High-resolution Experiments Research Articles

Mapping of Guest Localization in Mesoporous Silica Particles by Solid-State NMR and Ab Initio Modeling: New Insights into Benzoic Acid and p-Fluorobenzoic Acid Embedded in MCM-41 via Ball Milling

We present a novel strategy for mapping the localization of guest molecules (GMs) in mesoporous materials by combining mechanochemistry with solid-state nuclear magnetic resonance (ssNMR) spectroscopy. To this end, we consider model guest–host systems of benzoic acid (BA) and para-fluorobenzoic acid (4-FBA) embedded in mesoporous MCM-41 material and examine the recently proposed loading (MeLo) procedure for efficient encapsulation of molecular species. Application of high-resolution NMR experiments (1H and 19F NMR) has allowed detection of a multimodal distribution of the spectral signals ascribed to embedded GMs. This peculiarity reflects the presence of distinct molecular ensembles subjected to intrinsically different local environments and exhibiting different dynamical behavior. Furthermore, a considerable fraction of an amorphous phase has been found as a byproduct of the ball-milling. The stability of the phase mixture was further checked by subjecting the samples to chemical and physical stimuli, and a detailed interpretation of the NMR data was corroborated by theoretical calculations. To this end, we have undertaken a challenge to predict the NMR spectra of the GMs@MCM-41 using advanced ab initio molecular dynamics (AIMD) simulations, providing an accurate and exhaustive analysis of the NMR spectra. On the basis of the ab initio modeling validated against the experimental results, we find that the multimodal signal distribution reflects the level of the pore filling, and can be ascribed to the presence of both interface and fluid molecular species trapped inside the pores. This has been confirmed for both BA@MCM-41 and 4-FBA@MCM-41 systems. The presence of the third, amorphous fraction can be linked to interstitial space between randomly ordered crystallites, pointing at the importance of the external surface in further stabilization of encapsulated materials. Altogether, a consistent experimental and theoretical methodology has been presented, paving the way for a more accurate analysis of complex nanoconfined systems.

Evaluation of the Effects of Actuator Line Force Smearing on Wind Turbines Near-Wake Development

In the present study, the effects of the actuator line force smearing on the predicted near-wake development of a model wind turbine are evaluated. To exclude the uncertainties related to the blade loads computation, the loads of a corresponding blade-resolving simulation are imposed. Three force projection functions are applied, which all radially scale with the local chord length: an isotropic Gauss force distribution, an anisotropic Gauss distribution with distinct projection widths in the chord and thickness directions, and a Gauss-Gumbel projection function which creates an airfoil-like force distribution in the chord direction. Comparisons with corresponding blade-resolving simulations and experimental results show a good agreement of the tip vortex core size, position and circulation. Comparison of the mid-wake instantaneous streamwise velocity with blade-resolving simulations shows a thinner vortex sheet and stronger effects of the trailed vortices in the actuator line simulation. Although the bound vortex shape scales with the projection function, no significant difference was observed in the resulting tip vortices and near-wake flow field between the three projection functions used. It is attributed to the similar projection width in the tip vortex region for all cases considered. With the chosen grid refinement in the tip vortex area and a smearing width that scales with the chord length, tip vortices that compare with high-resolution experiments and high-fidelity blade-resolving simulation could be generated with the actuator line approach.

Summer afternoon precipitation associated with wind convergence near the Himalayan glacier fronts

Little is known about the effects of glacier-air interactions on the Himalayan glacier mass balance. Until this knowledge gap is filled, a reliable projection of the future changes in the Himalayan glaciers is hardly possible. Here, we describe the drying effect of the katabatic winds on the up-valley summer monsoon flows by creating favorable conditions for local convergence-induced precipitation to occur near the glacier fronts. We postulate that this retarding effect on the up-valley monsoon flows results in a negative feedback mechanism mediated by glacier-air interactions, in which glacial retreat pushes precipitation upwards as the down-valley katabatic winds weaken, resulting in greater local precipitation and enhanced snow accumulation across the upper parts of the Himalayan glaciers. Our analyses are based on the exclusive data recorded in the Khumbu valley and the Langtang valley in the Nepalese Himalayas. These data revealed higher afternoon precipitation in summer associated with surface wind convergence near the glacier fronts and a sharp decrease in the temperature lapse rate over the glacier surfaces. The principle of the observed phenomena was proven by our high-resolution modeling sensitive experiment, which involved two simulations, one with the present glaciers and the other without. This numerical experiment also supports the proposed negative feedback. Furthermore, we report a low deuterium excess near the glacier fronts, indicating below-cloud re-evaporation facilitated by the local convergence induced by the dry katabatic winds. Our study suggests that current models may overestimate the retreat of Himalayan glaciers because they have completely ignored the glacier-air interactions.

Double sample holder for efficient high-resolution studies of an insulator and a metal surface.

A double sample holder supporting both a metal sample and an insulator crystal for high-resolution scanning probe microscopy experiments is described. The metal sample serves as a substrate for tip preparation and tip functionalization to efficiently and reliably enable high-resolution studies of the adjacent insulator surface. Imaging of Ag(111)/mica, Au(111)/mica, CaF2(111), and calcite(104) surfaces is demonstrated at 5K, including images on calcite(104) produced with a CO terminated tip, which was prepared on the adjacent metal sample.

Magnetostructural coupling in ilmenite-type NiTiO3

We report the ground state magnetic structure and in-field magnetostrictive effects of $\mathrm{Ni}\mathrm{Ti}{\mathrm{O}}_{3}$ studied by means of zero-field and in-field single-crystal neutron diffraction, magnetization, and high-resolution dilatometry experiments. Zero-field neutron diffraction on $\mathrm{Ni}\mathrm{Ti}{\mathrm{O}}_{3}$ single crystals confirms an easy-plane antiferromagnet with a multidomain ground state. Upon application of external magnetic fields, neutron diffraction shows the evolution of domains with spins perpendicular to the applied field. The rotation of spins in the multidomain state exhibits pronounced lattice changes in the magnetostriction measurements. We see magnetization and magnetostriction measurements scale with each other in the multidomain state in accordance with phenomenological theories, revealing the strong coupling of spins to the lattice.

Cooperative Stabilization of Close-Contact Zones Leads to Sensitivity and Selectivity in T-Cell Recognition.

T cells are sensitive to 1 to 10 foreign-peptide-MHC complexes among a vast majority of self-peptide-MHC complexes, and discriminate selectively between peptide-MHC complexes that differ not much in their binding affinity to T-cell receptors (TCRs). Quantitative models that aim to explain this sensitivity and selectivity largely focus on single TCR/peptide-MHC complexes, but T cell adhesion involves a multitude of different complexes. In this article, we demonstrate in a three-dimensional computational model of T-cell adhesion that the cooperative stabilization of close-contact zones is sensitive to one to three foreign-peptide-MHC complexes and occurs at a rather sharp threshold affinity of these complexes, which implies selectivity. In these close-contact zones with lateral extensions of hundred to several hundred nanometers, few TCR/foreign-peptide-MHC complexes and many TCR/self-peptide-MHC complexes are segregated from LFA-1/ICAM-1 complexes that form at larger membrane separations. Previous high-resolution microscopy experiments indicate that the sensitivity and selectivity in the formation of closed-contact zones reported here are relevant for T-cell recognition, because the stabilization of close-contact zones by foreign, agonist peptide-MHC complexes precedes T-cell signaling and activation in the experiments.

Deciphering hierarchical organization of topologically associated domains through change-point testing

BackgroundThe nucleus of eukaryotic cells spatially packages chromosomes into a hierarchical and distinct segregation that plays critical roles in maintaining transcription regulation. High-throughput methods of chromosome conformation capture, such as Hi-C, have revealed topologically associating domains (TADs) that are defined by biased chromatin interactions within them.ResultsWe introduce a novel method, HiCKey, to decipher hierarchical TAD structures in Hi-C data and compare them across samples. We first derive a generalized likelihood-ratio (GLR) test for detecting change-points in an interaction matrix that follows a negative binomial distribution or general mixture distribution. We then employ several optimal search strategies to decipher hierarchical TADs with p values calculated by the GLR test. Large-scale validations of simulation data show that HiCKey has good precision in recalling known TADs and is robust against random collisions of chromatin interactions. By applying HiCKey to Hi-C data of seven human cell lines, we identified multiple layers of TAD organization among them, but the vast majority had no more than four layers. In particular, we found that TAD boundaries are significantly enriched in active chromosomal regions compared to repressed regions.ConclusionsHiCKey is optimized for processing large matrices constructed from high-resolution Hi-C experiments. The method and theoretical result of the GLR test provide a general framework for significance testing of similar experimental chromatin interaction data that may not fully follow negative binomial distributions but rather more general mixture distributions.

Interferometric optical fiber sensor for optoacoustic endomicroscopy

Optical fiber sensors can offer robust and miniaturized detection of wideband ultrasound, yielding high sensitivity and immunity to electromagnetic interference. However, the lack of cost-effective manufacturing methods prevents the disseminated use of these sensors in biomedical applications. In this study, we developed and optimized a simple method to create optical cavities with high-quality mirrors for acoustic sensing based on micro-manipulation of UV-curable optical adhesives and electroless chemical silver deposition. This approach enables the manufacturing of ultrasound sensors based on Fabry-Pérot interferometers on optical fiber tips with minimal production costs. Characterization and high-resolution optoacoustic imaging experiments show that the manufacturing process yielded a fiber sensor with a small NEP ( ) over a broad detection bandwidth (25 MHz), generally outperforming conventional piezoelectric based transducers. We discuss how the new manufacturing process leads to a high-performance acoustic detector that, due to low cost, can be used as a disposable sensor.

An early-morning gene network controlled by phytochromes and cryptochromes regulates photomorphogenesis pathways in Arabidopsis

Light perception at dawn plays a key role in coordinating multiple molecular processes and in entraining the plant circadian clock. The Arabidopsis mutant lacking the main photoreceptors, however, still shows clock entrainment, indicating that the integration of light into the morning transcriptome is not well understood. In this study, we performed a high-resolution RNA-sequencing time-series experiment, sampling every 2 min beginning at dawn. In parallel experiments, we perturbed temperature, the circadian clock, photoreceptor signaling, and chloroplast-derived light signaling. We used these data to infer a gene network that describes the gene expression dynamics after light stimulus in the morning, and then validated key edges. By sampling time points at high density, we are able to identify three light- and temperature-sensitive bursts of transcription factor activity, one of which lasts for only about 8 min. Phytochrome and cryptochrome mutants cause a delay in the transcriptional bursts at dawn, and completely remove a burst of expression in key photomorphogenesis genes (HY5 and BBX family). Our complete network is available online (http://www-users.york.ac.uk/∼de656/dawnBurst/dawnBurst.html). Taken together, our results show that phytochrome and cryptochrome signaling is required for fine-tuning the dawn transcriptional response to light, but separate pathways can robustly activate much of the program in their absence.

Oxidative and Proteolytic Inactivation of Alpha-1 Antitrypsin in Bronchopulmonary Dysplasia Pathogenesis: A Top-Down Proteomic Bronchoalveolar Lavage Fluid Analysis.

The study investigates the role of the oxidative and proteolytic inactivation of alpha-1 antitrypsin (AAT) in the pathogenesis of bronchopulmonary dysplasia (BPD) in premature infants. Bronchoalveolar lavage fluid (BALF) samples were collected on the 3rd day of life from mechanically ventilated neonates with gestational age ≤ 30 weeks and analyzed without previous treatment (top-down proteomics) by reverse-phase high-performance liquid chromatography-electrospray ionization mass spectrometry. AAT fragments were identified by high-resolution LTQ Orbitrap XL experiments and the relative abundances determined by considering the extracted ion current (XIC) peak area. Forty preterm neonates were studied: 20 (50%) did not develop BPD (no-BPD group), 17 (42.5%) developed mild or moderate new-BPD (mild + moderate BPD group), and 3 (7.5%) developed severe new-BPD (severe BPD group). Eighteen fragments of AAT and a fragment of AAT oxidized at a methionine residue were identified: significantly higher values of AAT fragments 25–57, 375–418, 397–418, 144–171, and 397–418 with oxidized methionine were found in the severe BPD group. The significantly higher levels of several AAT fragments and of the fragment 397–418, oxidized in BALF of preterm infants developing BPD, underlie the central role of an imbalance between proteases and protease inhibitors in exacerbating lung injury and inducing most severe forms of BPD. The study has some limitations, and between them, the small sample size implies the need for further confirmation by larger studies.

Signature of shape resonances on the differential cross sections of the S(1D)+H2 reaction.

Shape resonances appear when the system is trapped in an internuclear potential well after tunneling through a barrier. They manifest as peaks in the collision energy dependence of the cross section (excitation function), and in many cases, their presence can be observed experimentally. High-resolution crossed-beam experiments on the S(1D) + H2(j = 0) reaction in the 0.81-8.5 meV collision energy range reaction revealed non-monotonic behavior and the presence of oscillations in the reaction cross section as a function of the collision energy, as predicted by quantum mechanical (QM) calculations. In this work, we have analyzed the effect of shape resonances on the differential cross sections for this insertion reaction by performing additional QM calculations. We have found that, in some cases, the resonance gives rise to a large enhancement of extreme backward scattering for specific final states. Our results also show that, in order to yield a significant change in the state-resolved differential cross section, the resonance has to be associated with constructive interference between groups of partial waves, which requires not getting blurred by the participation of many product helicity states.

Uncertainty quantification for Multiphase-CFD simulations of bubbly flows: a machine learning-based Bayesian approach supported by high-resolution experiments

In this paper, we developed a machine learning-based Bayesian approach to inversely quantify and reduce the uncertainties of multiphase computational fluid dynamics (MCFD) simulations for bubbly flows. The proposed approach is supported by high-resolution two-phase flow measurements, including those by double-sensor conductivity probes, high-speed imaging, and particle image velocimetry. Local distributions of key physical quantities of interest (QoIs), including the void fraction and phasic velocities, are obtained to support the Bayesian inference. In the process, the epistemic uncertainties of the closure relations are inversely quantified while the aleatory uncertainties from stochastic fluctuations of the system are evaluated based on experimental uncertainty analysis. The combined uncertainties are then propagated through the MCFD solver to obtain uncertainties of the QoIs, based on which probability-boxes are constructed for validation. The proposed approach relies on three machine learning methods: feedforward neural networks and principal component analysis for surrogate modeling, and Gaussian processes for model form uncertainty modeling. The whole process is implemented within the framework of an open-source deep learning library PyTorch with graphics processing unit (GPU) acceleration, thus ensuring the efficiency of the computation. The results demonstrate that with the support of high-resolution data, the uncertainties of MCFD simulations can be significantly reduced. The proposed approach has the potential for other applications that involve numerical models with empirical parameters.

Turbulence properties in jets with fractal grid turbulence

Abstract

Evidence for fluvial and glacial activities within impact craters that excavated into a Noachian volcanic dome on Mars

Impact craters on Mars preserve diverse records of volcanic, fluvial, and glacial activities. Enigmatically, the preservation of these major activities or records altogether within impact craters is rare. We report one such new observation of impact craters that formed on a volcanic dome studied using data from the Mars Reconnaissance Orbiter's (MRO) Context Camera (CTX), High-Resolution Imaging Science Experiment (HiRISE), and Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), Mars Global Surveyor's Mars Orbiter Laser Altimeter (MOLA), and Mars Express' High-Resolution Stereo Camera (HRSC). A ~ 20 km diameter impact crater, informally named as Degana-A, is formed within the ~50 km diameter impact crater Degana. Mineralogical analysis reveals exposures of low-calcium pyroxene and olivine deposits, which occupy Degana-A's eastern walls, leading to the idea that pristine Noachian bedrocks might be exposed from beneath the volcanic dome. Degana-A floor is completely covered by alluvial fans from all sides with distributaries. Within Degana crater, multiple fans are observed along its eastern to southern side only. The most likely source of water was the accumulation of snow on Degana crater walls, which possibly melted as a result of the impact of Degana-A. We observed a ~ 1 km wide breach on the eastern wall of Degana-A and the estimated maximum flow velocity is ~2 m/s and a run-off ~2.25 mm/h. Over the south-facing walls, multiple moraine-like ridges superposed the fans, which suggests overprinting by glacial activities. The presence of fans and superposed moraine-like ridges located at the mid-latitudes (~23°S) implies atmosphere-derived snow/ice precipitation was possible. Chronologically, the dome is of Noachian age, whereas Degana crater formed in the Hesperian period and crater retention on the fans indicates late Hesperian to Amazonian ages. Overall, the preserved Noachian crustal material underneath a volcanic dome is rarely exposed in its pristine context, which offers a rare window into early igneous processes. This intriguing location also witnessed a climatic transition as implied by water/ice derived landforms formed by non-coeval events.

Mars3DNet: CNN-Based High-Resolution 3D Reconstruction of the Martian Surface from Single Images

Three-dimensional (3D) surface models, e.g., digital elevation models (DEMs), are important for planetary exploration missions and scientific research. Current DEMs of the Martian surface are mainly generated by laser altimetry or photogrammetry, which have respective limitations. Laser altimetry cannot produce high-resolution DEMs; photogrammetry requires stereo images, but high-resolution stereo images of Mars are rare. An alternative is the convolutional neural network (CNN) technique, which implicitly learns features by assigning corresponding inputs and outputs. In recent years, CNNs have exhibited promising performance in the 3D reconstruction of close-range scenes. In this paper, we present a CNN-based algorithm that is capable of generating DEMs from single images; the DEMs have the same resolutions as the input images. An existing low-resolution DEM is used to provide global information. Synthetic and real data, including context camera (CTX) images and DEMs from stereo High-Resolution Imaging Science Experiment (HiRISE) images, are used as training data. The performance of the proposed method is evaluated using single CTX images of representative landforms on Mars, and the generated DEMs are compared with those obtained from stereo HiRISE images. The experimental results show promising performance of the proposed method. The topographic details are well reconstructed, and the geometric accuracies achieve root-mean-square error (RMSE) values ranging from 2.1 m to 12.2 m (approximately 0.5 to 2 pixels in the image space). The experimental results show that the proposed CNN-based method has great potential for 3D surface reconstruction in planetary applications.

What It Takes to Compute Highly Accurate Rovibrational Line Lists for Use in Astrochemistry.

ConspectusWe review the Best Theory + Reliable High-Resolution Experiment (BTRHE) strategy for obtaining highly accurate molecular rovibrational line lists with InfraRed (IR) intensities. The need for highly accurate molecular rovibrational line lists is twofold: (a) assignment of the many rovibrational lines for common stable molecules especially those that exhibit a large amplitude motion, such as NH3, or have a high density of states such as SO2; and (b) characterization of the atmospheres of exoplanets, which will be one of the main areas of research in astronomy in the coming decades. The first motivation arises due to the need to eliminate lines due to common molecules in an astronomical observation in order to identify lines from new molecules, while the second motivation arises due to the need to obtain accurate molecular opacities in order to characterize the atmosphere of an exoplanet. The BTRHE strategy first consists of using high-quality ab initio quantum-chemical methods to obtain a global potential energy surface (PES) and dipole moment surface (DMS) that contains the proper physics. The global PES is then refined using a subset of the reliable high-resolution experimental data. The refined PES then gives energy-level predictions to an accuracy similar to the reproduction accuracy of the experimental data used in the refinement step in the interpolation region (i.e., within the range of the experimental data used in the refinement step). The accuracy of the energy levels will slowly degrade as they are extrapolated to spectral regions beyond the high-resolution experimental data used in the refinement step. However, because the degradation is slow, the predicted energy levels can be used to assign new high-resolution experiments, and the data from these can then be used in a subsequent refinement step. In this way, the global PES eventually can yield highly accurate energy levels for all desired spectral regions including to very high energies and high J values. We show that IR intensities computed with the BTRHE rovibrational wave functions and the DMS can be very accurate provided one has minimized the fitting error of the DMS and tested the completeness of the DMS. Some examples of our work on NH3, CO2, and SO2 are given to highlight the usefulness of the BTRHE strategy and to provide ideas on how to further improve its predictive power in the future. In particular, it is shown how successive refinement steps, once new high-resolution data are available, can lead to PESs that yield highly accurate transition energies to larger spectral regions. The importance of including nonadiabatic corrections to reduce the J-dependence of errors for H-containing molecules is shown with work on NH3. Another very important aspect of the BTRHE approach is the consistency across isotopologues, which allows for highly accurate line lists for any isotopologue once one is obtained for the main isotopologue (which has more high-resolution data available for refinement).

A Large Starphene Comprising Pentacene Branches.

Starphenes are attractive compounds due to their characteristic physicochemical properties that are inherited from acenes, making them interesting compounds for organic electronics and optics. However, the instability and low solubility of larger starphene homologs make their synthesis extremely challenging. Herein, we present a new strategy leading to pristine [16]starphene in preparative scale. Our approach is based on a synthesis of a carbonyl-protected starphene precursor that is thermally converted in a solid-state form to the neat [16]starphene, which is then characterised with a variety of analytical methods, such as 13 C CP-MAS NMR, TGA, MS MALDI, UV/Vis and FTIR spectroscopy. Furthermore, high-resolution STM experiments unambiguously confirm its expected structure and reveal a moderate electronic delocalisation between the pentacene arms. Nucleus-independent chemical shifts NICS(1) are also calculated to survey its aromatic character.

Localization in Flow of Non-Newtonian Fluids Through Disordered Porous Media

We combine results of high-resolution microfluidic experiments with extensive numerical simulations to show how the flow patterns inside a “swiss-cheese” type of pore geometry can be systematically controlled through the intrinsic rheological properties of the fluid. Precisely, our analysis reveals that the velocity field in the interstitial pore space tends to display enhanced channeling under certain flow conditions. This observed flow “localization”, quantified by the spatial distribution of kinetic energy, can then be explained in terms of the strong interplay between the disordered geometry of the pore space and the nonlinear rheology of the fluid. Our results disclose the possibility that the constitutive properties of the fluid can enhance the performance of chemical reactors and chromatographic devices through control of the channeling patterns inside disordered porous media.

Analysis of the Double-Bounce Interaction between a Random Volume and an Underlying Ground, Using a Controlled High-Resolution PolTomoSAR Experiment

The radar response of vegetated environments, and forested areas in particular, are usually modeled using a very simple structure made of a random volume, representing a cloud of vegetation particles, lying over a semi-infinite medium with a rough interface, associated with the underlying ground. This Random Volume over Ground model can efficiently handle double-bounce scattering mechanisms, or arbitrary volume reflectivity profiles. This paper proposes to analyze a specific component of the Random Volume over Ground simplified scattering model, which concerns the double-bounce interaction between the ground and the volume. This specific contribution is not considered by classical characterization techniques and is studied in this work using a controlled experiment involving a Synthetic Aperture Radar operated in a Polarimetric and Tomographic configuration in order to image in 3D a controlled miniaturized scene composed of volume lying over a ground. It is shown that ground/volume double-bounce scattering, which remains focused at the ground level even in 3D imaging mode, and has polarimetric patterns that differ largely from those usually expected from double-bounce reflections, with volume-like features, such as a strong cross-polarized reflectivity or decorrelation between co-polarized channels. Moreover, it is shown that the full rank polarimetric patterns of the ground-volume mechanism are tightly linked to the reflectivity of the volume and may mask the ground response. As a consequence, isolating the ground response using 3D imaging does not permit to avoid a generally very strong distortion of the soil response by the double-bounce reflection, and the estimation of different geophysical parameters of the ground, such as its humidity or roughness are significantly altered.

Assessment of the TROPOMI tropospheric NO<sub>2</sub> product based on airborne APEX observations

Abstract. Sentinel-5 Precursor (S-5P), launched in October 2017, carrying the TROPOspheric Monitoring Instrument (TROPOMI) nadir-viewing spectrometer, is the first mission of the Copernicus Programme dedicated to the monitoring of air quality, climate, and ozone. In the presented study, the TROPOMI tropospheric nitrogen dioxide (NO2) level-2 (L2) product (OFFL v1.03.01; 3.5 km × 7 km at nadir observations) has been validated over strongly polluted urban regions by comparison with coincident high-resolution Airborne Prism EXperiment (APEX) remote sensing observations (∼ 75 m × 120 m). Satellite products can be optimally assessed based on (APEX) airborne remote sensing observations, as a large amount of satellite pixels can be fully mapped at high accuracy and in a relatively short time interval, reducing the impact of spatiotemporal mismatches. In the framework of the S-5P validation campaign over Belgium (S5PVAL-BE), the APEX imaging spectrometer has been deployed during four mapping flights (26–29 June 2019) over the two largest urban regions in Belgium, i.e. Brussels and Antwerp, in order to map the horizontal distribution of tropospheric NO2. For each flight, 10 to 20 TROPOMI pixels were fully covered by approximately 2700 to 4000 APEX measurements within each TROPOMI pixel. The TROPOMI and APEX NO2 vertical column density (VCD) retrieval schemes are similar in concept. Overall, for the ensemble of the four flights, the standard TROPOMI NO2 VCD product is well correlated (R = 0.92) but biased negatively by −1.2 ± 1.2 × 1015 molec cm−2 or −14 ± 12 %, on average, with respect to coincident APEX NO2 retrievals. When replacing the coarse 1∘ × 1∘ the massively parallel (MP) version of the Tracer Model version 5 (TM5) a priori NO2 profiles by NO2 profile shapes from the Copernicus Atmospheric Monitoring Service (CAMS) regional chemistry transport model (CTM) ensemble at 0.1∘ × 0.1∘, R is 0.94 and the slope increases from 0.82 to 0.93. The bias is reduced to −0.1 ± 1.0 × 1015 molec cm−2 or −1.0 ± 12 %. The absolute difference is on average 1.3 × 1015 molec cm−2 (16 %) and 0.7 × 1015 molec cm−2 (9 %), when comparing APEX NO2 VCDs with TM5-MP-based and CAMS-based NO2 VCDs, respectively. Both sets of retrievals are well within the mission accuracy requirement of a maximum bias of 25 %–50 % for the TROPOMI tropospheric NO2 product for all individual compared pixels. Additionally, the APEX data set allows the study of TROPOMI subpixel variability and impact of signal smoothing due to its finite satellite pixel size, typically coarser than fine-scale gradients in the urban NO2 field. For a case study in the Antwerp region, the current TROPOMI data underestimate localized enhancements and overestimate background values by approximately 1–2 × 1015 molec cm−2 (10 %–20 %).