especially in the furnace and the WHB, many reactions may take place. In this work, a set of linearly independent reactions has been identified, and kinetic models of the furnace flame and anoxic zones, WHB, and catalytic reactors have been developed. To facilitate the modeling of the Claus furnace, a four-stage method was devised so as to determine which set of linearly independent reactions would best describe the product distributions from available plant data. Various approaches are taken to derive the kinetic rate expressions, which are either missing in the open literature or found to be inconsistent. A set of plant data is used for optimal estimation of the kinetic parameters. The final model agrees well with the published plant data. Using the developed kinetics models of the Claus reaction furnace, WHB, and catalytic stages, two optimization studies are carried out. The first study shows that there exists an optimal steam pressure generated in the WHB that balances hydrogen yield, oxygen demand, and power generation. In the second study, it is shown that an optimal H{sub 2}S/SO{sub 2} ratio exists that balances single-pass conversion, hydrogen yield, oxygen demand, and power generation. In addition, an operability study has been carried out to examine the operating envelope in which both the H{sub 2}S/SO{sub 2} ratio and the adiabatic flame temperature can be controlled in the face of disturbances typical for the operation of an IGCC power plant with CO{sub 2} capture. Impact of CO{sub 2} capture on the Claus process has also been discussed.

20 FOSSIL-FUELED POWER PLANTS↗
DOE OSTI2018

Four-Carbon Criegee Intermediate from Isoprene Ozonolysis: Methyl Vinyl Ketone Oxide Synthesis, Infrared Spectrum, and OH Production

The reaction of ozone with isoprene, one of the most abundant volatile organic compounds in the atmosphere, produces three distinct carbonyl oxide species (RR'COO) known as Criegee intermediates: formaldehyde oxide (CH 2 OO), methyl vinyl ketone oxide (MVK-OO), and methacrolein oxide (MACR-OO). The nature of the substituents (R,R' = H, CH 3 , CH＝CH 2 ) and conformations of the Criegee intermediates control their subsequent chemistry in the atmosphere. In particular, unimolecular decay of MVK-OO is predicted to be the major source of hydroxyl radicals (OH) in isoprene ozonolysis. Our study reports the initial laboratory synthesis and direct detection of MVK-OO through reaction of a photolytically generated, resonance-stabilized monoiodoalkene radical with O 2 . MVK-OO is characterized utilizing infrared (IR) action spectroscopy, in which IR activation of MVK-OO with two quanta of CH stretch at ca. 6000 cm –1 is coupled with ultraviolet detection of the resultant OH products. MVK-OO is identified by comparison of the experimentally observed IR spectral features with theoretically predicted IR absorption spectra. For syn -MVK-OO, the rate of appearance of OH products agrees with the unimolecular decay rate predicted using statistical theory with tunneling. This validates the hydrogen atom transfer mechanism and computed transition-state barrier (18.0 kcal mol –1 ) leading to OH products. Theoretical calculations display a further roaming pathway between the separating radical fragments, which results in other products. Master equation modeling yields a thermal unimolecular decay rate for syn -MVK-OO of 33 s –1 (298 K, 1 atm). For anti -MVK-OO, theoretical exploration of several unimolecular decay pathways predicts that isomerization to dioxole is the most likely initial step to products.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Liquid-Phase Effects on Adsorption Processes in Heterogeneous Catalysis

Aqueous solvation free energies of adsorption have recently been measured for phenol adsorption on Pt(111). Endergonic solvent effects of ~1 eV suggest solvents dramatically influence a metal catalyst’s activity with significant implications for the catalyst design. However, measurements are indirect and involve adsorption isotherm models, which potentially reduces the reliability of the extracted energy values. Computational, implicit solvation models predict exergonic solvation effects for phenol adsorption, failing to agree with measurements even qualitatively. In this study, an explicit, hybrid quantum mechanical/molecular mechanical approach for computing solvation free energies of adsorption is developed, solvation free energies of phenol adsorption are computed, and experimental data for solvation free energies of phenol adsorption are reanalyzed using multiple adsorption isotherm models. Explicit solvation calculations predict an endergonic solvation free energy for phenol adsorption that agrees well with measurements to within the experimental and force field uncertainties. Computed adsorption free energies of solvation of carbon monoxide, ethylene glycol, benzene, and phenol over the (111) facet of Pt and Cu suggest that liquid water destabilizes all adsorbed species, with the largest impact on the largest adsorbates.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Mesoscale Simulation of Proton Transport in Proton Exchange Membranes

Previous efforts to model proton transport through fuel cell membranes have largely focused on disparate length scales: molecular dynamics at the atomistic level and fuel cell stack engineering approaches at the macroscale. A new multiscale approach to bridge these extremes is proposed in this work which combines concepts from coarse-grained (CG) modeling with smoothed particle hydrodynamics (SPH) to capture the qualitative morphology and transport behavior of a proton exchange membrane at the length scale of tens of nanometers. This method allows for connection to atomistic simulations via the inclusion of transport coefficients from molecular dynamics and coarse-grained forces derived for the polymer backbone, side chain, proton, and water interactions. Information pertaining to macroscopic conductivity is obtained by volume averaging based on the flux and chemical potential fields within the membrane. Proton transport is effectively coarse-grained via introduction of a composition variable associated with each interacting site which carries the field information. By combining this technique with local electrostatics and coordinate dependent diffusion constants, the effects of double layer formation within the water pores and the influence of proximity to sulfonate groups on transport is recovered. The combined CG-SPH method is validated and subsequently applied to an equilibrated hydrated Nafion structure with a box length of 40 nm. The resulting conductivities calculated for the material agree very well with trends from experiment and provide insight into the complex interplay of morphology, proton distribution, and diffusion coefficients at a length scale that can be expanded beyond feasible atomistic molecular dynamics simulations to capture the effects of mesoscopic morphology on proton conduction.

Jorn, Ryan↗
DOE OSTI2013

Temperature Dependence of the Infrared Spectrum of Ammonia Borane: Librations, Rotations, and Molecular Vibrations

The absorbance of solid ammonia borane (AB) was measured in the energy range between 30 and 5000 cm(-1) and in the temperature range between 10 and 300 K. The intramolecular vibrations and their evolution through the structural phase transition around T-t approximate to 220 K fairly agree with previous measurements performed by means of Raman spectroscopy. In addition, we observed new vibrations centered in the far-infrared range, which can be tentatively ascribed to rotations and librations. The number of such modes does not agree with the calculations based on the group theory for both the tetragonal and the orthorhombic crystal structure of AB. We suggest that such a discrepancy is due to local reduction of the crystal symmetry compared to the one reported by X-ray diffraction and used to compute the number of IR active modes.

CRYSTAL↗
DOE OSTI2014

An Experimental and Computational Investigation into the Gas-Phase Acidities of Tyrosine and Phenylalanine: Three Structures for Deprotonated Tyrosine

Using mass spectrometry and correlated molecular orbital theory, three deprotonated structures were revealed for the amino acid tyrosine. The structures were distinguished experimentally by ion/molecule reactions involving proton transfer and trimethylsilyl azide. Gas-phase acidities from proton transfer reactions and from G3(MP2) calculations generally agree well. The lowest energy structure, which was only observed experimentally using electrospray ionization from aprotic solvents, is deprotonated at the carboxylic acid group and is predicted to be highly folded. A second unfolded carboxylate structure is several kcal/mol higher in energy and primarily forms from protic solvents. Protic solvents also yield a structure deprotonated at the phenolic side chain, which experiments find to be intermediate in energy to the two carboxylate forms. G3(MP2) calculations indicate that the three structures differ in energy by only 2.5 kcal/mol, yet they are readily distinguished experimentally. Structural abundance ratios are dependent upon experimental conditions, including the solvent and accumulation time of ions in a hexapole. Under some conditions, carboxylate ions may convert to phenolate ions. For phenylalanine, which lacks a phenolic group, only one deprotonated structure was observed experimentally when electrosprayed from protic solvent. This agrees with G3(MP2) calculations that find the folded and unfolded carboxylate forms to differ by 0.3 kcal/mol.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2012

Quantifying uncertainties in global and regional temperature change using an ensemble of observational estimates: The HadCRUT4 data set

Recent developments in observational near-surface air temperature and sea-surface temperature analyses are combined to produce HadCRUT4, a new data set of global and regional temperature evolution from 1850 to the present. This includes the addition of newly digitized measurement data, both over land and sea, new sea-surface temperature bias adjustments and a more comprehensive error model for describing uncertainties in sea-surface temperature measurements. An ensemble approach has been adopted to better describe complex temporal and spatial interdependencies of measurement and bias uncertainties and to allow these correlated uncertainties to be taken into account in studies that are based upon HadCRUT4. Climate diagnostics computed from the gridded data set broadly agree with those of other global near-surface temperature analyses. Fitted linear trends in temperature anomalies are approximately 0.07°C/decade from 1901 to 2010 and 0.17°C/decade from 1979 to 2010 globally. Northern/southern hemispheric trends are 0.08/0.07°C/decade over 1901 to 2010 and 0.24/0.10°C/decade over 1979 to 2010. Linear trends in other prominent near-surface temperature analyses agree well with the range of trends computed from the HadCRUT4 ensemble members.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2019

Reply to Comment by Robock et al. on “Climate Impact of a Regional Nuclear Weapon Exchange: An Improved Assessment Based on Detailed Source Calculations”

Robock et al. contend that the reasons for Reisner et al. simulations not producing enough black carbon (BC) in the upper atmosphere are as follows: (1) low fuel loads, (2) high surface winds, (3) omission of latent heat release by cloud formation, and (4) short simulation. We resolve (1) with additional simulations over a range of fuel loads, clarify (2) by noting that our surface winds of 2.4 m/s are below the firestorm threshold, agree that addressing (3) requires more research, and (4) short simulations are appropriate since this is when most of the BC emissions that can reach the upper atmosphere occur. While we agree with their assessment that a firestorm was not produced, we demonstrate here why a firestorm is unlikely in dense population centers found in India and Pakistan where concrete structures dominate. Our conversion of all fuel to BC is an upper bound and was the basis of the assertion on maximum global impacts. Furthermore, our study is the first to treat BC generation and lofting with physics–based models underscoring the need to initialize BC in global simulations properly. Reisner et al. and the responses identify critical gaps that must be filled to make any meaningful assessment of the probability of global nuclear winter.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2021

Revisiting AMOC Transport Estimates From Observations and Models

Abstract Reference level assumptions used to calculate the Atlantic meridional overturning circulation transports at the RAPID and Meridional Overturning Variability Experiment (MOVE) observing arrays are revisited in an eddying ocean model. Observational transport calculation methods are complemented by several alternative approaches. At RAPID, the model transports from the observational method and the model truth (based on the actual model velocities) agree well in their mean and variability. There are substantial differences among the transport estimates obtained with various methods at the MOVE site. These differences result from relatively large and time‐varying reference velocities at depth in the model, not supporting a level‐of‐no‐motion. The methods that account for these reference velocities properly at MOVE produce transports that are in good agreement with the model truth. In contrast with the observational estimates, the model transport trends at MOVE and RAPID largely agree with each other on pentadal to multi‐decadal time scales.

Danabasoglu, Gokhan↗
DOE OSTI2024

Improving BC Mixing State and CCN Activity Representation With Machine Learning in the Community Atmosphere Model Version 6 (CAM6)

Abstract Representing mixing state of black carbon (BC) is challenging for global climate models (GCMs). The Community Atmosphere Model version 6 (CAM6) with the four‐mode version of the Modal Aerosol Module (MAM4) represents aerosols as fully internal mixtures with uniform composition within each aerosol mode, resulting in high degree of internal mixing of BC with non‐BC species and large mass ratio of coating to BC ( R BC , the mass ratio of non‐BC species to BC in BC‐containing particles). To improve BC mixing state representation, we coupled a machine learning (ML) model of BC mixing state index trained on particle‐resolved simulations to the CAM6 with MAM4 (MAM4‐ML). In MAM4‐ML, we use R BC to partition accumulation mode particles into two new modes, BC‐free particles and BC‐containing particles. We adjust R BC to make the modeled BC mixing state index ( χ mode ) match the one predicted by the ML model ( χ ML ). On a global average, the mass fraction of BC‐containing particles in accumulation mode decreases from 100% (MAM4‐default) to 48% (MAM4‐ML). The globally averaged χ mode decreases from 78% (MAM4‐default) to 63% (MAM4‐ML, 19% reduction) and agrees well with χ ML (66%). The R BC decreases by 52% for accumulation mode and better agrees with observations. The hygroscopicity drops by 9% for BC‐containing particles in accumulation mode, leading to a 20% reduction in the BC activation fraction. The surface BC concentration increases most (6.9%) in the Arctic, and the BC burden increases by 4%, globally. Our study highlights the application of the ML model for improving key aerosol processes in GCMs.

Meteorology & Atmospheric Sciences↗