Simulating Microscale Urban Airflow and Pollutant Distributions Based on Computational Fluid Dynamics Model: A Review.

Urban surfaces exert profound influences on local wind patterns, turbulence dynamics, and the dispersion of air pollutants, underscoring the critical need for a thorough understanding of these processes in the realms of urban planning, design, construction, and air quality management. The advent of advanced computational capabilities has propelled the computational fluid dynamics model (CFD) into becoming a mature and widely adopted tool to investigate microscale meteorological phenomena in urban settings. This review provides a comprehensive overview of the current state of CFD-based microscale meteorological simulations, offering insights into their applications, influential factors, and challenges. Significant variables such as the aspect ratio of street canyons, building geometries, ambient wind directions, atmospheric boundary layer stabilities, and street tree configurations play crucial roles in influencing microscale physical processes and the dispersion of air pollutants. The integration of CFD with mesoscale meteorological models and cutting-edge machine learning techniques empowers high-resolution, precise simulations of urban meteorology, establishing a robust scientific basis for sustainable urban development, the mitigation of air pollution, and emergency response planning for hazardous substances. Nonetheless, the broader application of CFD in this domain introduces challenges in grid optimization, enhancing integration with mesoscale models, addressing data limitations, and simulating diverse weather conditions.

Fine particulate pollution driven by nitrate in the moisture urban atmospheric environment in the Pearl River Delta region of south China.

To identify potential sources of fine particles (PM2.5, with aerodynamic diameter (Da) ≤ 2.5 µm) in urban Dongguan of south China, a comprehensive campaign was carried out in the whole 2019. Hourly PM2.5 and its dominant chemical components including organic carbon (OC), elemental carbon (EC), water-soluble inorganic ions (WSIIs) and thirteen elements were measured using online instruments. Gaseous pollutants including NH3, HNO3, NO2, NO and O3 and meteorological parameters were also synchronously measured. PM2.5 was dominated by carbonaceous aerosols in summer and by WSIIs in the other seasons. PM2.5 and its dominant chemical components mostly peaked around noon (10:00-14:00 LST). Furthermore, high PM2.5 levels during the daytime were closely related with the increased NO3- levels. The high mass concentrations of NO3- in urban Dongguan during the daytime were likely related with regional transport of NO3- from suburban Dongguan, which was originated from the reaction between NO2 and O3 under the moisture condition during the nighttime. Seven major source factors for PM2.5 including secondary sulfate, ship emission, traffic emission, secondary nitrate, industrial processes, soil dust and coal combustion were identified by positive matrix factorization (PMF) analysis, which contributed 26 ± 14%, 16 ± 16%, 16 ± 10%, 14 ± 11%, 12 ± 11%, 8 ± 6% and 8 ± 6%, respectively, to annual PM2.5 mass concentration. Although secondary sulfate contributed much more than secondary nitrate to PM2.5 on annual basis, the latter exceeded the former source factor when daily PM2.5 mass concentration was higher than 60 µg m-3, indicating the critical role nitrate played in PM2.5 episode events.

Preparation of LDO@TiO2 core-shell nanosheets for enhanced photocatalytic degradation of organic pollution.

TiO2-based nanosheet materials with a core-shell structure are expected to be one of the promising photocatalysts for the degradation of organic pollution. It is a challenge to synthesize TiO2 by the desired nucleation and growth process, so most reported TiO2 core-shell photocatalysts are prepared using TiO2 as a core material. Layered double hydroxides (LDHs) are considered ideal platforms to grow TiO2in situ and further serve as additional components to improve the separation of photogenerated charge carriers. In this work, we report the design and fabrication of anatase TiO2-coated ZnAl-layered double oxide (LDO@TiO2) nanosheets, which involve the in situ growth of TiO2 on ZnAl-LDH followed by subsequent calcination treatment. The resulting LDO@TiO2 photocatalyst yields typical core-shell nanosheet morphology with a mesoporous structure, exhibiting excellent photodegradation and mineralization efficiency for organic pollution.

Relationship between summertime concurring PM2.5 and O3 pollution and boundary layer height differs between Beijing and Shanghai, China.

The rapid development in the economy during past decades has caused serious air pollution issues in China with high concentrations of PM2.5 and O3, particularly in the densely populous cities. To integrate PM2.5 and O3 controls, it is necessary to understand the impacts of meteorology on both pollutants. Thereby, the complex linkages between planetary boundary layer (PBL), synoptic forcing, regional transport, and heavy pollution in Beijing and Shanghai during summer were investigated using long-term measurements, simulations, and reanalysis. Influenced by the unfavorable meteorological conditions, PM2.5 pollution and O3 pollution often simultaneously occurred. In Beijing, the heavy concurring pollutions usually happened on the days with shallow afternoon PBL and southerly/southwesterly prevailing winds. Within the PBL, the pollutants emitted from the southern plains can be transported to Beijing and accumulated on the windward side of the mountains. At the top of PBL, the synoptic southerly warm advections can strengthen the elevated thermal inversion layer and suppress the development of PBL, leading to worse pollution. Contrarily, the heavy pollutions in Shanghai usually occurred on the days with deep afternoon PBL and southwesterly warm advections within the PBL. Although the warm advections were more favorable to the PBL development than the movements of cool marine air mass, the input of pollutants from the southwest can overweigh this advantage, resulting in poor air quality in Shanghai. The occurrence of heavy pollution or clean condition in Shanghai was primarily determined by the synoptic forcing rather than the local PBL structure. This comparative study indicates that the relationship between PBL height and pollution level is changeable and complicated, which needs to be elucidated from the synoptic perspective.

Changes in ammonia and its effects on PM2.5 chemical property in three winter seasons in Beijing, China.

NH3, SO2, NOx and the inorganic ions of PM2.5 in winter 2009, 2014 and 2016 were examined to investigate the change in NH3 and aerosol chemistry in Beijing, China. NH3 concentrations showed an increase by 59% on average, in contrast to the decrease of SO2 by 63% from winter 2009 to 2016. The mean mass ratio of NH3/NHx was 0.83 ± 0.12 in 2016, which is higher than those obtained in 2009 and 2014, implying more NHx remaining as free NH3 in 2016 winter. Our findings suggest that vehicles exhaust emissions are an important NH3 source in urban central atmosphere of Beijing in winter. Despite the observed NOx presenting declining trends from 2014 to 2016, nitrate concentrations even exhibited a significant increasing trend, which may be largely attributable to high NH3 levels. An in-depth analysis of measured NH3 and aerosol species in a heavy pollution episode in December 2016, combined with the acidity predicted by ISORROPIA II model demonstrated abundant NH3 most of the time in air, where NH3 is not only a precursor for NH4+ but also effect the neutralization of SO42- and NO3- in PM2.5. With high RH and low photochemical activity, elevated NO3- concentration was attributed to an enhanced heterogeneous conversion of NOx to HNO3 to form NH4NO3 in pollution transport stage. The decrease in NOx from high level and the increase in NH3, with peaks of SO42- occurring were observed in pollution cumulative stage. The aqueous-phase oxidation of SO2 by NO2 to sulfate might play an important role with high pH values. Our results suggested that the simultaneous control of NH3 emissions in conjunction with SO2 and NOx emissions would be more effective in reducing particulate matter PM2.5 formation.

Synoptic pattern and planetary boundary layer structure associated with aerosol pollution during winter in Beijing, China.

The day-to-day variations in the planetary boundary layer (PBL) structure and air quality are closely governed by large-scale synoptic forcings. Partly due to the lack of long-term PBL observations during the winter in Beijing, the complex relationships between the large-scale synoptic patterns, local PBL structures/processes, and PM2.5 pollution have not been fully understood. Thus, this study systematically investigated these linkages by combining aerosol measurements, surface meteorological observations, radiosonde data, reanalysis, long-term three-dimensional meteorological simulations, and idealized meteorology-chemistry coupled simulations. Based on the validated long-term simulation results, the boundary layer height (BLH) in Beijing during two winters from 2013 to 2015 was calculated and compared with PM2.5 measurements. A significant anti-correlation was found between the daily BLH and PM2.5 concentration in Beijing, indicating the importance of the PBL structure on the variations in the aerosol pollution levels. Those days with low BLHs are often accompanied by a strong elevated thermal inversion layer. Based on the daily 900-hPa geopotential height fields, seven synoptic patterns were identified using an objective approach, in which two types were found to be associated with heavy PM2.5 pollution in Beijing. One pattern was characterized by weak northwesterly prevailing winds and a strong elevated thermal inversion layer over Beijing, and the local emissions of aerosols played a decisive role in the formation of heavy pollution. The other pattern was associated with southerly prevailing winds, which could transport the pollutants emitted from southern cities to Beijing. According to the meteorology-chemistry coupled simulations, southerly regional transportation can contribute approximately 56% of the PM2.5 in Beijing. The results of this study have important implications for understanding the crucial roles that multiscale meteorological factors play in modulating the aerosol pollution in Beijing during the winter.

Influence of Boundary Layer Structure and Low-Level Jet on PM2.5 Pollution in Beijing: A Case Study.

Beijing experiences frequent PM2.5 pollution, which is influenced by the planetary boundary layer (PBL) structure/process. Partly due to a lack of appropriate observations, the impacts of PBL on PM2.5 pollution are not yet fully understood. Combining wind-profiler data, radiosonde measurements, near-surface meteorological observations, aerosol measurements, and three-dimensional simulations, this study investigated the influence of PBL structure and the low-level jet (LLJ) on the pollution in Beijing from 19 to 20 September 2015. The evolution of the LLJ was generally well simulated by the model, although the wind speed within the PBL was overestimated. Being influenced by the large-scale southerly prevailing winds, the aerosols emitted from the southern polluted regions could be easily transported to Beijing, contributing to ~68% of the PM2.5 measured in Beijing on 20 September. The relative contribution of external transport of PM2.5 to Beijing was high in the afternoon (≥80%), which was related to the strong southerly PBL winds and the presence of thermally-induced upslope winds. On 20 September, the LLJ in Beijing demonstrated a prominent diurnal variation, which was predominant in the morning and after sunset. The occurrence of the LLJ could enhance the dilution capacity in Beijing to some extent, which favors the dilution of pollutants at a local scale. This study has important implications for better understanding the complexity of PBL structure/process associated with PM2.5 pollution in Beijing.

Linkages between aerosol pollution and planetary boundary layer structure in China.

China suffers from high levels of PM2.5 pollution, which is often exacerbated by unfavorable planetary boundary layer (PBL) structures. Partly due to a lack of appropriate observations, the PBL-aerosol linkages in China are not clearly understood. Thus, we investigated these linkages from a national perspective using sounding data collected from 2014 to 2017. Correlation analyses revealed a significant anti-correlation between monthly boundary layer height (BLH) and aerosol pollution that was ubiquitous across China, indicating the important role of the PBL in regulating the seasonal variations of pollution in China. Besides, the day-to-day variations in pollution were modulated by the daily variabilities in the PBL structure. During winter, highly polluted days in most of the Chinese cities studied were associated with a low BLH, strong thermal stability, and weak PBL winds. In the North China Plain and Northeast China, the wintertime heavy pollution was often related to southerly winds and moister PBL. This study has important implications for understanding the crucial role that the PBL plays in modulating aerosol pollution in China.

Turning a native or corroded Mg alloy surface into an anti-corrosion coating in excited CO2.

Despite their energy-efficient merits as promising light-weight structural materials, magnesium (Mg) based alloys suffer from inadequate corrosion resistance. One primary reason is that the native surface film on Mg formed in air mainly consists of Mg(OH)2 and MgO, which is porous and unprotective, especially in humid environments. Here, we demonstrate an environmentally benign method to grow a protective film on the surface of Mg/Mg alloy samples at room temperature, via a direct reaction of already-existing surface film with excited CO2. Moreover, for samples that have been corroded obviously on surface, the corrosion products can be converted directly to create a new protective surface. Mechanical tests show that compared with untreated samples, the protective layer can elevate the yield stress, suppress plastic instability and prolong compressive strains without peeling off from the metal surface. This environmentally friendly surface treatment method is promising to protect Mg alloys, including those already-corroded on the surface.

Unraveling the relationships between boundary layer height and PM2.5 pollution in China based on four-year radiosonde measurements.

Most cities in China experience frequent PM2.5 pollution, in relation to unfavorable planetary boundary layer (PBL) conditions. Partly due to the limited appropriate PBL observations, the explicit relationships between PBL structure/process and PM2.5 pollution in China are not yet clearly understood. Using the fine-resolution sounding measurements from 2014 to 2017, the relationships between boundary layer height (BLH) and PM2.5 pollution in China were systematically examined. Four regions of interest (ROIs) featured with dense population and heavy pollution were studied and compared, including Northeast China (NEC), North China Plain (NCP), East China (EC), and Sichuan Basin (SCB). From 2014 to 2017, the heaviest PM2.5 pollution happened in NCP with an annual average concentration of 84 µg m-3, followed by NEC (60 µg m-3), SCB (57 µg m-3), and EC (54 µg m-3). Correlation analyses revealed a significant anti-correlation between BLH and daily PM2.5 concentrations across China, independent of ROIs. During an annual cycle, the pollution was heaviest in winter, followed by fall and spring, and reached its minimum in summer. Such a seasonal variation of pollution was not only modulated by the emissions, but also the seasonal shifts of BLH. The low BLH in winter was often associated with strong near-surface thermal stability. Moreover, certain synoptic conditions in winter can exacerbate the pollution, leading to concurrent drops of BLH and synchronous increases of PM2.5 concentration in different cities of a ROI. In NCP and SCB, the mountainous terrains could further worsen the pollution by blocking effects and lee eddies.

Elucidating the relationship between aerosol concentration and summertime boundary layer structure in central China.

Wuhan, a megacity in central China, suffers from frequent aerosol pollution and is accompanied by meteorological factors at both synoptic and local scales. Partly due to the lack of appropriate observations of planetary boundary layer (PBL), the associations between synoptic conditions, PBL, and pollution there are not yet fully understood. Thus, systematic analyses were conducted using the fine-resolution soundings, surface meteorological measurements, and aerosol observations in Wuhan during summer for the period 2013-2016, in combination with T-mode principal component analysis and simulations of backward trajectory. The results showed that the variations of boundary layer height (BLH) not only modulated the diurnal variation of PM2.5 concentration in Wuhan, but also the daily pollution level. Five different synoptic patterns during summer in Wuhan were identified from reanalysis geopotential height fields. Among these synoptic patterns, two types characterized by northeasterly prevailing winds, were found to be associated with heavy pollution in Wuhan. Driven by the northeasterly winds, the polluted air mass from the heavily polluted regions could be easily transported to Wuhan, such as North China Plain and Yangtze River Delta. Such regional transports of pollutants must be partly responsible for the aerosol pollution in Wuhan. In addition, these two synoptic patterns were also featured by the relatively high cloud cover and low boundary layer height in Wuhan, which would favor the occurrence of pollution there. Overall, this study has important implications for understanding the important roles of meteorological factors in modulating aerosol pollution in central China.

Impacts of meteorological conditions on wintertime PM2.5 pollution in Taiyuan, North China.

Taiyuan frequently experiences heavy PM2.5 pollution in winter under unfavorable meteorological conditions. To understand how the meteorological factors influence the pollution in Taiyuan, this study involved a systematic analysis for a continuous period from November 2016 to January 2017, using near-surface meteorological observations, radiosonde soundings, PM2.5 measurements, and three-dimension numerical simulation, in combination with backward trajectory calculations. The results show that PM2.5 concentration positively correlates with surface temperature and relative humidity and anti-correlates with near-surface wind speed and boundary layer height (BLH). The low BLH is often associated with a strong thermal inversion layer capping over. In addition to the high local emissions, it is found that under certain synoptic conditions, the southwesterly and southerly winds could bring pollutants from Linfen to Taiyuan, leading to a near-surface PM2.5 concentration higher than 200 µg m-3. Another pollution enhancing issue is due to the semi-closed basin of Taiyuan affecting the planetary boundary layer (PBL): the surrounding mountains favor the formation of a cold air pool in the basin, which inhibits vertical exchanges of heat, flux, and momentum between PBL and the free troposphere, resulting in stagnant conditions and poor air quality in Taiyuan. These findings can be utilized to improve the understanding of PM2.5 pollution in Taiyuan, to enhance the accuracy of forecasting pollution, and to provide scientific support for policy makers to mitigate the pollution.

Effect of Each Layer on Anisotropic Magnetic Properties of Nd/Fe/Polyamide 66 Three-Layer Coaxial Nanocables.

One-dimensional (1D) Nd/Fe/polyamide 66 (Nd/Fe/PA66) three-layer coaxial nanocable arrays with high aspect ratio and highly anisotropic magnetization were successfully prepared via layer-by-layer deposition in the anodic aluminum oxide template. The morphology, chemical composition, and magnetic properties of Nd/Fe/PA66 nanocables were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and vibrating sample magnetometry . The effects of 1D nanocables on the magnetic properties of the assembled ordered arrays have been systematically investigated. The structural properties of these nanostructures are investigated as a function of the geometrical parameters. The magnetic anisotropy of Nd/Fe/PA66 nanocables has been significantly enhanced owing to the doped Nd that possesses a larger spin-orbital coupling and leads to a synergistic effect with Fe to enhance anisotropy energy.

Trans-Pacific transport of dust aerosols from East Asia: Insights gained from multiple observations and modeling.

East Asia is one of the world's largest sources of dust and anthropogenic pollution. Dust particles originating from East Asia have been recognized to travel across the Pacific to North America and beyond, thereby affecting the radiation incident on the surface as well as clouds aloft in the atmosphere. In this study, integrated analyses are performed focusing on one trans-Pacific dust episode during 12-22 March 2015, based on space-borne, ground-based observations, reanalysis data combined with Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT), and the Weather Research and Forecasting Model coupled with Chemistry (WRF-Chem). From the perspective of synoptic patterns, the location and strength of Aleutian low pressure system largely determined the eastward transport of dust plumes towards western North America. Multi-sensor satellite observations reveal that dust aerosols in this episode originated from the Taklimakan and Gobi Deserts. Moreover, the satellite observations suggest that the dust particles can be transformed to polluted particles over the East Asian regions after encountering high concentration of anthropogenic pollutants. In terms of the vertical distribution of polluted dust particles, at the very beginning, they were mainly located in the altitudes ranging from 1 km to 7 km over the source region, then ascended to 2 km-9 km over the Pacific Ocean. The simulations confirm that these elevated dust particles in the lower free troposphere were largely transported along the prevailing westerly jet stream. Overall, observations and modeling demonstrate how a typical springtime dust episode develops and how the dust particles travel over the North Pacific Ocean all the way to North America.

Numerical Study on the Stomatal Responses to Dry-Hot Wind Episodes and Its Effects on Land-Atmosphere Interactions.

The wheat production in midland China is under serious threat by frequent Dry-Hot Wind (DHW) episodes with high temperature, low moisture and specific wind as well as intensive heat transfer and evapotranspiration. The numerical simulations of these episodes are important for monitoring grain yield and estimating agricultural water demand. However, uncertainties still remain despite that enormous experiments and modeling studies have been conducted concerning this issue, due to either inaccurate synoptic situation derived from mesoscale weather models or unrealistic parameterizations of stomatal physiology in land surface models. Hereby, we investigated the synoptic characteristics of DHW with widely-used mesoscale model Weather Research and Forecasting (WRF) and the effects of leaf physiology on surface evapotranspiration by comparing two land surface models: The Noah land surface model, and Peking University Land Model (PKULM) with stomata processes included. Results show that the WRF model could well replicate the synoptic situations of DHW. Two types of DHW were identified: (1) prevailing heated dry wind stream forces the formation of DHW along with intense sensible heating and (2) dry adiabatic processes overflowing mountains. Under both situations, the PKULM can reasonably model the stomatal closure phenomena, which significantly decreases both evapotranspiration and net ecosystem exchange of canopy, while these phenomena cannot be resolved in the Noah simulations. Therefore, our findings suggest that the WRF-PKULM coupled method may be a more reliable tool to investigate and forecast DHW as well as be instructive to crop models.

Modeling the feedback between aerosol and boundary layer processes: a case study in Beijing, China.

Rapid development has led to frequent haze in Beijing. With mountains and sea surrounding Beijing, the pollution is found to be influenced by the mountain-plain breeze and sea-land breeze in complex ways. Meanwhile, the presence of aerosols may affect the surface energy balance and impact these boundary layer (BL) processes. The effects of BL processes on aerosol pollution and the feedback between aerosol and BL processes are not yet clearly understood. Thus, the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) is used to investigate the possible effects and feedbacks during a haze episode on 23 September 2011. Influenced by the onshore prevailing wind, sea-breeze, and upslope breeze, about 45% of surface particulate matter (PM)2.5 in Beijing are found to be contributed by its neighbor cities through regional transport. In the afternoon, the development of upslope breeze suppresses the growth of BL in Beijing by imposing a relatively low thermal stable layer above the BL, which exacerbates the pollution. Two kinds of feedback during the daytime are revealed as follows: (1) as the aerosols absorb and scatter the solar radiation, the surface net radiation and sensible heat flux are decreased, while BL temperature is increased, resulting in a more stable and shallower BL, which leads to a higher surface PM2.5 concentration in the morning and (2) in the afternoon, as the presence of aerosols increases the BL temperature over plains, the upslope breeze is weakened, and the boundary layer height (BLH) over Beijing is heightened, resulting in the decrease of the surface PM2.5 concentration there.

Numerical study of the effects of Planetary Boundary Layer structure on the pollutant dispersion within built-up areas.

The effects of different Planetary Boundary Layer (PBL) structures on pollutant dispersion processes within two idealized street canyon configurations and a realistic urban area were numerically examined by a Computational Fluid Dynamics (CFD) model. The boundary conditions of different PBL structures/conditions were provided by simulations of the Weather Researching and Forecasting model. The simulated results of the idealized 2D and 3D street canyon experiments showed that the increment of PBL instability favored the downward transport of momentum from the upper flow above the roof to the pedestrian level within the street canyon. As a result, the flow and turbulent fields within the street canyon under the more unstable PBL condition are stronger. Therefore, more pollutants within the street canyon would be removed by the stronger advection and turbulent diffusion processes under the unstable PBL condition. On the contrary, more pollutants would be concentrated in the street canyon under the stable PBL condition. In addition, the simulations of the realistic building cluster experiments showed that the density of buildings was a crucial factor determining the dynamic effects of the PBL structure on the flow patterns. The momentum field within a denser building configuration was mostly transported from the upper flow, and was more sensitive to the PBL structures than that of the sparser building configuration. Finally, it was recommended to use the Mellor-Yamada-Nakanishi-Niino (MYNN) PBL scheme, which can explicitly output the needed turbulent variables, to provide the boundary conditions to the CFD simulation.

Numerical study of the effects of local atmospheric circulations on a pollution event over Beijing-Tianjin-Hebei, China.

Currently, the Chinese central government is considering plans to build a trilateral economic sphere in the Bohai Bay area, including Beijing, Tianjin and Hebei (BTH), where haze pollution frequently occurs. To achieve sustainable development, it is necessary to understand the physical mechanism of the haze pollution there. Therefore, the pollutant transport mechanisms of a haze event over the BTH region from 23 to 24 September 2011 were studied using the Weather Research and Forecasting model and the FLEXible-PARTicle dispersion model to understand the effects of the local atmospheric circulations and atmospheric boundary layer structure. Results suggested that the penetration by sea-breeze could strengthen the vertical dispersion by lifting up the planetary boundary layer height (PBLH) and carry the local pollutants to the downstream areas; in the early night, two elevated pollution layers (EPLs) may be generated over the mountain areas: the pollutants in the upper EPL at the altitude of 2-2.5 km were favored to disperse by long-range transport, while the lower EPL at the altitude of 1 km may serve as a reservoir, and the pollutants there could be transported downward and contribute to the surface air pollution. The intensity of the sea-land and mountain-valley breeze circulations played an important role in the vertical transport and distribution of pollutants. It was also found that the diurnal evolution of the PBLH is important for the vertical dispersion of the pollutants, which is strongly affected by the local atmospheric circulations and the distribution of urban areas.