Transition Regime Research Articles

Ultrafast Structural Dynamics of a Photoexcited Mn−Fe Charge‐Transfer Material in the Polaronic and Phase Transition Regimes

AbstractThe Rb0.94Mn0.94Co0.06[Fe(CN)6]0.98 material was designed to exhibit a 75 K wide thermal hysteresis around room temperature, associated with a bistability between the low temperature MnIIIFeII tetragonal phase and the high temperature MnIIFeIII cubic phase. We have recently shown that a single laser pulse can be used to drive, at room temperature, the photoinduced phase transition from the MnIIIFeII tetragonal phase to the MnIIFeIII cubic phase. By using a specific powder sample streaming technique, we could monitor by time‐resolved X‐ray diffraction how the ultrafast out‐of‐equilibrium dynamics, imposed by the structural relaxation around the local photoinduced charge transfer, changes with the fluence of the laser pulse. In this paper we present a detailed structural analysis of the crystalline reorganizations associated with the charge‐transfer in different regimes of photoexcitation. In the low excitation polaronic regime, the photoinduced charge transfer is local and accompanied by an anisotropic distortion of the Mn coordination sphere and of the tetragonal lattice. In the high excitation regime, a macroscopic and cooperative charge‐transfer phase transition occurs towards the cubic lattice, where the coordination around the Mn reaches octahedral symmetry.

Size-Resolved Shape Evolution in Inorganic Nanocrystals Captured via High-Throughput Deep Learning-Driven Statistical Characterization.

Precise size and shape control in nanocrystal synthesis is essential for utilizing nanocrystals in various industrial applications, such as catalysis, sensing, and energy conversion. However, traditional ensemble measurements often overlook the subtle size and shape distributions of individual nanocrystals, hindering the establishment of robust structure-property relationships. In this study, we uncover intricate shape evolutions and growth mechanisms in Co3O4 nanocrystal synthesis at a subnanometer scale, enabled by deep-learning-assisted statistical characterization. By first controlling synthetic parameters such as cobalt precursor concentration and water amount then using high resolution electron microscopy imaging to identify the geometric features of individual nanocrystals, this study provides insights into the interplay between synthesis conditions and the size-dependent shape evolution in colloidal nanocrystals. Utilizing population-wide imaging data encompassing over 441,067 nanocrystals, we analyze their characteristics and elucidate previously unobserved size-resolved shape evolution. This high-throughput statistical analysis is essential for representing the entire population accurately and enables the study of the size dependency of growth regimes in shaping nanocrystals. Our findings provide experimental quantification of the growth regime transition based on the size of the crystals, specifically (i) for faceting and (ii) from thermodynamic to kinetic, as evidenced by transitions from convex to concave polyhedral crystals. Additionally, we introduce the concept of an "onset radius," which describes the critical size thresholds at which these transitions occur. This discovery has implications beyond achieving nanocrystals with desired morphology; it enables finely tuned correlation between geometry and material properties, advancing the field of colloidal nanocrystal synthesis and its applications.

Experimental investigations of flow boiling heat transfer performance in finned micro-channels

The flow boiling heat transfer and pressure drop in rectangular-finned micro-channels (i.e. micro-channel with rectangular fins) were experimentally investigated at five different fin-heights. At three different mass fluxes and a range of heat fluxes, heat transfer coefficients and pressure drops in the micro-channels were measured. The flow patterns during the flow boiling process were visualized to estimate the flow regime transitions in the micro-channels. The experimental results showed that the micro-channel with 500 μm height fins (i.e. tip clearance = 100 μm) performs the highest heat transfer and shows the highest resistance to flow boiling instability among the studied cases. Compared to the smooth micro-channel, the averaged heat transfer coefficient in the micro-channel with 500 μm height fins has been improved by up to 133.9 % at the same wall temperature. Compared to the micro-channel with 600 μm height fins (i.e. tip clearance = 0 μm), the averaged heat transfer coefficient in the micro-channel with 500 μm height fins has been improved by up to 45.8 %. At the initial point of flow instability, the micro-channel with 500 μm height fins has the highest pressure drop fluctuation frequency, the highest surface wetting frequency, and the lowest pressure drop variation coefficient among the studied cases. However, the micro-channel with 600 μm height fins has the lowest pressure drop fluctuation frequency and highest pressure drop variation coefficient, hence experiencing the longest dry-out process among the studied cases.

The Transition from Aerosol- to Updraft-Limited Susceptibility Regime in Large-Eddy Simulations with Bulk Microphysics

Large-eddy simulation (LES) is often used as a benchmark simulation in climate science and is suggested as a fundamental tool to examine, e.g., marine cloud brightening. Therefore, it is necessary to critically evaluate if these high-resolution models can skillfully simulate expected physical phenomena. This study focuses on the first indirect aerosol effect in warm stratocumulus clouds. We investigate if an LES code with explicit aerosol-cloud interactions and a widely used two-moment bulk microphysical scheme can reproduce well-known cloud droplet number susceptibility regimes previously identified by observations and supported by detailed parcel model simulations—the updraft-limited regime (typically occurring at high aerosol number concentrations and low updraft speeds) and the aerosol-limited regime (typically occurring at low aerosol number concentrations and high updraft speeds). Our simulations show that the LES in its default configuration cannot reproduce the two regimes if the initial droplet radius of newly activated droplets (rid) is estimated by integrating the wet aerosol size distribution. The main reason is related to the relatively coarse (but commonly used) time step in the model (Δt ≈ 1s), which is too long to resolve relevant microphysical processes adequately at high aerosol concentrations. A regime transition does occur if the timestep is decreased to Δt ≈ 0.1s and if a renormalization procedure is applied, which limits the number of activated droplets so that the water mass of the newly activated droplets cannot exceed the available amount of supersaturated water vapor. Another way to obtain a regime transition is to increase rid to values >1 µm. However, a clear recommendation for the choice of rid cannot be made upon physical arguments. An alternative solution could be to introduce a sub-time-stepping or adaptive time-stepping algorithm to calculate droplet formation and growth, particularly for updraft-limited conditions. Our study highlights the importance of critically evaluating LES results to guarantee that relevant physical processes are properly represented.

Performance of Rough-Ribbed Low-Pressure Turbine Blades Under Varying Loading and Operating Conditions

Abstract In this research, we pursue the efficacy of riblets in reducing blade profile loss of Low-Pressure Turbine (LPT) blades under various design and off-design conditions. We adopt a strategy in which surface roughness is employed in the transitional regime to minimize the separation bubble-related losses and flush mounted riblets downstream to mitigate the boundary layer losses due to an increase in the turbulent wetted area. Several high-fidelity scale resolving simulations are carried out to test the efficacy of this ‘rough-ribbed’ LPT blade for loadings ranging from low-lift (LL), high-lift (HL) and ultra-high-lift (UHL) conditions. Furthermore, two exit Reynolds numbers - 83,000 and 166,000 pertaining to engine-relevant design and off-design conditions, respectively, are considered. The streamwise evolution of skin friction coefficient, boundary layer integral parameters, instantaneous flow features and second-order statistics are analyzed to determine the design and off-design performance of riblets. The results demonstrate the efficacy of scallop-shaped riblets in reducing the profile loss and the blockage due to boundary layers with loading and Reynolds number. This resulted in a net skin-friction reduction of 3.4% for LL and 8% for UHL loading at cruise and a corresponding reduction in the trailing edge momentum thickness from 10% to 15%. Furthermore, the study highlights the necessity to optimize the riblet ramp to achieve skin friction reduction under off-design conditions.

Lattice Boltzmann Model for Rarefied Gaseous Mixture Flows in Three-Dimensional Porous Media Including Knudsen Diffusion

Numerical modeling of gas flows in rarefied regimes is crucial in understanding fluid behavior in microscale applications. Rarefied regimes are characterized by a decrease in molecular collisions, and they lead to unusual phenomena such as gas phase separation, which is not acknowledged in hydrodynamic equations. In this work, numerical investigation of miscible gaseous mixtures in the rarefied regime is performed using a modified lattice Boltzmann model. Slip boundary conditions are adapted to arbitrary geometries. A ray-tracing algorithm-based wall function is implemented to model the non-equilibrium effects in the transition flow regime. The molecular free flow defined by the Knudsen diffusion coefficient is integrated through an effective and asymmetrical binary diffusion coefficient. The numerical model is validated with mass flow measurements through microchannels of different cross-section shapes from the near-continuum to the transition regimes, and gas phase separation is studied within a staggered arrangement of spheres. The influence of porosity and mixture composition on the gas separation effect are analyzed. Numerical results highlight the increase in the degree of gas phase separation with the rarefaction rate and the molecular mass ratio. The various simulations also indicate that geometrical features in porous media have a greater impact on gaseous mixtures’ effective permeability at highly rarefied regimes. Finally, a permeability enhancement factor based on the lightest species of the gaseous mixture is derived.

Influence of the gas phase on a large-scale bubble column fluid dynamics: Gas holdup, flow regime transitions, and bubble size distributions

We present the results of an experimental study of a large-diameter bubble column operating in batch mode with air or CO2 as the gas phase and water as the liquid phase. The bubble column has an inner diameter of 0.24 m and is 5.3 m in height. The superficial gas velocity varied between 0.0037 m/s and 0.0233 m/s. The experimental investigation consists of gas holdup measurements and image analysis. Flow regime transitions were detected by gas holdup measurements, and image analysis was used to investigate the bubble size distributions. The results suggest that the homogeneous flow regime is stabilized when CO2 is used as the gas phase. The gas holdup is higher for the CO2-water system at fixed superficial gas velocity, especially in the heterogeneous flow regime. In the homogeneous flow regime, the mean bubble diameter and the fraction of large bubbles are lower for the CO2-water system, explaining the higher gas holdup.

Experimental analysis of eigenmode frequency of an hydrofoil on trailing edge hydroelastic coupling generating tonal noise

The majority of anthropogenic noise in the oceans is due to commercial, military or tourist motorized navigation. Lifting surfaces such as hydrofoils, propeller blades, rudders and stabilizers can be prone to flow induceed vibrations and, under certain conditions, to produce tonal noise. Reducing vibration and noise generated by this type of structures, is therefore an important issue to limit noise pollution and to improve the acoustic discretion of ships. This study, carried out at the French Naval Academy Research Institute, highlights the hydroelastic coupling mechanism responsible of trailing edge vibrations and of lifting surface tonal noise evolving in laminar to turbulent transition regime. This experimental research was carried out in a hydrodynamic tunnel with a NACA0015 hydrofoil, for angles of incidence ranging from 0° to 12° and Reynolds numbers varying from 200,000 to 1,200,000. Laser vibrometry was used to specify the vibrational behavior of two identical hydrofoil made of aluminum and brass in order to estimate the influence of material and eigenfrequency on tonal noise generation. The results show that, contrary to the same mechanism in air, tonal noise of hydrofoil was strongly dependent of the natural frequency of the profile.

Magneto-Stokes flow in a shallow free-surface annulus

In this study, we analyse ‘magneto-Stokes’ flow, a fundamental magnetohydrodynamic (MHD) flow that shares the cylindrical-annular geometry of the Taylor–Couette cell but uses applied electromagnetic forces to circulate a free-surface layer of electrolyte at low Reynolds numbers. The first complete, analytical solution for time-dependent magneto-Stokes flow is presented and validated with coupled laboratory and numerical experiments. Three regimes are distinguished (shallow-layer, transitional and deep-layer flow regimes), and their influence on the efficiency of microscale mixing is clarified. The solution in the shallow-layer limit belongs to a newly identified class of MHD potential flows, and thus induces mixing without the aid of axial vorticity. We show that these shallow-layer magneto-Stokes flows can still augment mixing in distinct Taylor dispersion and advection-dominated mixing regimes. The existence of enhanced mixing across all three distinguished flow regimes is predicted by asymptotic scaling laws and supported by three-dimensional numerical simulations. Mixing enhancement is initiated with the least electromagnetic forcing in channels with order-unity depth-to-gap-width ratios. If the strength of the electromagnetic forcing is not a constraint, then shallow-layer flows can still yield the shortest mixing times in the advection-dominated limit. Our robust description of momentum evolution and mixing of passive tracers makes the annular magneto-Stokes system fit for use as an MHD reference flow.

Collision frequencies across collision regimes in two-component systems

Agglomeration is the most important growth process in particle systems but modelling efforts typically assume mono-disperse primary particle distributions for the closure of the collision frequency that determines the growth rates. Real systems such as sooting flames, however, involve poly-disperse primary particle distributions. Also, systems with multiple components, where primary particles are of distinct but different sizes, cannot be treated as mono-disperse. Here, we introduce bi-disperse primary particle distributions and use Langevin dynamics simulations to develop closures for the collision kernels that are applicable over a wide range of agglomerate characteristics. The simulations cover fractal dimensions from 1.4 to 2.2, primary particle diameters from 5 nm to 50 nm, primary particle size ratios from 2 to 10 and agglomerates of up to a size of 200 primary particles with varying particle compositions. The Langevin dynamics simulations cover all collision regimes from ballistic to diffusive and allow to deduce expressions for the respective collision diameters, the hydrodynamic radii and the projected area as functions of particle characteristics. It is shown that existing expressions for the transition regime that were developed for the modelling of the collision kernel of spherical particles continue to hold for collision kernels of agglomerates in two-component systems under the condition that the collision diameters and drag coefficients are modelled accurately. An example ‘a posteriori’ simulation for a particle size ratio of 6 uses the population balance equation and demonstrates that bi-variate kernels are needed for the accurate prediction of the growth rates. Errors in predicted number density at the end of the simulations are less than 12 % while mono-variate kernels developed for mono-disperse primary particle systems overpredict the growth rate by 46%.

Bubble generation in a 3-D flow-focusing microchannel: From squeezing to jetting

Fine bubbles have been applied in many fields; however, the controllable preparation of monodispersed fine bubbles remains a challenge. In this study, a three-dimensional (3-D) flow-focusing microfluidic device was designed. Compared with bubble formation in one-dimensional (1-D) and two-dimensional (2-D) flow-focusing, the 3-D flow-focusing microchannel owns the features of making bubbles with much smaller sizes and wider operation range. Four different gas–liquid flow regimes (squeezing, squeezing-dripping transition, dripping, and jetting) and three different gas–liquid dispersion states (mono-dispersion, bi-dispersion, and poly-dispersion) were observed in the 3-D flow-focusing microchannels. Importantly, the bi-dispersion in the squeezing-dripping transition regime experimentally verified that the squeezing and viscous shearing mechanisms separately control bubble formation in the transition region based on the time sequence. The intrinsic reason for poly-dispersion in the squeezing-dripping transition and jetting regimes was revealed. Finally, a mathematical model is proposed to predict the bubble size in the flow-focusing microchannels.

Immiscible non-Newtonian displacement flows in stationary and axially rotating pipes

We examine immiscible displacement flows in stationary and rotating pipes, at a fixed inclination angle in a density-unstable configuration, using a viscoplastic fluid to displace a less viscous Newtonian fluid. We employ non-intrusive experimental methods, such as camera imaging, planar laser-induced fluorescence (PLIF), and ultrasound Doppler velocimetry (UDV). We analyze the impact of key dimensionless numbers, including the imposed Reynolds numbers (Re, Re*), rotational Reynolds number (Rer), capillary number (Ca), and viscosity ratio (M), on flow patterns, regime classifications, regime transition boundaries, interfacial instabilities, and displacement efficiency. Our experiments demonstrate distinct immiscible displacement flow patterns in stationary and rotating pipes. In stationary pipes, heavier fluids slump underneath lighter ones, resulting in lift-head and wavy interface stratified flows, driven by gravity. Decreasing M slows the interface evolution and reduces its front velocity, while increasing Re* shortens the thin layer of the interface tail. In rotating pipes, the interplay between viscous, rotational, and capillary forces generates swirling slug flows with stable, elongated, and chaotic sub-regimes. Progressively, decreasing M leads to swirling dispersed droplet flow, swirling fragmented flow, and, eventually, swirling bulk flow. The interface dynamics, such as wave formations and velocity profiles, is influenced by rotational forces and inertial effects, with Fourier analysis showing the dependence of the interfacial front velocity's dominant frequency on Re and Rer. Finally, UDV measurements reveal the existence/absence of countercurrent flows in stationary/rotating pipes, while PLIF results provide further insight into droplet formation and concentration field behavior at the pipe center plane.

Experimental study on grid-patterned low thermal conductivity coatings for fast chill-down of flat plate in liquid nitrogen pool

This study investigates a fast chill-down technology utilizing grid-patterned low-thermal-conductivity (low-k) coatings. Experiments were conducted on the transient pool boiling of liquid nitrogen on stainless steel plates under various conditions: bare, partially coated, and fully coated surfaces with coating thicknesses ranging from 50 to 200 μm. Fully coated surfaces exhibited a 52–58 % shorter chill-down time owing to the rapid transition from film to transition boiling. However, these surfaces reduced thermal performance in the nucleate boiling regime owing to the added thermal resistance of the low-k coating. To address this, a partially coated surface with a grid-patterned design was proposed and examined for various pattern sizes of d = 2.5, 5, and 10 mm. The rapid regime transition was most effectively facilitated when the pattern size was comparable to the wavelength determined by the Taylor instability. In the nucleate boiling regime, thermal performance improved as the pattern size approached the capillary length. Consequently, the chill-down time for the partially coated surface with d = 2.5 mm was reduced by 79 %, owing to both the rapid regime transition and improved performance throughout the entire process. Finally, the grid-patterned coatings showed a twofold increase in coating effectiveness (∼4.8) compared to the fully coated surfaces (∼2.4). This study establishes a new design guideline for low-k coatings, optimizing cooling performance across all boiling regimes.

Partition function zeros of zeta-urns

We discuss the distribution of partition function zeros for the grand-canonical ensemble of the zeta-urn model, where tuning a single parameter can give a first or any higher order condensation transition. We compute the locus of zeros for finite-size systems and test scaling relations describing the accumulation of zeros near the critical point against theoretical predictions for both the first and higher order transition regimes.

Aerodynamic breakup of emulsion droplets in airflow

AbstractAerodynamic breakup refers to the process where large droplets are fragmented into small droplets by the aerodynamic force in airflow, which plays a vital role in fluid atomization and spray applications. Previous research has primarily concentrated on the aerodynamic breakup of single‐component droplets, but investigations into the breakup of emulsion droplets are limited. This study experimentally investigated the aerodynamic breakup of water‐in‐oil emulsions in airflow, utilizing high‐speed photography to observe the breakup process and digital in‐line holography to measure fragment sizes. Comparative analyses between emulsion droplets and single‐component droplets are conducted to examine the breakup morphology, breakup regime, deformation characteristics, and fragment size distributions. The emulsion droplets exhibit higher apparent viscosity and shorter stretching lengths of the bag film and peripheral rim due to the presence of a dispersed phase. The breakup regime transitions of emulsions are modeled by integrating the viscosity model of emulsions and the transition model of the pure fluid. The fragment sizes of emulsion droplets are larger due to the shorter lengths of the bag film and peripheral rim.

Numerical appraisal of the role of heat transfer regimes on transient response of carbon dioxide based supercritical natural circulation loop during power upsurge

Appearance of steep property gradients with change in temperature is a fascinating feature of any supercritical fluid, which can instigate intricate dynamics in supercritical natural circulation loops by modulating the effective forces. While most of the relevant literature focuses on stability evaluation, anticipation regarding the transient response of the system during power transition is of utmost significance, especially in high-power applications. Present study aims at furnishing insight on the same by developing a one-dimensional numerical model of a rectangular loop with supercritical CO2 as the working medium, and characterizing the temporal trends over a wide range of heating power. Two different profiles of power upsurge have been tested for different regimes of heat transfer, unearthing intriguing characteristics. The combination of initial and final regimes during any transformation is found to be the most crucial factor. Single-step rise in power, in general, is the most vulnerable one, specifically during large-scale change of the order of 1000 W, and better be employed only at low-power regime. Even single-step change ∼ 25 W can inflict instability and flow transition within the transition regime. Power transformation following linear ramp profile with transition periods of 5, 10 and 20 s is identified to be the most suitable one across all the regimes. It can successfully mitigate instability even in the later parts of the transition regime, albeit at the expense of greater time requirement to attain the final stable state and possibly a greater period of transformation. Change through multiple small steps (about 250 W for large change in low power regime and 15 W within transition regime) can also be a feasible option for avoiding the growth of unstable oscillations at higher powers.

Coupled Wall Boiling and Population Balance Model for High Void Fraction Flows Under Sub-Cooled Nucleate Boiling Regime: Model Development and Validation

Abstract The subcooled flow boiling of water in a vertical annulus channel is studied numerically at low-pressure conditions. The two-fluid model is developed with flow-regime dependent interfacial transfers for mass, momentum, and energy using the algebraic interfacial area density (AIAD) framework. A discrete population balance model is used to mechanistically determine the vapor bubble diameter in the flow channel by considering the bubble aggregation and breakup effects. Energy balance at the heated wall for the subcooled nucleate boiling is handled using a suitable wall boiling model. A coupling is achieved between the discrete population balance and the wall boiling model for the nucleation and the growth rate of the vapor bubbles along the heated wall. The developed model simulates the reference experimental cases of flow boiling in a vertical channel for various flow and thermal conditions. At low wall heat flux, the wall boiling generates vapor bubbles near the heated wall and within the bubbly flow regime. With an increase in the wall heat flux, the aggregation and evaporation cause the formation of larger bubbles, which progress toward the flow channel core region, a phase that is representative of the transitional flow regime. The model's capability to predict such flow regime transition is validated with the experimental results. The bubble aggregation is found to be dominant compared to the breakup, and thus, proper choice of the aggregation factor is important for the accurate prediction of vapor parameters for the subcooled flow boiling at low-pressure conditions.

Disentangling the effects of meteorology and emissions from anthropogenic and biogenic sources on the increased surface ozone in Eastern China

China's Clean Air Actions have substantially improved ambient PM2.5 air quality since 2013. However, ozone (O3) pollution in urban areas has worsened in recent years, particularly in the eastern region. The formation of O3 in these high emission areas is highly complex and regulated by numerous factors. Utilizing the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), this study conducted a comprehensive analysis through nine sensitivity experiments for the years 2013 and 2017, aimed at disentangling the relative contributions of major factors (e.g., meteorological factors, anthropogenic emissions, biogenic emissions, and aerosol feedback mechanisms) to the observed O3 concentration trends in Jiangsu, a developed region of China with increasing trend of O3 level. Our findings indicate a pronounced increase in mean daily maximum 8-h average (MDA8) O3 levels in the economically vibrant southern Jiangsu, contrasted with a decrease in the less developed northern regions. The study identifies anthropogenic emissions change as the primary driver of O3 variations, with significant impacts also attributed to meteorological conditions and aerosol feedback effects, while biogenic emission shifts play a lesser role. In terms of meteorological factors, we discovered that the alteration in meteorological thermal factors (enhanced solar radiation and temperatures) in 2017, compared to 2013, exerted a more pronounced influence on the formation of O3 than the change in thermally driven dynamic factors (boundary layer height and wind speed). Moreover, the study observes a shift in O3 formation sensitivity from VOC-sensitive to transitional or NOX-sensitive regimes, signifying a notable transformation in the regional atmospheric chemistry conducive to O3 generation. Aerosol feedback effects, through complex pathways including photolysis rate alterations and modifications in the vertical O3 distribution, further compound the challenge of mitigating O3 levels. Our research underscores the necessity for adaptive, region-specific strategies to mitigate O3 pollution, providing potential insights for policymakers to formulate effective control measures.

The impact of turbulent transport efficiency on surface vertical heat fluxes in the Arctic stable boundary layer predicted from similarity theory and machine-learning

Abstract We analyzed 14 days of observations from sonic anemometry and high-resolution fiber optic distributed sensing collected in the stable polar boundary layer (SBL). The study sought to evaluate if and under which conditions the sensible heat flux is related to the temperature gradient. Machine learning methods were employed to identify drivers of and model heat fluxes. We found the recently proposed coupling metric Ω defined as the ratio of the buoyancy length scale and measurement height to delineate physically meaningful transport regimes. The regime transition marks the point where static stability in addition to the vertical turbulence strength control the heat transport, which is rather gradual than abrupt. The maximum downward heat flux is reached when one third of turbulent eddies exceed the opposing buoyancy forces in the SBL. We found evidence that even for large Ω a substantial fraction of the turbulent transport is non-equilibrium. The non-dimensional temperature gradient is better explained by variations in Ω than ζ = zL−1 from Monin-Obukhov Similarity theory. Its continuous organization with Ω across stabilities suggest that the vertical heat transport always remains coupled to the surface, but its efficiency and the resulting flux vary. 43% of the total enthalpy is exchanged during conditions of limited transport efficiency in the very SBL despite the small flux magnitude of ≤ 7 W m−2, which underlines the importance of quantifying the weak surface exchange for polar regions. When predicting sensible heat fluxes using mean quantities from weather stations, the net longwave radiative forcing and the horizontal wind speed are the most important predictors representing stratification and bulk shear.

Study on the characteristics of urban background ozone pollution based on long-term observations from mountain sites in China during 2015–2022

In recent years, Chinese cities have faced persistent O3 pollution challenges. O3 concentrations at urban mountain sites, serving as proxies for urban background O3 levels, can help address data gaps in O3 research in China. This study, for the first time, analyzed O3 data from ten urban mountain sites during 2015–2022 using statistical methods like the Mann-Kendall test, Theil-Sen slope estimation, and Pearson correlation analysis to reveal characteristics and trends in urban background O3 and proposed a metric for assessing mountain sites as urban background stations. Results indicate that O3 at mountain sites were generally higher than at plain sites, regardless of mean, 10th percentile, or 90th percentile the average maximum daily 8-h average (MDA8) O3. Although mountain sites exhibited similar annual and diurnal variations as plain sites, their amplitudes were significantly lower. Significant upward trends (P < 0.05) in MDA8 O3 were observed mainly at plain sites, whereas trends at mountain sites were insignificant (P > 0.05) during 2015–2022. The increase in urban ground-level O3 was largely due to changes in VOCs and NOx emission structures, which reduce nighttime titration effects while enhancing daytime O3 production. The insignificant trends observed at mountain sites suggest that these mountain sites may be in NOx-limited or transitional regimes rather than VOCs-limited regimes, which is also associated with their lower NOx concentrations compared to plain sites. These findings suggest that if China continues to prioritize the reduction of NOx emissions, urban ground-level O3 could further increase and approach the O3 at mountain sites. Finally, this study proposes that the correlation in nighttime O3 concentrations between mountain and plain stations can serve as a valuable indicator for evaluating the suitability of mountain stations as regional background stations in urban areas. It recommends mountain sites in cities such as Changji, Nanping for urban background O3 monitoring stations.