Moist Air Research Articles

Natural volatile organic compounds (NVOCs) and airborne microorganisms in different stands of urban forests

Natural volatile organic compounds (NVOCs) and airborne microorganisms are important elements in urban forest air that affect air quality and human health. In this study, the Zhuyu Bay urban forest in Yangzhou was selected as the research object, and gas chromatography–mass spectrometry (GC–MS) was used to detect the composition of NVOCs in different forest stands. Terpenes, heterocyclic compounds, and esters accounted for the highest proportions. We then explored the effects of NVOCs on the physiological health of each forest stand and used Kyoto Encyclopedia of Genes and Genome enrichment analysis to identify beneficial secondary metabolites. Among the identified compounds, alpha-phellandrene 1, azulene, and other terpenoids were found to possess antibacterial, anti-inflammatory, and antioxidant properties. Heterocyclic compounds, such as 4-Pyridinecarboxylic acid and visnagin, showed significant effects in the treatment of diseases. In addition, we collected and analyzed culturable airborne microorganisms in different forest stands and found that the bamboo forest had the lowest number of culturable airborne microorganisms. To further explore the influence of urban microclimates on air microorganisms and NVOCs, a partial least squares path modelling (PLS-PM) analysis was conducted. Air negative oxygen ion is an important factor affecting NVOCs, and Air moisture has a significant positive effect on bacteria proportion.

Multifunctional Janus nanofibrous membrane with unidirectional water transport and pH-responsive color-changing for wound dressing

Chronic wounds often produce a significant volume of exudate, posing a substantial obstacle to healing. Consequently, there is a pressing demand for a versatile dressing capable of effectively managing exudate in chronic wounds. In this context, a Janus smart dressing is proposed, featuring unidirectional water transport and a pH-responsive color-changing for exudate management and wound monitoring. The dressing’s mechanisms for unidirectional water transport and color changing are elucidated. The Janus dressing consists of a polyacrylonitrile (PAN)/sodium polyacrylate (SPA)/anthocyanin (An) hydrophilic layer with antioxidant and pH-sensitive functions and a poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) hydrophobic layer, enabling unidirectional exudate drainage without reverse osmosis. Studies indicate that the Janus dressing exhibits excellent air permeability, moisture permeability, mechanical properties, cytocompatibility, and antioxidant performance while promptly responding to color variations in solutions of varying pH levels. In vivo studies demonstrated the excellent wound healing ability of Janus PHBV-PAN/SPA/An membrane. Consequently, this study provides a promising solution to develop wound dressings that can effectively manage excessive wound exudate and dressing changes.

Experimental study of thermoconcentration convection in air–water and air–undecane mixtures

A quantitative experimental comparison was conducted between thermal convection in dry air and thermoconcentration convection in two gas mixtures: air–water vapor and air–undecane vapor, within the temperature range of 0–80 °C at normal atmospheric pressure. Convection in these mixtures is driven by temperature and concentration gradients of water (or undecane) vapor in the air. The stationary thermoconcentration convection is accompanied by continuous phase transitions of the fluids. The quantitative results of the experiments are represented in terms of the Nusselt number Nu and the effective Rayleigh number RaE, which is the sum of the thermal RaT and concentration RaC Rayleigh numbers. Quantitative laboratory measurements were performed using the thermocouple method and were supplemented with qualitative data from visual monitoring of transparent fluid flows using holographic interferometry. The cubic and quadratic temperature dependencies of RaCRaT−1 were determined experimentally for moist air and for the air–undecane vapor mixture, respectively. The significant role of moisture phase transitions in air convection is established. Neglecting these effects at 25 °C and using the ordinary RaT instead of actual RaE would result in an error exceeding 30%. At 38 °C, this error would increase to nearly 100%. At around 80 °C, thermoconcentration convection becomes almost entirely concentration-driven, as the high molecular disordered thermal motion is suppressed by the ordered convective motion generated by evaporation and condensation of water on the opposite heat exchangers of the convective cell.

Acoustics of wet porous media with evaporation/condensation

This paper investigates acoustic wave propagation in wet rigid-frame porous media accounting for evaporation and condensation. At equilibrium, the solid walls are covered by a thin water film, and water vapor in the air is at its temperature-dependent saturation pressure. Small acoustic perturbations cause water to vaporize or condense, which together with the reversibility of the phase change, lead to a linear problem where the usual local poro-acoustics physics is enriched with the (i) Clapeyron relation linking liquid-wall temperature, vapor pressure, and latent heat of vaporization, (ii) latent heat transfer in the solid frame, (iii) diffusion equation for water vapor in air, and (iv) water vapor's equation of state. The equilibrium temperature highly influences the vapor concentration and the physical parameters of saturated moist air. Using the two-scale asymptotic homogenization method, it is shown that the dynamic permeability is determined similarly to classical porous media, while the effective compressibility is modified by evaporation/condensation and the equilibrium temperature. This modification is influenced by vapor mass and heat flows associated with phase changes through a local fully coupled heat transfer and water vapor diffusion problem, with specific boundary conditions at the gas–water interface. The analysis identifies dimensionless parameters and characteristic frequencies defining the upscaled model's features. Depending on equilibrium temperature, the theory qualitatively and quantitatively determines the characteristics of acoustic waves propagating through the media. The results are illustrated and discussed with analytically developed models.

Antibacterial, antioxidant and barrier properties of clay-doped electrospun fibers

Nowadays, packaging technologies have been extensively developed, including active and intelligent packages able to promote quality, safety, and the product shelf life. At this purpose, the aim of the study was focused on the design and development of polyvinyl alcohol-based nanofibers doped with two natural clays, montmorillonite or clinoptilolite, and loaded with chlorogenic acid, as nanofibrous membranes to control microbial contamination and oxidative state upon direct contact. The results showed that the 1 % w/w clay doping was the optimal concentration to obtain continuous and uniform nanofibers. In addition, chlorogenic acid loading (5 % w/w) into the nanofibers did not change the systems morphology, resulting in a chlorogenic acid sustained release, achieving 100 % within 24 h for all the systems with enhanced antioxidant and antimicrobial activities. Depending on their structure, the presence of the clays mineral was able to affect the aqueous vapor permeability. These results suggested the promising applications of the clay doped polyvinyl alcohol-based nanofibrous membranes as smart advanced material able to control microbial contamination and oxidative degradation to be used in biomedical field or as active packaging.

Effect of surface orientation on the corrosion behavior of Ni-based single-crystal alloy in a simulated marine atmosphere at 750 °C

DD5 single-crystal alloy samples with different surface orientations were obtained by machining the alloy ingot perpendicular (PP) and parallel (P), respectively, to its solidification direction (SD). The corrosion behaviors of the PPSD and PSD alloy samples were investigated in the atmosphere of moist air + NaCl aerosol at 750 °C. Results showed that during initial corrosion, γ′ phase developed a thin NiO layer with Al internal corrosion product. However, a NiO nodule with an internal Cr2O3 layer was formed on γ phase. As corrosion progressed, the corrosion product gradually grew to a NiO/(Al, Cr)2O3 bilayer, accompanied by internal corrosion consisting of Al2O3. It was also found that the PSD sample with a lower fraction and smaller size of γ’ phases exhibited better corrosion resistance than the PPSD sample. The effect of alloy surface orientation on the corrosion mechanism was discussed.

Hollow Hydrogels for Excellent Aerial Water Collection and Autonomous Release

Air moisture is a valuable and omnipresent resource of fresh water. However, traditional water collectors come with an enduring problem of the water‐release step, which requires special devices and additional energy to remove the water from the adsorbent, such as heat, sunlight, or both. Herein, we report the first composite conical hollow hydrogel to harvest water from aerial humidity. This hollow hydrogel device can rapidly collect water from humid air to a saturation point, whereupon it automatically and continually releases fresh water at room temperature. The entire water collection and release process doesn't require any external assistance. As an exemplary demonstration, positioning the hollow hydrogel device next to a plant turns into an individualized system for irrigation. Since the device is biodegradable, it eventually decomposes and becomes an organic fertilizer after the water supply is not required. For long‐term application, the water‐release process can be monitored in real time by an electronic device to indicate the amount of collected water. The hollow hydrogel combines the humidity‐absorbing capacity with autonomous water release that carries the potential for harvesting water from humidity to address the shortage of fresh water, especially in arid locations where other sources of water are scarce or inaccessible.

The 2019–21 drought in southern Madagascar

Two consecutive failed rainy seasons in the southern part of Madagascar in 2019–21 had devastating impacts on the population, including an amplification of the ongoing food insecurity in the area. The drought events were second in severity only to the 1990–92 drought and were estimated in a previous study to have a return period of 135 years. In this study, the physical mechanisms that led to these consecutive drought events are investigated.We found that the anomalously cold sea surface temperatures (SSTs) that persisted to the south of Madagascar between December 2019 and December 2020 led to a decrease in the transport of moist air over land. These cold SST anomalies were the most negative anomalies in the past four decades and intensified the rainfall deficit resulting from a negative Subtropical Indian Ocean Dipole (SIOD) mode during the rainy season of December 2019 to March 2020 and during December 2020. We also found that the rainfall response to the SST anomaly south of Madagascar was three times greater than that of a canonical SIOD.A weak Mozambique Channel Trough and a strong Angola low system, on the other hand, modulated the expected above-normal rainfall from a La Niña event in January–February 2021. Our study demonstrates how local factors can modulate the impacts of large-scale drivers, and that both local and global drivers, and their interactions, should be considered when producing seasonal forecasts and advisories, as well as climate change adaptation and mitigation plans for southern Madagascar.

Thermomechanical motion in single crystals triggered by de-solvation induced structural transformation: Molecular insights and actuation

Stimuli-responsive molecular crystals have interesting applications as actuators and energy-harvesting materials. Herein, we report a nitrobenzoate stabilized nitropentaamminecobalt(III) complex (1H), exhibits thermo-mechanical motion ∼120 °C. During the process, the crystal converts collective molecular displacements within the lattice into a macroscopic movement due to dehydration-induced structural transformation (monoclinic-orthorhombic) forming a partially dehydrated species (1D) which was confirmed by single-crystal-to-single-crystal (SCSC) analysis. The dehydration process can be reversed when 1D was placed in moist air (RH > 80 %) reforming 1H. A three-state stability model analysis based on DFT calculations suggests that de-solvation of 1H leads to a large instability in the system, resulting in an intermediate state (1-int), which undergoes spontaneous relaxation through the movement of cations and anions forming 1D. Macroscopic transfer of thermo-mechanical energy from crystal to composite films was successfully established by fabrication of a hybrid polymer composite using 1H and polyvinyl di-fluoride (PVDF) that shows excellent actuating behavior upon heating. The present work provides a detailed experimental and theoretical molecular-level insights into thermo-mechanical motion and thereby brings a new dimension towards designing stimuli-responsive crystals as promising actuators.

A Multi-Scale Analysis of the Extreme Precipitation in Southern Brazil in April/May 2024

Since 2020, southern Brazil’s Rio Grande do Sul (RS) State has been affected by extreme precipitation episodes caused by different atmospheric systems. However, the most extreme was registered between the end of April and the beginning of May 2024. This extreme precipitation caused floods in most parts of the state, affecting 2,398,255 people and leading to 183 deaths and 27 missing persons. Due to the severity of this episode, we need to understand its drivers. In this context, the main objective of this study is a multi-scale analysis of the extreme precipitation between 26 April and 5 May, i.e., an analysis of the large-scale patterns of the atmosphere, a description of the synoptic environment, and an analysis of the mesoscale viewpoint (cloud-top features and lightning). Data from different sources (reanalysis, satellite, radar, and pluviometers) were used in this study, and different methods were applied. The National Center for Monitoring and Early Warning of Natural Disasters (CEMADEN) registered accumulated rainfall above 400 mm between 26 April and 5 May using 27 pluviometers located in the central-northern part of RS. The monthly volumes reached 667 mm and 803 mm, respectively, for April and May 2024, against a climatological average of 151 mm and 137 mm for these months. The maximum precipitation recorded was 300 mm in a single day on 30 April 2024. From a large-scale point of view, an anomalous heat source in the western Indian Ocean triggered a Rossby wave that contributed to a barotropic anticyclonic anomalous circulation over mid-southeastern Brazil. While the precipitant systems were inhibited over this region (the synoptic view), the anomalous stronger subtropical jet southward of the anticyclonic circulation caused uplift over RS State and, consequently, conditions leading to mesoscale convective system (MCS) development. In addition, the low-level jet east of the Andes transported warm and moist air to southern Brazil, which also interacted with two cold fronts that reached RS during the 10-day period, helping to establish the precipitation. Severe deep MCSs (with a cloud-top temperature lower than −80 °C) were responsible for a high lightning rate (above 10 flashes km−2 in 10 days) and accumulated precipitation (above 600 mm in 10 days), as observed by satellite measurements. This high volume of rainfall caused an increase in soil moisture, which exceeded a volume fraction of 0.55, making water infiltration into the soil difficult and, consequently, favoring flood occurrence.

The establishment of superlubricity in engineering field for H-DLC composite in multi-environments

The application of hydrogenated diamond-like carbon (H-DLC) with superlubricity can significantly decrease friction and largely save energy dissipation in mechanical systems. Thus, the realization of the near-frictionless state at the engineering scale in both inert and active atmospheres for H-DLC film is of interest. However, the film can achieve the state only under inert and vacuum conditions, while it fails in active atmospheres, such as oxygen and moist air, which seriously hinders wide application of the superlow friction. In previous studies, we have successfully solved this challenge by combining H-DLC film with MoS2 flakes. Nevertheless, the life of the superlubricity achieved by the MoS2/H-DLC composite in an inert gas was extremely short, which was attributed to the failure of MoS2-rich transfer films. To establish the near-frictionless state in various atmospheres, graphene oxide with numerous oxygen-containing groups was used to prepare a GO/MoS2/H-DLC triple composite in this study. The tribological performances were investigated systematacially. The results demonstrated that the triple composite could establish superlubricity in inert gases and ultralow friction in active atmosphere by forming robust transfer films at sliding interfaces. The structures at frictional interfaces including wear tracks and scars were thoroughly investigated. This work provides a valuable approach for the H-DLC composite to achieve an extremely low friction in various atmospheres and can guide the application of superlow friction in engineering fields.

Differentiated influences of anomalous subtropical high on extreme persistent precipitation and heatwave events in the Yangtze River Valley

AbstractIn the summers of 2020 and 2022, the Western Pacific Subtropical High (WPSH) intensified extremely and extended westward. However, in summer 2020, the Yangtze River Valley (YRV) witnessed record‐breaking floods, while in 2022, an unprecedented and prolonged heatwave occurred. Distinctly, these two extreme events were caused by different effects of the WPSH: one is enhancement of the transportation of water vapor and the other is adiabatic heating caused by the descending airflow. In June–July 2020, the stable extension of the WPSH ridge line to South China directed a southwesterly airflow along its northwest flank, leading to sustained precipitation in the YRV. Additionally, the midlatitude circulation pattern featured two troughs and two ridges. Such a circulation configuration, combined with the strong and westward WPSH, enabled the continuous southward intrusion of cold air and northward transport of warm moist air, converging over the YRV, and thus influenced extreme persistent precipitation. In contrast, the WPSH covered the YRV almost entirely during summer 2022. Under this influence, the clear‐sky condition and descending airflow through adiabatic warming directly resulted in the heatwave. In addition, local land–atmosphere feedback was crucial in its development and persistence. The soil moisture deficit induced by high temperatures increased the sensible heat flux between the soil and atmosphere upward, further enhanced the surface air temperature and strengthened the heat dry condition.

AEA-RDCP: An Optimized Real-Time Algorithm for Sea Fog Intensity and Visibility Estimation

Sea fog reduces visibility to less than 1 km and is a major cause of maritime accidents, particularly affecting the navigation of small fishing vessels as it forms when warm, moist air moves over cold water, making it difficult to predict. Traditional visibility measurement tools are costly and limited in their real-time monitoring capabilities, which has led to the development of video-based algorithms using cameras. This study introduces the Approximating and Eliminating the Airlight–Reduced DCP (AEA-RDCP) algorithm, designed to address the issue where sunlight reflections are mistakenly recognized as fog in existing video-based sea fog intensity measurement algorithms, thereby improving performance. The dataset used in the experiment is categorized into two types: one consisting of images unaffected by sunlight and another consisting of maritime images heavily influenced by sunlight. The AEA-RDCP algorithm enhances the previously researched RDCP algorithm by effectively eliminating the influence of atmospheric light, utilizing the initial stages of the Dark Channel Prior (DCP) process to generate the Dark Channel image. While the DCP algorithm is typically used for dehazing, this study employs it only to the point of generating the Dark Channel, reducing computational complexity. The generated image is then used to estimate visibility based on a threshold for fog density estimation, maintaining accuracy while reducing computational demands, thereby allowing for the real-time monitoring of sea conditions, enhancing maritime safety, and preventing accidents.

Switchable Pseudo‐Triaxial Structure Enabled Mechanosensory Textiles with Ultra‐Wide Detection Range for Flexible E‐Wearables

AbstractFlexible sensors hold significant promise for wearable monitoring and rehabilitation training applications. However, current flexible strain sensors struggle to compatible their sensing performance and fabrication with textile substrate and weaving technologies, which limits the practical applications of flexible sensors in commercial markets for electronic wearables. Differing from traditional strategies that rely on sophisticated construction of functional materials, leveraging industrial braiding, and weaving technologies, an all‐textile‐based pseudo‐triaxial mechanosensory textile (PTMT) is designed by engineering the wrapping pattern, yarn twist, and fabric architecture. The switchable hierarchically‐structured morphing of the PTMT enables an ultra‐wide strain detection range (up to 140%) along with desirable sensitivity. Moreover, the PTMT shows outstanding air permeability (1915 mm s−1), moisture permeability (1922 g m−2 h−1), high washability, and electromagnetic interference shielding (20.25 dB). The potential applications of PTMT are also demonstrated, such as in simulating fetal‐movement in pregnant women, proving its effectiveness in fetal‐movement health monitoring. Furthermore, by integrating the PTMT into shoe vamps and combining it with machine learning algorithms (CNN, RF, and PSO‐SVM), it is proved that PSO‐SVM outperformed CNN and RF in accuracy and stability, achieving a combined recognition accuracy of 95.42%.

Hydroentangled waste cotton non-woven based alginate hydrogel wound dressing for high wound exudates

Hydrogels are used in modern wound dressings due to their ability to provide comfort with quick healing. However, poor mechanical properties of hydrogels limit their availability in commercial wound dressings. Nonwovens are highly porous, strong, and flexible structures that can provide support to hydrogels without compromising their properties. In this study, a cost-effective and sustainable hydroentangled nonwoven from industrial cotton waste was prepared and incorporated into alginate hydrogel for wound dressings. The novel composite of hydroentangled cotton nonwoven and alginate hydrogel was synthesized by a simple sol-gel technique. The effect of concentration of alginate hydrogel (0.5 wt%, 1 wt%, 1.5 wt %) and drying temperature (20 °C, 40 °C, 60 °C) of composite was analyzed for high wound exudates. The properties of prepared composite samples were characterized by scanning electron microscopy (SEM), XRD, tensile strength, tear strength, Air permeability, moisture management wound exudate test, and quantitative antimicrobial testing. Moreover, the comfort performance of these samples was evaluated by air permeability and moisture management testing. The properties of developed composites are highly dependent on the concentration of alginate and drying temperature. The results showed that maximum fluid absorbency (%) of 650 was achieved with good comfort properties. This study can help to increase the commercial availability of hydrogel-based wound dressings.

Extending Cucumber Shelf Life: A Comparative Study of Low-Cost Drip Cooling Systems Using Different Fabric Coverings

Aims: Postharvest losses of cucumbers pose significant challenges for small-scale farmers in developing regions due to limited access to modern cold storage. This study investigated the effectiveness of low-cost indigenous drip coolers with different fabric coverings as an alternative preservation method. Study Design: Completely Randomized Design (CRD) with five treatments and three replications. Place and Duration of Study: Department of Crop Science, Surigao del Norte State University – Mainit Campus, Mainit, Surigao del Norte, Philippines on March 1-20, 2024. Methodology: Cucumber fruits were stored in indigenous drip coolers constructed with various recycled fabric coverings. Treatment combinations were: T1 - Control ambient temperature, T2 - Alpha Gina fabric, T3 - Silk satin bedsheets, T4 – Cotton comforter, and T5 - Cotton Blanket. The study evaluated color change, firmness, shriveling index, visual quality rating, weight loss, and damage rate over 20 days storage period. Results: Significant differences in cucumber fruit quality emerged at different time points across various parameters. Differences in color change and firmness were observed on day 15 (p = 0.023), while shriveling and visual quality showed differences by day 20 of storage (p = 0.042 and p = 0.042, respectively). However, in terms of weight loss, the Alpha gina fabric significantly had the highest loss among fabric covers used (p = 0.038). Consistently, the control ambient group exhibited the poorest quality, while treatments like the cotton comforter and silk satin bedsheet were most effective in preserving cucumber quality. Conclusion: This study demonstrates the effectiveness of indigenous drip coolers in preserving cucumber quality, with silk satin bedsheets and cotton comforter coverings consistently outperforming other materials and ambient conditions. These coolers were effective in slowing color change, maintaining firmness, reducing shriveling, and preserving overall visual quality. The study highlights the importance of fabric selection in drip cooler design, with cotton comforter and silk satin bedsheet coverings likely offering better air circulation, moisture retention, and insulation.

Responses of Methane Emission and Bacterial Community to Fertilizer Reduction Plus Organic Materials over the Course of an 85-Day Leaching Experiment

Methane produced from paddy fields has a negative impact on global climate change. However, the role of soil bacterial community composition in mediating methane (CH4) emission from waterlogged paddy soil using the column experiment is poorly known. In the present study, various fertilization treatments were adopted to investigate the effects of fertilizer reduction combined with organic materials (CK: control; CF: conventional fertilization; RF: 20% fertilizer reduction; RFWS: RF plus wheat straw amendment; RFRS: RF plus rapeseed shell amendment; RFAS: RF plus astragalus smicus amendment) on CH4 emission and soil bacterial community during an 85-day leaching experiment. We hypothesized that the fertilizer reduction plus the organic materials could enrich the bacterial communities and increase CH4 emission. The average CH4 flux varied from 0.03 μg m−2 h−1 to 76.19 μg m−2 h−1 among all treatments in the nine sampling times, which may account for the experimental conditions such as air temperature, moisture, and anthropogenic factors. In addition, high-throughput sequencing was utilized to investigate the alteration of the soil bacterial community structure. It was revealed that the diversity and composition of the bacterial community in the topsoil amended with organic materials underwent significant shifts after the 85-day leaching experiment. Proteobacteria was identified as the dominant phylum of the soil bacteria, with an average proportion of 35.2%. For Firmicutes, the proportion of RFRS (11%) was higher than that in the CK (8%), RF (8%), RFWS (7%), RFAS (6%), and CF (5%) treatments. Additionally, Gammaproteobacteria and Alphaproteobateria were supposed to be the major class bacterial communities, with average proportions of 12.8% and 12.2%, respectively. For the RFWS treatment, the contribution of Alphaproteobateria was the highest among all the bacterial relative abundance. According to the correlation heatmap analysis, the top ten bacterial communities were positively related to soil microbial biomass carbon (MBC) and ammonia nitrogen (NH4+-N) (p < 0.01). The findings also indicated that the RFRS treatment was the favorable management to alleviate CH4 emission during an 85-day leaching experiment or possibly in paddy production. Collectively, these results predict that the impacts of different treatments on CH4 production are strongly driven by soil microbial communities and soil properties, with soil bacteria being more prone to the crop residue degradation stage and more sensitive to soil properties. The discoveries presented in this study will be useful for assessing the efficacy and mechanisms of organic material amendments on CH4 emissions in paddy soil.

Moisture cycles in Jezero crater, Mars

Diurnal and annual water cycles are studied during the first year of Perseverance rover in Jezero, using observations from the Mars Environmental Dynamics Analyzer (MEDA) and column modeling. Areal values for the ground thermal inertia (TI) and albedo are first found at one site by fitting model temperatures to the observed air temperatures. Areal soil porosity and the initial water vapor volume mixing ratios (vmr) are next found via model-vmr fits to the observation-based vmr. The meteorology and physics of the modeled air and subsurface diurnal moisture cycle at the site is then discussed in detail. The process of fit to observations is finally extended to fourteen sites along the Perseverance track, resulting in estimates for areal TI, albedo and porosity at these sites, and in MEDA-based initial estimates for the annual and diurnal moisture cycles at Jezero during MY36.

Impacts of urban expansion on air temperature and humidity during 2022 mega-heatwave over the Yangtze River Delta, China

The Yangtze River Delta (YRD) experienced record-breaking heat in the summer of 2022. However, the urban heat pattern and the role of urban expansion over the last two decades in this hot summer have not been explored. Using the advanced mesoscale Weather Research and Forecasting (WRF) model, we reproduced the fine spatial features and investigated the urban heat island (UHI) and dry island (UDI) effects during the two identified heatwaves in 2022. We further replace the current (2020) land use with the historical (2001) land use in WRF to evaluate the impacts of urban expansion from 2001 to 2020 on air temperature and moisture. Our finding revealed that the conversion of land use resulted in near-surface warming and drying, with pronounced diurnal variations, especially during the July heatwave. The analysis of surface energy balance demonstrated that the substantial decrease in evapotranspiration (ET) was the primary driver of daytime warming, elevating temperatures by 7 °C (July heatwave) and 2 °C (August heatwave). This ET reduction also led to the strong daytime coupling of warming and drying effects over new urban areas. At night, the release of stored heat resulted in the temperature increase of 2 °C (1 °C) during July (August) heatwave, highlighting the nighttime as a critical period for heightened thermal risk. Additionally, urban expansion at the periphery contributed modestly to the warming of urban cores, exacerbating conditions in an already hot environment. This study enhances understanding of the impacts of urban expansion on air temperature and humidity during extreme heatwaves, thereby supporting targeted adaptation and mitigation for extreme events within large cities.

Obvious difference of dominant circulation patterns between dry‐type and humid‐type heatwaves in North China

AbstractBased on the observed maximum temperature (Tmax), relative humidity (RH) and NCEP/NCAR reanalysis data during 1981–2021, basic temporal features and dominant atmospheric circulation patterns of dry‐type and humid‐type heatwaves in North China are investigated and compared. Statistical results indicate the dry heatwaves occur mainly in early summer (from early June to early July), that is, before the rainy season of North China, while the humid heatwaves have a high frequency in mid‐July to mid‐August. During the research period, the increasing trend of dry heatwaves is 0.67 days·decade−1, while the humid heatwaves increase at a greatly higher rate of 1.85 days·decade−1. For the dry heatwave, a high ridge in the subtropical westerlies plays the main role, and the northerly wind in the east of the ridge reduces the air moisture convergence over the region. However, for the humid heatwave, the westward and northward propagations of the western Pacific subtropical high (WPSH) may make the major contribution, and the southerly wind anomalies in the west of the WPSH enlarge the water vapour to the region. The adiabatic heating in subsiding air at all levels and horizontal temperature advection at lower troposphere are stronger for dry heatwaves than for humid heatwaves, which cause a higher Tmax for the former type. These results highlight the diversity of the heatwaves in North China, which suggests that multiple local and large‐scale subseasonal circulations should be considered to improve the subseasonal to seasonal forecast skills for heat extremes.