Variation Of Conductivity Research Articles

Copper Precursor-Driven Variations in Structural, Optical and Electrical Properties of SILAR-Deposited CTS Thin Films

Abstract This pioneering study elucidates, for the first time, the profound impact of copper precursors (copper acetate, copper sulfate, and copper chloride) on the structural, optical, and electrical properties of Copper Tin Sulfide (CTS) thin films synthesized via successive ionic layer adsorption and reaction (SILAR) deposition. Comprehensive characterization using X-ray diffraction (XRD), cross-sectional scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, and electrical and optical measurements revealed significant variations in crystallite size (59.3-76.4 nm), film thickness (1.235-4.75 µm), and conductance (3.7-6.4 × 10-11 S). Notably, copper chloride-derived films exhibited enhanced surface morphology, maximum bandgap energy (3.7 eV), and highest conductance, with displaying unique optical properties, including zero transmittance below 300 nm, low absorption above 300 nm, and a balanced absorption profile, making them ideal for photovoltaic cells, optical sensors, and UV-blocking applications. These results demonstrate the critical influence of copper precursors on CTS thin film properties, paving the way for tailored material design and enhanced device performance, with significant implications for the development of efficient and sustainable photovoltaic technologies.

BIOT NUMBER ERROR IN LOW TEMPERATURE INCONEL OVERALL EFFECTIVENESS EXPERIMENTS

Abstract To predict the performance of turbine materials at engine conditions, experiments are performed at lower temperature laboratory conditions. To ensure the results accurately predict the nondimensionalized surface temperature, the Biot number must be matched, requiring the ratio of the thermal conductivity of the material to that of air is matched. With alloys such as Inconel, it is sometimes assumed that the Biot number is matched since Inconel's thermal conductivity variation scales similarly to that of air. However, the thermal conductivity ratio does not scale perfectly sosome Biot number error exists, with the problem exacerbated at lower temperatures. To date, there has been no quantification of the error in the overall effectiveness, ϕ, that might be caused by this Biot number error. Ti-6Al-4V is predicted to allow for a better Biot number match, better simulating Inconel at engine conditions in a low temperature experiment. In this research, we utilized geometrically identical models constructed of Ti-6Al-4V and Inconel 718 to evaluate the error in ϕ that might occur through using an actual engine nickel alloy part at experimental conditions. While the Ti-6Al-4V model has nearly perfectly matched Biot number, the Inconel model's value was 73% higher. The theoretically more Biot number appropriate Ti-6Al-4V model produced area averaged ϕ values that differed by only 0.01 from its Inconel counterpart. These results suggest that typical nickel superalloy turbine components may be tested at low temperature without the use of a surrogate material.

Dynamic and thermodynamic behavior of supercooled Na–Ba metaphosphate liquids: Role of modifier cation ordering

AbstractThe Q‐speciation and the role of modifier dynamics on network relaxation in the supercooled mixed‐alkali–alkaline‐earth (MAAE) Na–Ba metaphosphate liquids are investigated using a combination of 31P nuclear magnetic resonance (NMR) spectroscopy, calorimetric, electrical conductivity, and rheological measurements. Progressive replacement of Na with Ba in these glasses is shown to result in an increasing disproportionation of Q2 species via the reaction: 2Q2 = Q1 + Q3. Unlike mixed‐alkali liquids, the Na–Ba metaphosphate liquids display a monotonic variation in isothermal electrical conductivity, glass transition temperature, calorimetric and kinetic fragility, and isothermal viscosity. It is hypothesized that this monotonic variation arises from the lack of elastic facilitation of network relaxation via coupled hopping of Na–Ba pairs as these modifier cations are prohibited from mixing randomly due to the differences between their size, mass, charge, and mobility. Isobaric heat capacity measurements provide supporting evidence in favor of a such a nonrandom mixing between the modifier cations in Na–Ba metaphosphate glasses and liquids.

Behavior of PGS/apatite foam scaffolds during incubation in SBF, PBS, Ringer's solution, artificial saliva, and distilled water.

Poly(glycerol sebacate) is a polymeric material with potential biomedical application in the field of tissue engineering. In order to act as a biodegradable scaffold, its incubation study is vital to simulate its behavior. This study explores the degradation of porous poly(glycerol sebacate)/hydroxyapatite scaffolds subjected to incubation in various physiological solutions. The research involved monitoring pH and conductivity values over a 14-day period, as well as analyzing the swelling capacity and mass alterations of the scaffolds. In simulated body fluid (SBF) and phosphate-buffered saline (PBS), the pH levels remained relatively stable, whereas Ringer's solution caused a pH decrease. Conversely, artificial saliva demonstrated an increase in pH, and distilled water caused a slight decrease. The conductivity values remained stable in SBF and Ringer's solution, slightly decreased in PBS, increased in artificial saliva, and significantly increased in distilled water. The swelling capacity of the scaffolds varied depending on the solution used, with the lowest equilibrium swelling observed in SBF and PBS. The effect of the presence of ceramics on this parameter was also observed. The mass changes of the scaffolds indicated deposition of particles or salts from the incubation solutions, and subsequent rinsing in distilled water led to a decrease in mass. Scanning electron microscopy (SEM) imaging and elemental analysis confirmed the presence of crystallized salts on the scaffold surfaces after incubation in SBF. Surface roughness measurements revealed changes in roughness depending on the solution, with deposition of additional layers in SBF and degradation in artificial saliva. In summary, the scaffolds exhibited biodegradation in physiological solutions, with variations in pH, conductivity, swelling capacity, mass changes, and surface morphology depending on the specific solution and scaffold composition.

Sensitivity of M2tidal magnetic signals to seasonal and spatial variations of ocean electric conductivity

Electrical conductivity of the Earth’s oceans is an important oceanographic parameter related through its dependence on temperature and salinity to the state of the ocean. The tidally induced magnetic field then provides a directly and globally observable physical variable affected by the ocean conductivity spatial and temporal distribution. This contribution addresses two topics of the impact of the ocean conductivity variations on the principal lunar semi-diurnal magnetic signals. First, using high-resolution forward modelling, we investigate the sensitivity of the magnetic field to seasonal conductivity variations. Here, we find that the differences between magnetic signatures calculated for individual monthly conductivity climatologies are small, and localized to the marginal seas of the global ocean. Second, we formulate an inverse method to provide a constraint for the ocean conductivity in the upper 1000 m of the ocean, and test it using a synthetic dataset, demonstrating a proof-of-concept for such an approach.This article is part of the theme issue ‘Magnetometric remote sensing of Earth and planetary oceans’.

Study of Cooling Performance of Liquid-Cooled EV Battery Module According to the TIM Compression Ratio

AbstractThis study examines the coolant and heat flows in electric vehicle (EV) battery pack that employs a thermal interface material (TIM). The overall temperature distribution of the battery pack that consists of many battery modules is precomputed based on the cooling circuit design, and the battery module that is most strongly influenced by cooling circuit is selected. To investigate the detailed effects of the TIM’s performance, we measure its thermal conductivity based on its compression ratio and consider the detailed shape of the battery cell module for incorporating the TIM’s thermal conductivity in the battery assembly. The TIM, which functions as part of the cooling system within the confined space of the battery cell module, is included in the CFD analysis to assess the effect of thermal conductivity variation resulting from the compression ratio of the TIM on the cooling performance of the battery pack. Various cases of confined spaces in the battery module are compared to optimize the cooling efficiency of the EV battery pack.

Conductivity Estimation of Metallic Tubes Using Ferrite Core Horizontal Probe with Eddy Current Method

Conductivity evaluation is crucial for monitoring the condition of metallic tubes during inspections and for assessing their service life. Eddy current testing (ECT) is a precise, non-contact method used for this purpose. Industries such as aerospace, power generation, and petrochemicals rely on precise condition monitoring, which requires a deep understanding of the relationship between conductivity variations and probe impedance of metallic tubes. However, experimental analysis of conductivity changes using coil impedance has limitations in terms of cost and complexity. Encircling coil methods provide overall information about the average conductivity of specimens but have limited ability to capture local variations such as hot spots. To address these issues, we propose using a finite element method (FEM) based approach to predict a ferrite core horizontal probe signal near non-ferromagnetic tubes. The probe data can be used to estimate conductivity through regression methods, and the impact of lift-off on ECT results is also investigated.

A comparative study of LiTa2PO8 ceramics prepared with different lithium sources

In advancing lithium-ion batteries, solid electrolytes like LiTa2PO8, a Li-ion conductor with high bulk conductivity (~10−3 S/cm), show promise for all-solid-state battery technology. Here, we investigate the impact of different Li sources and their excess on the structural, microstructural and electrical properties of LiTa2PO8 ceramics. Employing diverse characterization techniques, including XRD, MAS NMR, TMA, SEM/EDS, IS, DC potentiostatic polarization, and density measurements, we unveil significant impact of the source of lithium ions and its excess on electrical properties. The sample without excess LiNO3 source exhibited the highest σtot = 5.2×10−4 S/cm at 30°C. Notably, the grain conductivity remained constant at ca. 3 mS/cm, irrespective of the chosen lithium source and excess. The microstructural analysis uncovered correlations between variations in total ionic conductivity and changes in the nature and concentration of secondary phases. Additionally, it revealed the influence of the arrangement of grains affecting the grain boundaries and porosity regions.

Effect of temperature and frequency on the dielectric behaviour of cellulose nanocrystals

Nanocellulose is a promising material for advanced applications due to its renewable nature, non-toxicity, eco-friendliness, mechanical strength and other unique properties. This study investigates the dielectric properties of cellulose nanocrystals (CNCs) derived from wastepaper through acid hydrolysis, across a temperature range of 30°C to 100°C and a frequency range of 1 Hz to 10 MHz. Using theoretical models such as the modified Cole-Cole model, Jonscher's universal dielectric response (UDR) model and modified Kohlrausch-Williams-Watts (KWW) model, a detailed analysis is performed. The results indicate that the dielectric properties depend on temperature, with the frequency dispersion of the real (ε′) and imaginary parts (ε″) of the dielectric permittivity fitting the modified Cole-Cole equation. These values show an initial increase up to 70°C (ε′∼ 6000 and dielectric loss <1) followed by a decrease at higher temperatures, which is linked to the hydroxyl groups. A temperature dependence is also observed in space charge conductivity, free charge conductivity and relaxation time. Non-Debye type behaviour is attributed to asymmetrical features in the electric modulus spectra, as explained by the modified KWW model at various temperatures. The frequency variation of AC conductivity adheres to Jonscher’s power law and the temperature variation of the frequency exponent (0≤ s ≤1) indicates a correlated barrier hopping conduction mechanism for charge carriers. Overall, these findings highlight CNCs as excellent candidate for antistatic applications, demonstrating conductivity ranging from 10−5 to10−9 Scm−1. The results also suggest CNCs can be used in energy storage applications due to their outstanding dielectric properties and low dielectric loss (<1).

Multifrequency image reconstruction for electrical impedance tomography

Abstract Electrical Impedance Tomography (EIT) is a medical imaging technique that is primarily used to monitor the respiration of a patient. Because EIT is based on electrical measurements, it is a safe, non-invasive, and cost-effective imaging technique. However, the EIT image reconstruction is a severely ill-posed problem that gives low spatial resolution where only large variations in tissue conductivity can be visualized. Furthermore, widely used time difference EIT relies on a single frequency alternating current measurement which does not allow for discrimination of different tissues on their conductivity spectra. Here we show the application of a new EIT reconstruction algorithm which correlates measurements taken at different frequencies to include the spectral dependency of the tissue properties. The algorithm is tested on a simulated phantom using data for muscle and lung tissue from the literature. It shows that contrary to a standard EIT image reconstruction, the frequency dependence of the tissues is retained, which can be used to further improve distinguishability in EIT images.

Study on the change of the oil-paper insulation’s AC conductivity characteristics based on dielectric response in frequency domain

Oil-immersed power equipment (transformers and high voltage bushings) are key components of power systems. Oil-paper insulation has endured complex and harsh operating conditions when used as a primary insulation system, so it is essential to determine the insulation state of oil-paper insulation system to ensure a stable operation for the power grid. The oil-paper insulation’s conductivity behavior can effectively characterize the degree of insulation deterioration. However, the relationship between the alternating current (AC) and direct current (DC) conductivity behavior for oil-paper insulation systems is still unclear in current research, resulting in limitations in the traditional evaluation model’s application. Therefore, this paper uses oil-paper insulation materials under different aging to carry out relevant research about the changing features of the complex AC conductivity under different test temperature environments. The influence of oil-paper insulation aging, test temperature, and excitation frequency on the complex AC conductivity is verified. Considering the correlation between low-frequency AC conductivity and DC conductivity, the frequency range of AC conductivity is extended by means of frequency temperature and conductance double translation. The temperature dependence of the AC conductivity behavior is defined over a wide frequency test range. The relaxation polarization process dominated the AC conductivity variation law is obtained. The impact of experimental temperature and insulation aging on the cumulative characteristic parameters of AC ionic mobility is clarified. This study theoretically corrects the traditional calculation model for AC conductivity of oil-paper insulation. The bias in conductivity calculations caused by only considering a single relaxation polarization process is eliminated. The conductivity model that can characterize multiple relaxation polarization processes in the dielectric is established. This model provides theoretical support for the later assessment of oil-paper insulation state depending on conductance behavior.

Impact of crown architecture on light availability, gas exchange, flowering and fruiting in jamun (Syzygium cumini [L.] Skeels)

Experiments were conducted to assess the impact of crown architecture on light availability beneath the trees, flowering, fruiting, yield and quality of jamun (Syzygium cumini [L.] Skeels). Trees were maintained as control, palmette and open centre crown. Impact was evaluated for three consecutive years, i.e. 2017–2019. Diffuse light beneath the trees ranged from 69.7 ± 2.22 to 45.9 ± 1.45%, whereas direct light varied from 30.4 ± 0.97 to 54.1 ± 1.78%. At flowering and fruit development stage (June), photosynthesis rate (A) in control trees was 12.5 ± 0.43 μmol CO2/m2/s; however, at fruit maturity and dormancy (August), it was only 9.5 ± 0.35 μmol CO2/m2/s. Similarly, in palmette and open centre trees, photosynthesis rate at flowering and fruit development stage was 13.5 ± 0.46 and 15.7 ± 0.54 μmol CO2/m2/s, respectively; whereas at fruit maturity and dormancy, photosynthesis rate dropped to 10.5 ± 0.39 and 11.7 ± 0.43 μmol CO2/m2/s, respectively. Substantial variation in stomatal conductance (gs), vapour pressure deficit (VPD) and transpiration rate (E) was also found. Days to start flowering ranged from 92 ± 0.33 to 98 ± 0.33. Similarly, days to end flowering varied from 99 ± 0.07 to 107 ± 0.36, days to fruit set 132 ± 0.33 to 139 ± 0.33 and days to fruit maturity 176 ± 0.48 to 184 ± 0.63. Significant variation in fruit length, fruit width and fruit weight was also found. Total soluble solids in fruit pulp varied from 9.0 ± 0.15 to 12.2 ± 0.149°Brix and fruit yield 62.3 ± 1.5 to 86.7 ± 1.33 kg per tree. Noteworthy variation in fruit quality traits was also recorded. This study illustrates that crown architecture has considerable impact on gas exchange parameters, flowering, fruiting, yield and quality of jamun.

Scanning Microwave Impedance Microscopy for Characterization of Graphene Systems Encapsulated by Hexagonal Boron Nitride

Scanning microwave impedance microscopy (sMIM) is a near‐field technique that enables the characterization of conductive materials with a resolution down to 1 nm. In hexagonal boron nitride (hBN)‐encapsulated devices, microwaves emitted from the sMIM tip penetrate and reach with the underlying 2D materials, allowing for the mapping of local conductivity variations. Using twisted bilayer graphene, it is demonstrated that this technique can characterize moiré patterns through hBN flakes up to 2.5 nm thick. Additionally, it is showed that sMIM can distinguish between conductive and insulating materials in samples encapsulated by hBN layers exceeding 15 nm. A key finding is that signal decay due to encapsulation is intrinsically linked to the tip's condition. This work overcomes limitations in the application of twistronics, enabling the verification of the periodicity of moiré patterns in high‐quality encapsulated devices between electrical contacts.

Can storage-discharge characteristics of karst matrix system quantified through recession analysis be reliable?

Storage and subsequent release of water in matrix system is a key function of karst catchments that controlling baseflow variation. Hydrograph recession analysis is currently the economic way to assess storage-discharge characteristics broadly at the catchment scale. However, there is large uncertainty in the related quantification due to recession data extraction. This study, by combining recursive digital filters with different automatic extraction methods, slow flow recession segments were extracted for hydrograph recession analysis and the further dynamic matrix storage (DMS) and related recession time (RT) assessment. The diversity and consistence among DMS and RT derived from different recession extraction methods (REMs) were analyzed, using hydrometric data in 20 catchments in the Wujiang river Basin in southwest China. Results indicate that the estimates of DMS and RT remarkedly varied between different REMs, however the order in which they ranked was mostly consistent. Moreover, the relationships between the derived DMS and RT with catchment physical features that potentially control storage and release processes are mostly consistent. Larger DMS are strongly associated with catchments featured as lower soil saturated hydraulic conductivity and smoother hydrographs. Longer RT are mostly related with drier catchments characterized as less variation of elevation and lower soil saturated hydraulic conductivity. This study highlights not only the uncertainty in quantifying storage and accompanied release characteristics, but also the reliability of storage-discharge characteristics quantified through recession analysis in terms of catchment comparison. Considering the diversity among catchments, ensemble of multi-method estimates of DMS and RT can enhance our understanding of the storage-discharge processes in the karst matrix system.

TCAD Simulation of Resistive Switching Devices: Impact of ReRAM Configuration on Neuromorphic Computing.

This paper presents a method for modeling ReRAM in TCAD and validating its accuracy for neuromorphic systems. The data obtained from TCAD are used to analyze the accuracy of the neuromorphic system. The switching behaviors of ReRAM are implemented using the kinetic Monte Carlo (KMC) approach. Realistic ReRAM characteristics are obtained through the use of the trap-assisted tunneling (TAT) model and thermal equations. HfO2-Al2O3-based ReRAM offers improved switching behaviors compared to HfO2-based ReRAM. The variation in conductance depends on the structure of the ReRAM. The conductance extracted from TCAD is validated in the neuromorphic system using the MNIST (Modified National Institute of Standards and Technology) dataset.

Thermal Hydraulic Performance and Characteristics of a Microchannel Heat Exchanger: Experimental and Numerical Investigations

Abstract This paper presents extensive fluid flow and Heat Transfer studies conducted through an experimental setup followed by a detailed three-dimensional (3D) numerical analysis of the same setup using a commercial package for computational fluid dynamics (CFD), known as cfd-ace® for additive-manufactured counterflow AlSi10 Mg microchannel heat exchangers (MCHEs). A detailed 3D computational model of the experimentally tested MCHEs was built and analyzed using the commercial software cfd-ace® for the same experimentally tested operating conditions. The computational model results are in good agreement with experimental data of tested MCHE within +2% to +7% and ∼0% to −13.5% variation for cold and hot fluids for the entire set of design of experiments (DoEs). This percentage disagreement may be due to various factors, such as manufacturing deviation within tolerance, longitudinal conduction, variation in the thermal conductivity of the material after heat treatment, variation in environmental temperature, sensor deviation, and surface roughness of internal channels. Instead of Stainless steel (SST), AlSi10 Mg was used because of its lower manufacturing cost because AlSi10 Mg was lighter than SST, though its thermal conductivity is almost ∼8–10 times more than that of SST. A higher thermal conductivity is not good for MCHEs because it leads to higher longitudinal conduction, which eventually degrades the performance of MCHEs in terms of effectiveness. MCHE effectiveness is also reduced by ∼12% to 18% owing to longitudinal conduction from ideal effectiveness.

Numerical study of the synergetic current drive by lower hybrid waves and loop voltage

Abstract Thanks to the tail of fast electrons generated by the lower hybrid (LH) waves, the plasma conductivity changes significantly with respect to the ohmic heating status (Fisch 1985 Phys. Fluids 28 245). In this paper, the synergy effects of lower hybrid current drive (LHCD) in the presence of a DC electric field (namely, the loop voltage V loop ≠ 0) on the EAST tokamak are studied numerically using the ray tracing and Fokker–Planck tools of C3PO/LUKE. In addition to the normal case of a positive loop voltage with an asymmetric power spectrum (namely, co-current LHCD), the synergy effects of negative loop voltage with an asymmetric power spectrum (namely, counter LHCD), and positive/negative loop voltage with a symmetric power spectrum are also investigated, which are previously rarely reported. It is found that the total plasma conductivity can be increased by a factor of 1.68–2.18 (depending on the value of V loop) when a 1.1 MW LH power at 2.45 GHz with a symmetric power spectrum is injected. Furthermore, unlike the asymmetric power spectrum, the direction of the sum of the pure LH current (I lh0) and the synergetic current (I ad) resulting from the variation in plasma conductivity is always in line with the ohmic current (I oh) for a symmetric power spectrum, which is favorable for AC tokamak operation. These simulated results are helpful for LHCD applications, especially in current ramp-up and AC operation.

The Influence of Radiofrequency Heating on the Quality and Shelf of Life of Pepper

Pepper is one of the common vegetable food items that constitute most African dishes with beneficial health properties. This study investigates the influence of radiofrequency (RF) heating on the quality and shelf life of pepper. Utilising the measured dielectric properties of pepper, the research evaluates how microwave exposure at varying frequencies affects pepper's nutritional content and preservation duration. Fresh samples of pepper were subjected to radio waves ranging from 5 MHz to 25 MHz, and their dielectric properties, proximate composition, and shelf life were analysed. The dielectric properties were measured using a parallel plate capacitance method, revealing significant variations in conductivity, dielectric constant, and loss factor across different frequencies. Results demonstrated that RF treatment effectively extended the shelf life of pepper, with the highest frequency (25 MHz) achieving a shelf life of 36 days, compared to 20 days for the control. Moreover, the study observed minimal changes in the proximate composition of pepper, ensuring the retention of its nutritional quality. This research provides valuable insights into the use of RF heating as a viable technique for extending the shelf life of pepper while maintaining its nutritional integrity.

Dynamics and synchronization of the Morris-Lecar model with field coupling subject to electromagnetic excitation

In this study, we extend the Morris-Lecar (M-L) neuron model to design a neuronal network model with field effects. Using Lyapunov theory and the master stability function to evaluate the stability of synchronous manifolds. The Hamilton energy function of a single neuron is derived using Helmholtz's theorem to equivalently describe its internal field energy, and bioelectric activities are analyzed in conjunction with synchronization error functions. A comprehensive analysis was conducted using various numerical methods, including time series diagrams, bifurcation diagrams, two-parameter plane synchronization error diagrams, and similarity function diagrams. The observations indicate that variations in conductivity and equilibrium potential directly impact the firing patterns and synchronization states of neurons. In various coupling methods, an increase in coupling strength induces different degrees of oscillation within the coupled system. The effects of three coupling methods on the synchronization of the neuronal network are examined using spatiotemporal evolution diagrams and energy evolution diagrams. Global synchronization error and synchronization factors are introduced to quantify the level of synchrony. Numerical results indicate that an appropriate coupling strength and ionic conductance enhance network synchrony. Numerical results indicate that appropriate coupling strength promotes network synchrony. The insights gained from this study contribute to providing a more coherent framework for constructing neuronal network models under field-coupled conditions, thereby enhancing the understanding of fundamental principles in neuroscience and offering new perspectives for the treatment and rehabilitation of neurological disorders.

Spatiotemporal distribution in chemical composition of wet atmospheric deposition in Bandung Indonesia.

Bandung, Indonesia, represents the complex interactions between climate variability, basin topography, and deposition processes. This study conducted a long-term spatiotemporal analysis, including pH distribution and pollutant accumulation monitoring, to observe the chemical composition of wet deposition in Bandung as part of the Acid Deposition Monitoring Network in East Asia (EANET). The results revealed that and were the predominant ions, followed by , with their distribution varying across different sites due to local emissions and atmospheric processes. The interactions between these ions, particularly the formation of secondary inorganic aerosols such as ammonium sulfate and ammonium nitrate, were closely linked to the basin's localized sources and topographical features. Areas experiencing high traffic congestion were classified as acidic regions due to their low pH levels. In contrast, a rural site exhibited a basic pH due to the high concentration of ion . Variations in pH and conductivity, along with the impacts of climatic events such as El Niño and La Niña, emphasized the role of weather patterns in shaping wet deposition dynamics. Seasonal trends indicated elevated total ion concentrations during the dry season, driven by sea salt contributions, as supported by strong correlations between Na+ and and between Na+ and Mg2+. Additionally, geological materials and atmospheric reactions contributed to the strong correlations observed between soil-derived cations and acidic species. The increasing trend in and the contrasting decrease in concentrations in rural areas suggest evolving emission sources and environmental conditions.