Transport paths and vertical exchange characteristics of haze pollution in Southern China

Spatiotemporal dynamics of near-surface ozone concentration and potential source areas in northern China during 2015-2020

Near-surface ozone (O3) pollution has become one of the main factors hampering urban air quality in northern China. However, on a spatiotemporal scale, dynamic transport paths and potential source areas of O3 in northern China are ambiguous. In addition, we suspect that the contribution of transportation activities to urban O3 concentrations developed in northern China may be underestimated. In this study, the HYSPLIT, PSCF, CWT and GTWR model were used to study the transmission paths, potential source areas and driving factors of urban O3 concentration on a spatiotemporal scale. The average annual concentration of surface O3 (the 90th percentile of MDA8) was 172 ± 29 μg/m3 in northern China from 2015 to 2020. In terms of inter-annual variation, the urban O3 concentration increased from 2015 to 2018, and decreased after 2018. On the spatial scale, the areas with high O3 concentration were mainly clustered in industrial cities (Tangshan, Baoding, Shijiazhuang, Xingtai and Handan). During the study period, the area with high O3 concentration in northern China shifted from northwest to southeast. From 2015 to 2020, the influence of long-distance air mass trajectories from Xinjiang and Siberi on airflow transport in Beijing city dominates (78.60%) The average percentage of short-distance transport trajectories from Shandong Peninsula region is about 21.40%. The core potential source areas of O3 pollution shifted from northwest to southeast, but the contribution to O3 pollution in Beijing gradually weakened during the same period. Temperature and relative humidity were the main meteorological driving factors affecting O3 concentration in the study area, while population density, the proportion of secondary industry in GDP, industrial smoke (dust) emissions, and passenger traffic were the main non-meteorological factors. During the period study, the influence of industrial and traffic emissions had a more significant impact on O3 concentration in northern China, which will require that more attention be paid to emission mitigation in the regional industrial and passenger transportation sector, as well as the joint prevention and control of O3 pollution in northern China in the future.

Response of PM2.5 pollution to meteorological and anthropogenic emissions changes during COVID-19 lockdown in Hunan Province based on WRF-Chem model

In December 2019, the New Crown Pneumonia (the COVID-19) outbroke around the globe, and China imposed a nationwide lockdown starting as early as January 23, 2020. This decision has significantly impacted China's air quality, especially the sharp decrease in PM_2.5 (aerodynamic equivalent diameter of particulate matter less than or equal to 2.5 μm) pollution. Hunan Province is located in the central and eastern part of China, with a "horseshoe basin" topography. The reduction rate of PM_2.5 concentrations in Hunan province during the COVID-19 (24.8%) was significantly higher than the national average (20.3%). Through the analysis of the changing character and pollution sources of haze pollution events in Hunan Province, more scientific countermeasures can be provided for the government. We use the Weather Research and Forecasting with Chemistry (WRF-Chem, V4.0) model to predict and simulate the PM_2.5 concentrations under seven scenarios before the lockdown (2020.1.1-2020.1.22) and during the lockdown (2020.1.23-2020.2.14). Then, the PM_2.5 concentrations under different conditions is compared to differentiate the contribution of meteorological conditions and local human activities to PM_2.5 pollution. The results indicate the most important cause of PM_2.5 pollution reduction is anthropogenic emissions from the residential sector, followed by the industrial sector, while the influence of meteorological factors contribute only 0.5% to PM_2.5. The explanation is that emission reductions from the residential sector contribute the most to the reduction of seven primary contaminants. Finally, we trace the source and transport path of the air mass in Hunan Province through the Concentration Weight Trajectory Analysis (CWT). We found that the external input of PM_2.5 in Hunan Province is mainly from the air mass transported from the northeast, accounting for 28.6%-30.0%. To improve future air quality, there is an urgent need to burn clean energy, improve the industrial structure, rationalize energy use, and strengthen cross-regional air pollution synergy control.

Understanding the inter-city causality and regional transport of atmospheric PM2.5 pollution in winter in the Harbin-Changchun megalopolis in China: A perspective from local and regional

Harbin-Changchun megalopolis (HCM) is the typical cold urban agglomeration in China, where PM_2.5 pollution is still serious in winter against the backdrop of continuous improvement in annual air quality in China. To further understand interactions of atmospheric pollution among HCM cities, inter-city causality and regional transport of PM_2.5 in the winter in the HCM were comprehensively investigated by using convergent cross mapping (CCM) and CMAQ-BFM methods. CCM analysis results suggest strong bidirectional causal relationships between cities in the HCM, and the causality during polluted episodes were significantly larger than that during clean period. In addition, the influence on local PM_2.5 from the HCM western cities were larger than that from cities in the southeast. Inter-city and regional transport contributions results demonstrated that although local emission were the largest contributors among 14 sub-regions for most HCM cities, interactions among cities were strong. Regional transport (42.8%-77.4%) largely contributes to HCM cities' PM_2.5 concentrations. Among three regions outside the HCM, NMG (including part of inner Mongolia and Baicheng city in Jilin, 9.1%) was the largest contributor to the PM_2.5 concentration in the whole HCM, followed by JLS (including Liaoning Province, Tonghua and Baishan cities in Jilin province, 5.1%) and HLJ (including cities of Heihe, Yichun, Jiamusi, Hegang, Shuangyashan, Jixi, Qitaihe in the Heilongjiang province, 3.8%). Regional transport contribution to the most HCM cities increased significantly from excellent to heavily polluted days. Furthermore, close relationships between transport paths/intensity and wind direction/speed in studied region suggests that we can quantitatively guide the regional joint emergency prevention and control before and during heavily polluted events based on regional weather forecasts in the future.

Numerical Simulation of Topography Impact on Transport and Source Apportionment on PM2.5 in a Polluted City in Fenwei Plain

The unique energy structure, high intensity of coal production, and complex terrain, make Fenwei Plain a highly polluted region in China. In this study, we characterized the transport characteristic and sources of PM2.5 (the fraction of particulate matter ≤ 2.5 μm) in Sanmenxia, a polluted city in canyon terrain. The results showed that special topography in Sanmenxia had an important role in the transport of particulates. Sanmenxia is located between two northeast-southwest facing mountains, showing a special local circulation. The local circulation was dominated by a downslope wind at nighttime, while the cross−mountain airflow and zonal wind were dominant during the daytime in the canyon terrain. PM2.5 accumulated near Sanmenxia with the influence of downslope, zonal wind, and topography. The main regional transport paths could be summarized into an eastern path, a northern path, and a western path during the severe haze episodes. The PM2.5 source apportionment revealed by an on-line tracer-tagged of the Nested Air Quality Prediction Model System (NAQPMS) showed that the main regional sources of Sanmenxia were Yuncheng, Sanmenxia, and Weinan. The contribution to PM2.5 concentration in Sanmenxia was 39%, 25%, and 11%, respectively. The northern path had the most important impact on Sanmenxia. The results can provide scientific basis for the establishment of severe haze control in Sanmenxia and regional joint control.

Source apportionment and health risk due to PM10 and TSP at the surface workings of an underground coal mine in the arid desert region of northwestern China

The surface operations area of an underground coal mine near Lingwu in Ningxia Hui Autonomous Region was selected for this study. Particulate matter (PM) was sampled in the coal screening plant during the day and night in Spring and Winter, 2019. Twelve trace metals and eight water-soluble ions in particulate matter up to 10 μm in diameter (PM₁₀) and total suspended particles (TSP) were analyzed using ICP-OES and ion chromatography, respectively. The enrichment factor (EF) and positive matrix fraction (PMF) were used to identify potential sources of particulate metals. The forward trajectory model was used to analyze the main migration pathways of particles. Results showed that higher concentrations of PM₁₀ and TSP were found in Spring than in Winter; the concentration of PM at night was lower than during the day. Most of the trace metals in TSP were greater than in PM₁₀, while the content of most water-soluble ions in TSP was lower than in PM₁₀. The EF analysis confirmed that particulate metals were attributable to anthropogenic emission. PMF model results further demonstrated that the main sources of PM in both seasons were regional suspended dust, traffic emission, industrial emission and coal burning. Air mass dispersion analysis showed that PM generated by these operations may spread to eastern and southeastern China within 72 h in Spring, while mainly to southeastern and southern China in Winter. These suggest a need for greater focus on strengthening the monitoring and early warning of the presence of atmospheric PM in southern Shanxi, China. Because of the risks that particulate metals pose to human health, the protection of children should be strengthened around the surface operation area of an underground coal mine. Moreover, monitoring of the concentrations of Cr in PM₁₀ and Mn in TSP in Spring should be strengthened, and the opposite procedure should be adopted in Winter. These findings are useful for providing a theoretical basis for the prevention and control of pollutants in underground mining areas and the construction of cleaner production lines.

Direct and Inverse Reduced-Form Models for Reciprocal Calculation of BC Emissions and Atmospheric Concentrations

Atmospheric black carbon (BC) concentrations are governed by both emissions and meteorological conditions. Distinguishing these effects enables quantification of the effectiveness of emission mitigation actions by excluding meteorological effects. Here, we develop reduced-form models in both direct (RFDMs) and inverse (RFIMs) modes to estimate ambient BC concentrations. The models were developed based on outputs from multiyear simulations under three conditional scenarios with realistic or fixed emissions and meteorological conditions. We established a set of probabilistic functions (PFs) to quantify the meteorological influences. A significant two-way linear relationship between multiyear annual emissions and mean ambient BC concentrations was revealed at the grid cell scale. The correlation between them was more significant at grid cells with high emission densities. The concentrations and emissions at a given grid cell are also significantly correlated with emissions and concentrations of the surrounding areas, respectively, although to a lesser extent. These dependences are anisotropic depending on the prevailing winds and source regions. The meteorologically induced variation at the monthly scale was significantly higher than that at the annual scale. Of the major meteorological parameters, wind vectors, temperature, and relative humidity were found to most significantly affect variation in ambient BC concentrations.

Cross-regional transport of PM2.5 nitrate in the Pearl River Delta, China: Contributions and mechanisms

Cross-regional transport potentially contributes to PM_2.5 nitrate (pNO₃), and this can occur as indirect transport, through which pNO₃ precursors are transported to targeted regions, wherein they subsequently react with locally emitted ones to produce pNO₃. However, the process has been rarely studied, which limits its comprehensive understanding. We applied the CMAQ model to study the contributions and mechanisms of pNO₃ transport during autumn in the Pearl River Delta (PRD), a metropolitan region under the growing influence of cross-regional transport on PM_2.5 pollution. Results showed that cross-regional transport contributed to 58% pNO₃ monthly in the PRD, and this mostly occurred as indirect transport contributions (accounting for 43% among all contributions). For the first time, we identified the mechanism of indirect pNO₃ transport in the PRD, which mainly involves transported O₃ and locally emitted NO_x reacting to produce pNO₃ through N₂O₅ heterogeneous hydrolysis. pNO₃ contributions in different periods and regions indicated differences in the indirect transport contributions to N₂O₅ heterogeneous hydrolysis under varying O₃ availability conditions, which are determined by wind fields and the intensity of NO_x emissions. On the regional scale, the pNO₃ level is controlled by both transported O₃ and local NO_x emissions, but pNO₃ sensitivity to these two precursors varies among cities. This study demonstrates the notable effect and complex process of cross-regional pNO₃ transport in the PRD. Considering the important role of transported O₃ for pNO₃, O₃ reduction within a larger scale is required to achieve PM_2.5 pollution control target.

Asymmetric and Spatial Non-Stationary Effects of Particulate Air Pollution on Urban Housing Prices in Chinese Cities

Fine particulate matter(PM2.5) pollution will affect people’s well-being and cause economic losses. It is of great value to study the impact of PM2.5 on the real estate market. While previous studies have examined the effects of PM2.5 pollution on urban housing prices, there has been little in-depth research on these effects, which are spatially heterogeneous at different conditional quantiles. To address this issue, this study employs quantile regression (QR) and geographically weighted quantile regression (GWQR) models to obtain a full account of asymmetric and spatial non-stationary effects of PM2.5 pollution on urban housing prices through 286 Chinese prefecture-level cities for 2005–2013. Considerable differences in the data distributions and spatial characteristics of PM2.5 pollution and urban housing prices are found, indicating the presence of asymmetric and spatial non-stationary effects. The quantile regression results show that the negative influences of PM2.5 pollution on urban housing prices are stronger at higher quantiles and become more pronounced with time. Furthermore, the spatial relationship between PM2.5 pollution and urban housing prices is spatial non-stationary at most quantiles for the study period. A negative correlation gradually dominates in most of the study areas. At higher quantiles, PM2.5 pollution is always negatively correlated with urban housing prices in eastern coastal areas and is stable over time. Based on these findings, we call for more targeted approaches to regional real estate development and environmental protection policies.

Spatial–Temporal Heterogeneous Evolution of Haze Pollution in China as Deduced with the Use of Spatial Econometrics

Haze Pollution, consisting essentially of PM2.5 and PM10, has been arousing wide public concern home and abroad. It has become a universal urgency for atmospheric researchers, governments, organizations, institutions, and the general public to conduct corresponding actions. Therefore, this paper aims to explore the institutional distribution and the regional evolution trend of path characteristics of haze pollution in China under the spatial–temporal heterogeneity on the basis of spatial econometrics, by incorporating the spatial element into the framework of the Multiple Influencing Factors mechanism. The results show that it has been abating under the governance year by year, though with a decreasing intensity; the major polluted regions have been moving from the East to the central and western area; there is significant spatial autocorrelation among the highly polluted area, but the effective local regulations of les- polluted regions do not impact the neighboring regions correspondingly; among the impacting factors, industrial structure, energy intensity, and traffic pollution have a significant Positive Impact on haze pollution, and the level of urbanization has a Negative Impact, while economic growth and innovation performance have no significant Positive Impact and are both weak in promotion. This research, theoretically and practically, offers reference for the Chinese government to integrate regional effective systems into multiregional diversified environmental governance, so as to realize its Green Ecology Transformation Development Strategy.

Source and exposure apportionments of ambient PM2.5 under different synoptic patterns in the Pearl River Delta region

PM_2.5 is one of the most notorious ambient pollutants in the Pearl River Delta (PRD) region during episodic conditions. In this work, the Comprehensive Air Quality Model with extension (CAMx) was used together with the Particulate Source Apportionment Technology (PSAT) module to analyze the influences of different sources on PM_2.5 concentration in the PRD region under different synoptic patterns (sea high pressure, sub-tropical high pressure and equalizing pressure field). The result shows that the PM_2.5 concentration increases to different degrees under the three synoptic patterns. The emissions outside the PRD region contribute more than 54% under episodic conditions. The source category contribution varies little under different synoptic patterns. Area (46%), mobile (21%) and industry point source (16%) are the major contributors over the three episodic cases. The regional source contributions (from other cities within the PRD) to Foshan, Zhongshan and Zhaoqing are larger and can reach up to 33%. People living in the PRD region are more exposed to pollutants produced from the area and mobile sources. About 80% of the population is exposed to PM_2.5 levels exceeding the IT-3 standard during the pollution episodes.

Computing payments for wind erosion prevention service incorporating ecosystem services flow and regional disparity in Yanchi County

Simulating the flows of ecosystem services (ESs) can help understand their spatiotemporal flow paths from generation to use, thereby facilitating payments from beneficiaries to providers of ESs. In this study, an analytical framework incorporated with ES flows and regional disparity was established to compute payments for wind erosion prevention service (WEPS). The results showed that between 2010 and 2015, both the potential and actual wind erosion amounts in Yanchi County decreased considerably, and the total amount of sand fixed by WEPS decreased significantly from 3.71 × 10⁹ kg to 0.08 × 10⁹ kg; additionally, the economic value of the WEPS also decreased from CNY 479.46 million to CNY 10.22 million. Based on the spatiotemporal movements of the physical and economic value flows of the WEPS, this study revealed spatiotemporal relationships between areas providing and benefiting from the WEPS of Yanchi County and provided a direct, scientific basis for decision makers to formulate payment systems for WEPS. The total amount paid for WEPS by beneficiaries in China should theoretically be CNY 38.16 million in 2010 and CNY 1.00 million in 2015 based on the economic value flow of WEPS and the regional disparity coefficient. This framework can provide a scientific and objective basis for establishing horizontal ecological compensation policies.

Meteorological and chemical impacts on PM2.5 during a haze episode in a heavily polluted basin city of eastern China

Haze formation involves many interacting factors, such as secondary aerosol formation, unfavourable synoptic conditions and regional transport. The interaction between these factors complicates scientific understanding of the mechanism behind haze formation. In this study, we investigated the factors resulting in haze events in Longyou, a city located in a basin in China. Aerosol samples of PM_2.5 were collected for subsequent chemical composition analysis between 11 January and 5 February 2018. The impacts of wind on PM_2.5, SO₂ and NO₂ concentrations were analysed. Besides, the origin of air parcels and potential sources of PM_2.5 were analysed by backward trajectory, potential source contribution function (PSCF) and concentration-weighted trajectories (CWT). Among the water-soluble ions identified, NO₃^- had the highest concentration, with further analysis demonstrating the haze evolution was mainly driven by the reactions involving NO₃^- formation. The dramatic increase of nitrate is mainly due to the homogeneous reaction of nitric acid with ammonia, while sulfate is likely due to heterogeneous reactions of NO₂, SO₂ and NH₃. The average wind speed was less than 2 m/s during the aerosol sampling period, which could be considered as a stagnant state. Pollutants emitted by industrial area located in the northeast Longyou were probably brought to observation sites by continuous wind from northeast and accumulated gradually. Air parcels originating from the northeast of Zhejiang province also had large effects on haze pollution in Longyou. Together, our results showed that rapid secondary aerosol formation and unfavourable synoptic conditions are the main factors resulting in haze pollution in Longyou.

Transport Paths and Identification for Potential Sources of Haze Pollution in the Yangtze River Delta Urban Agglomeration from 2014 to 2017

Besides local emissions, long-range transportation of polluted air masses also has a huge impact on haze pollution. In this study, the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used to determine the transport paths and potential sources of haze pollution in the Yangtze River Delta Urban Agglomeration. Haze days were determined by setting the threshold of meteorological elements. Shanghai, Hangzhou, Nanjing and Hefei were selected as four representative cities to calculate the −72 h backward transport trajectory of haze air mass; thus, the main transport path was obtained after clustering. A potential source contribution function and concentration weighted field were used to identify potential pollution sources of the study. The results showed that the number of haze days in the northern Yangtze River Delta Urban Agglomeration is much higher than that in the south. Haze days and Fine particulate matter (PM2.5) concentration showed a downward trend. The transport paths could be summarized as long-range transports from the northwest and coastal direction during the dry season and short-distance transports from all directions. −72 h air flow trajectories come from the higher altitudes in dry season than these in wet season. The main sources of potential pollution are Hebei, Shandong, Anhui and northern Jiangsu.

The Spatio-Temporal Disparities of Areas Benefitting from the Wind Erosion Prevention Service

Ecosystem services are closely linked to human welfare. The flow of ecosystem service can establish spatio-temporal relationships between ecosystem service provision areas (SPAs) and service beneficiary areas (SBAs). In this study, the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used to simulate the spatial flow path of the wind erosion prevention (WEP) service in Yanchi County. The frequency at which the simulated trajectories passed through each grid cell was used as a proxy for spatio-temporal disparities in SBAs, and the distribution of benefitting land cover, population, and gross domestic product (GDP) reflected the effects of the WEP flow. The flow paths of the Yanchi County WEP in 2010 mainly extended to eastern and central China, North Korea, South Korea, Japan, Mongolia, and eastern Russia, and were more intensive and longer in spring and winter than in autumn and winter. The SBAs covered an area of 1153.2 × 10⁴ km² in 2010, with dominant service beneficiary areas (DSBAs) comprising 185.1 × 10⁴ km² and accounting for 16.1% of the total beneficiary area of the year. The areas through which the flow paths passed with a high frequency (≥10%) were mainly located in Shaanxi, Shanxi, Henan, western Shandong, Hebei, Beijing, and northern Hubei, and the spatial scale of these areas varied, demonstrating obvious seasonal changes, and was the largest in spring. The benefitting land cover was mainly cropland across all of the SBAs, with one billion benefitting people (accounting for 77.11% of the total population of China) associated with a gross domestic product (GDP) of 26.8 trillion RMB (Chinese currency; as of 2018-06-22, 6.497 RMB = US $1, accounting for 87.90% of the total GDP of China). Furthermore, the population and socio-economic development in the DSBAs (21 million people and 0.53 trillion RMB GDP) were no longer affected by wind erosion from Yanchi County. This study revealed the spatio-temporal disparities of the SBAs of WEP in Yanchi County from an ecosystem services flow perspective and provides a scientific and effective basis for policymakers to perform standard ecological compensation accounting and to formulate ecological protection policies.