Regional Transport Of Air Pollutants Research Articles

Role of air stagnation in determining daily average PM2.5 concentrations in areas with significant impact of long-range transport

In areas where the regional transport of air pollutants exerts a significant impact, ascertaining whether short-term air stagnation affects PM2.5 concentrations is crucial for accurate air quality forecasting and effective management planning. However, this research area remains underexplored. In this study, we analyzed the relationship between stagnant atmospheric conditions and daily average PM2.5 concentrations in areas with a substantial long-range transport impact (LRT). Specifically, we focused on days with elevated PM2.5 concentrations (daily average ≥ 35 μg/m3) from January to March 2019 in South Korea. The analysis was performed using the Weather Research and Forecasting and the Community Multiscale Air Quality Modeling System. Stagnant conditions were quantified using ventilation index (VI), calculated as the product of planetary boundary layer height and 10-m wind speed. The correlation coefficient between daily average VI and PM2.5 concentration (r = −0.23) was lower than that between VI and NO2 concentration (r = −0.60). This can be attributed to the fact that LRT was 2.9-fold higher than local emission impact (LEI) during days of elevated PM2.5 concentrations. Notably, the correlation coefficient between LRT and VI (r = −0.03) was considerably lower than that of LEI (r = −0.70). Hence, predicting the daily average PM2.5 concentration solely based on VI proved challenging in an area characterized by substantial LRT. For future research, in areas where LRT plays a substantial role, accurate prediction of PM2.5 concentrations requires distinguishing the effects of LRT and LEI on PM2.5 concentrations.

Application of complete ensemble empirical mode decomposition based multi-stream informer (CEEMD-MsI) in PM2.5 concentration long-term prediction

Nowadays, air pollution has become one of the most serious environmental problems facing humanity and an inescapable obstacle limiting the sustainable development of cities and society. Although air quality sensing and management systems based on artificial intelligence and signal analysis are evolving as essential parts of intelligent cities, the mixture of local emission sources and regional transport of air pollutants still makes PM2.5 long-term prediction challenging, especially under complex geographical and climatic conditions. In this paper, the complete ensemble empirical mode decomposition based multi-stream informer (CEEMD-MsI) is proposed to predict the hourly PM2.5 concentration, and extensive testing and comparisons are carried out in four typical cities in Shandong, China. Firstly, CEEMD is used for signal pre-processing to construct the intrinsic mode functions (IMFs) based multi-channel representations. Then MsI is specifically designed to learn both temporal and spatial features, and complete the PM2.5 concentration prediction. To the best of our knowledge, this is the first attempt to predict long-term PM2.5 concentrations using a deep learning model driven by data collected from monitoring stations spanning long distances and diverse terrains. Finally, test results demonstrate that CEEMD-MsI achieves the best PM2.5 prediction performance by comparing it with state-of-the-art methods.

Identifying the impacts of warming anomalies in the Arctic region and the Tibetan Plateau on PM2.5 pollution and regional transport over China

Arctic region (AR) and Tibetan plateau (TP) are the most sensitive regions to climate change and have synchronously experienced accelerated warming over recent decades. Based on the multi-year observational data of PM2.5 and meteorology, we investigated the modulation of AR and TP's anomalous warming on the variations of PM2.5 pollution and regional transport over central and eastern China (CEC), including major PM2.5 pollution regions in northern China (NC), the Yangtze river delta (YRD), central China (CC), and southern China (SC). Results show that the thermal effects of TP and AR modulated PM2.5 pollution in different ways respectively with prominently influencing the interannual changes of PM2.5 concentrations and heavy PM2.5 pollution occurrences. The impacts of anomalous warming in TP and AR on PM2.5 pollution consistently enhanced regional PM2.5 transport from NC to YRD and SC, while the warming anomalies over AR and TP respectively enhanced and inhibited the southward transport of PM2.5 from NC. In association with the synergistic impact of warming anomalies in AR and TP, the regional PM2.5 transport over CEC presented the inverse patterns of the northward anomalies between NC and CC and the southward anomalies from the eastern part of NC, YRD to SC, forming the regional distribution with negative PM2.5 anomalies in NC and CC and positive PM2.5 anomalies in YRD and SC. Our results highlight the effect of warming anomalies in AR and TP on the PM2.5 pollution over China with the implication of source–receptor relationship of regional transport of air pollutants.

Meteorological mechanisms of regional PM2.5 and O3 transport in the North China Plain driven by the East Asian monsoon

The regional transport of air pollutants significantly affects downwind air quality in the North China Plain (NCP), which forms part of the East Asian monsoon region. Here, we investigated the meteorological regional transport mechanisms that lead to wintertime PM2.5 and summertime O3 pollution episodes in downwind regions over the NCP during the East Asian monsoon. First, we analyzed the spatiotemporal patterns of PM2.5 and O3 pollution during the summers and winters of 2015–2020 in the NCP, as well as the influence of meteorological conditions. Subsequently, we identified the typical regional transport patterns of wintertime PM2.5 and summertime O3 using the multivariable empirical orthogonal function with multi-source data. The results showed that prevailing southerly and southeasterly winds in summer aggravated PM2.5 and O3 pollution over the northern NCP. Conversely, prevailing northerly and northwesterly winds in winter exacerbated PM2.5 and O3 pollution over the southern NCP. Wintertime PM2.5 pollution events in Henan (on the southern edge of the NCP) were induced by the southward movement of weak cold airflow following northerly winds governed by the East Asian winter monsoon. Additionally, a warm air mass to the south of Henan and the high surrounding terrain inhibited further southward invasion of weak cold air, causing heavy PM2.5 pollution. Conversely, summertime O3 pollution episodes in Beijing (on the northern edge of the NCP) were associated with southerly and southeasterly winds, which drove the transport of O3 and its precursors from the southern NCP to Beijing. The mountain barrier in northern Beijing aggravated O3 pollution. During the O3 pollution events, Beijing was in a transitional regime, and HCHO and NO2 increased by 17.16% and 20.43%, respectively. Furthermore, the vertical structure of the atmosphere facilitated the downward mixing of PM2.5 and O3 transported during pollution episodes. These results provide a theoretical reference for predicting and controlling heavy air pollution in the NCP.

Aggravated chemical production of aerosols by regional transport and basin terrain in a heavy PM2.5 pollution episode over central China

Heavy aerosol pollution has occurred frequently over China during wintertime in recent years. Compared to source regions with relatively developed economy (e.g., eastern China), the pollution receptor regions with special topography, such as the Twain-Hu Basin (THB) over central China, have been less analyzed in terms of the main drivers of pollution episodes. In this study, we used the Weather Research and Forecasting model with Chemistry (WRF-Chem) with improved integrated process rate (IPR) analysis to evaluate the contributions of individual physical and chemical processes to PM2.5 variation for a typical aerosol pollution episode in THB in January 2019, and quantified the effects of regional transport and basin terrain on chemical production of PM2.5. Regional transport of air pollutants from the north was estimated to increase the PM2.5 concentration in THB by 43.5%, attributed mainly to the elevated advection and chemical processes. Furthermore, regional transport enhanced the chemical production of PM2.5 concentration by 110.2 μg m−3, and the number would decline to 7.1 without the effect of basin terrain. Our findings highlighted the critical roles of regional transport of gases precursors and basin terrain on chemical production of aerosols in THB, and emphasized the importance of regional joint controls of emissions for air quality improvement.

Vertical changes of PM2.5 driven by meteorology in the atmospheric boundary layer during a heavy air pollution event in central China

Regional PM2.5 transport is a crucial factor affecting air quality, and the meteorological mechanism in the atmospheric boundary layer (ABL) has not been fully understood over the receptor region in the regional transport of air pollutants. Based on the intensive vertical measurements of air pollutants and meteorology in the ABL during a transport-induced heavy air pollution event in Xiangyang, an urban site over a receptor region in central China, we investigated the meteorological mechanism in vertical PM2.5 changes in the ABL for heavy air pollution over the receptor region. Driven by northerly winds, regional PM2.5 transport was built from upstream northern China to downstream central China, where the observed ABL structures were unstable throughout the air pollution event. We assessed the ABL structures with meteorological and PM2.5 profiles at growth, maintenance, and dissipation stages, and elucidated the mechanism of regional PM2.5 transport inducing air pollution over the receptor region with the contribution of thermal and mechanical factors. The regional PM2.5 transport was concentrated in the upper ABL over the downwind receptor region with high PM2.5 concentrations at altitudes of 600–800 m, where the transported PM2.5 peaks were downwards mixed by vertical wind shear, forming the vertical PM2.5 transport from the upper ABL to near-surface in the growth stage; the weakened winds and less unstable structures in the ABL favored the sustained pollution with slight vertical PM2.5 changes in the maintenance stage, which was dominated by thermal factors with 87 % contribution; the removal of PM2.5 was triggered by increasing winds from the upper ABL, activating the dissipation of heavy PM2.5 pollution with the mechanical effect accounting for 60 % in the dissipation stage. These findings could improve our understanding of ABL's influence on air pollution over the receptor region with implications for the regional transport of air pollutants in environmental changes.

Synergistic Effect of Atmospheric Boundary Layer and Regional Transport on Aggravating Air Pollution in the Twain-Hu Basin: A Case Study

The impact of structural variations in the atmospheric boundary layer (ABL) during the regional transport of air pollutants on its local pollution changes deserves attention. Based on multi-source ABL detection and numerical simulation of air pollutants over the Twain-Hu Basin (THB) during 4–6 January 2019, the mechanism of the rapid growth of atmospheric pollutant concentrations in Xianning by the synergistic effect of regional transport and ABL evolution is explored, and the main conclusions are obtained as follows. The vertically stratified atmosphere is noticeable at nighttime, and the heavy humidity of near-surface fog within the stable boundary layer (SBL) promoted the generation and cumulative growth of secondary PM2.5 components during the pollution formation stage. The horizontal transport characteristics of atmospheric pollutant concentration peak were observed in the residual layer (RL) of 500–600 m. At the pollution maintenance stage, the convective boundary layer (CBL) developed during the daytime, and northerly wind transported high-concentration pollutants from the north to the THB. Under the combined action of horizontal transport and turbulent mixing, the high-concentration atmospheric pollutants in the mixing layer (ML) from the ground to the 500 m height were mixed uniformly and maintained accumulation growth. The next day, the strong vertical turbulent mixing caused the downward transport of high-concentration pollutants in the RL during nighttime due to the development of the CBL again, resulting in a doubling of near-surface pollutant concentration in a short time. With the development of ABL turbulence, local pollution dissipated rapidly without the continuous input of pollutants from external regions. This study emphasizes the importance of multi-scale processes impact on pollution variation, that is, regional transport of atmospheric pollutants at the CBL development stage for the rapid growth of PM2.5 concentration in the ML.

Analysis of pollutants transport in heavy air pollution processes using a new complex-network-based model

A transport analysis model of air pollutants based on complex networks is proposed to study the spatiotemporal variation characteristics of air pollution during heavy air pollution processes and further analyze the contribution and interrelationship of air pollution of different cities. To construct the transmission network for each heavy air pollution process, a link is established between two cities according to the correlation coefficient of pollutants, the angle between the direction of cities and the wind direction, and its weight is assigned by integrating the factors of pollution, meteorology and geography. Then the PageRank algorithm is improved to identify the main contribution areas, and the label propagation algorithm is improved to find the associated air pollution areas. Finally, the period from January 1, 2014 to February 28, 2021 in Beijing–Tianjin–Hebei (BTH) region are used to demonstrate the performance of this model. The results show that a total of 58 heavy air pollution processes occur during the study period, the shortest lasting 3 days and the longest 13 days. These pollution processes occur mainly in winter, especially in January, and there are sufficient data to show that pollution levels decline year on year. Long-term correlation analysis for single-pollutant between different cities shows that geography is one of the most important factors affecting the regional transport of air pollutants. Moreover, according to the statistical analysis of each heavy pollution process, Tangshan, Tianjin and Langfang are the main contribution areas in the pollutant regional transport. In addition, apart from that in Zhangjiakou, Chengde and Qinhuangdao, the pollutants in the other cities in the BTH region have an effect on one another so that these cities are formed as an associated contaminated area. This study can provide better insights into local control and joint prevention of atmospheric pollution.

Effect of Vertical Wind Shear on PM2.5 Changes over a Receptor Region in Central China

Vertical wind shear (VWS) significantly impacts the vertical mixing of air pollutants and leads to changes in near-surface air pollutants. We focused on Changsha (CS) and Jingmen (JM), the upstream and downstream urban sites of a receptor region in central China, to explore the impact of VWS on surface PM2.5 changes using 5-year wintertime observations and simulations from 2016–2020. The surface PM2.5 concentration was lower in CS with higher anthropogenic PM2.5 emissions than in JM, and the correlation between wind speed and PM2.5 was negative for clean conditions and positive for polluted conditions in both two sites. The difference in the correlation pattern of surface PM2.5 and VWS between CS and JM might be due to the different influences of regional PM2.5 transport and boundary layer dynamics. In downstream CS, the weak wind and VWS in the height of 1–2 km stabilized the ABL under polluted conditions, and strong northerly wind accompanied by enhanced VWS above 2 km favored the long-range transport of air pollutants. In upstream JM, local circulation and long-range PM2.5 transport co-determined the positive correlation between VWS and PM2.5 concentrations. Prevailed northerly wind disrupted the local circulation and enhanced the surface PM2.5 concentrations under polluted conditions, which tend to be an indicator of regional transport of air pollutants. The potential contribution source maps calculated from WRF-FLEXPART simulations also confirmed the more significant contribution of regional PM2.5 transport to the PM2.5 pollution in upstream region JM. By comparing the vertical profiles of meteorological parameters for typical transport- and local-type pollution days, the northerly wind prevailed throughout the ABL with stronger wind speed and VWS in transport-type pollution days, favoring the vertical mixing of transported air pollutants, in sharp contrast to the weak wind conditions in local-type pollution days. This study provided the evidence that PM2.5 pollution in the Twain-Hu Basin was affected by long-distance transport with different features at upstream and downstream sites, improving the understanding of the air pollutant source–receptor relationship in air quality changes with regional transport of air pollutants.

Regulation of Synoptic Circulation in Regional PM2.5 Transport for Heavy Air Pollution: Study of 5‐year Observation Over Central China

AbstractRegional transport of air pollutants is modulated by large‐scale synoptic circulation, which has been poorly understood for heavy air pollution in the downstream receptor region. In the present study of 5‐year (2015–2019) observation, we targeted the Twain‐Hu Basin (THB), a region of heavy PM2.5 pollution over central China, to investigate the regulation of synoptic circulation governing regional PM2.5 transport for heavy air pollution. It was found that regional transport of PM2.5 predominated 65.2% of the heavy pollution events (HPEs) over the THB based on the statistics of observational environment and meteorology. By employing the FLEXPART‐WRF model, the regional transport of PM2.5 from upwind areas in central and eastern China (CEC) to receptor region in the THB was identified with three prominent pathways in the northerly, northeasterly, and easterly directions respectively. We then used T‐mode principal component analysis in conjunction with the K‐means cluster method to categorize synoptic circulations, and it was recognized that three regional PM2.5 transport pathways for the HPEs over central China were governed respectively by three patterns of synoptic circulation over CEC with (a) weak high air pressure to the north (13.3%), (b) strong high air pressure to the northeast (46.7%), and (c) weak high air pressure to the east (40.0%). This study reveals a significant modulation of large‐scale synoptic circulation for regional transport of air pollutants in environmental change.

Impact of Inter-Annual Variation in Meteorology from 2010 to 2019 on the Inter-City Transport of PM2.5 in the Beijing–Tianjin–Hebei Region

Air pollution has become a great challenge to achieving sustainable development. Among the pollutants, aerosols significantly affect human health and play an important role in global climate change. The concentration of aerosols in the ambient air is influenced strongly by the regional transport of pollutants and their precursors and may vary considerably under different meteorological conditions in different years. This inter-annual variation in meteorology may yield conflicting results in the quantification of the contribution from regional transport of air pollutants. It creates uncertainty for local governments to develop pollution control measures to reduce the challenges to sustainable development. Previous studies on this issue are often year-specific or cover short time spans, and the inter-city transport of air pollutants in the long term is still not fully understood. Therefore, in this study, the Weather Research and Forecasting (WRF) model and Community Multiscale Air Quality (CMAQ) model was used to assess inter-annual variations in the contribution of inter-city transport to the PM2.5 concentration in the Beijing–Tianjin–Hebei region from 2010 to 2019. To highlight the impact of inter-annual variations in meteorology, the authors used the same emission inventory and the same model configurations for the 10-year simulation. The major findings can be summarized as follows: (1) Both PM2.5 concentration and inter-city transport in the Beijing–Tianjin–Hebei (BTH) region were influenced by the inter-annual variation in meteorological conditions. (2) The simulated annual average concentrations in 13 cities in BTH are highly variable, with fluctuations ranging from 30.8% to 54.1%, and more evident variations were found in seasonal results. (3) Seven out of thirteen cities have a contribution from regional transport exceeding 50%, which are located in the eastern half of the Beijing–Tianjin–Hebei region. (4) The magnitude of the regional transport contribution varies significantly among the cities of BTH, on an annual basis, from a minimum inter-annual fluctuation of 8.9% to a maximum of 37.2%, and seasonal fluctuation is even more strongly evident. These results indicate that, when formulating pollution control strategies, inter-annual changes in meteorological conditions should not be ignored.

Two Typical Patterns of Regional PM2.5 Transport for Heavy Air Pollution Over Central China: Rapid Transit Transport and Stationary Accumulation Transport

The regional transport of air pollutants has been identified as a critical factor that affects air quality over downwind receptor areas; however, the regional transport patterns for air pollution episodes remain poorly understood. In this study, we identified two distinct patterns of regional transport: rapid transit transport (RTT) and stationary accumulation transport (SAT), both of which induced severe PM2.5 pollution episodes in central China during 2015–2020. The differences and similarities between the two regional transport patterns of air pollution were characterized by their influencing meteorological factors. Similar meteorological conditions trigger the RTT and SAT, with a high-PM2.5 concentration air mass from the upstream regions transported to the Twain-Hu Basin (THB) by the strong northerly winds with a southward advance of cold air mass. The average rate of increase in the PM2.5 concentration of the RTT (12.5 μg/m3/h) is more than that of the SAT (5.7 μg/m3/h). However, meteorological conditions evolved differently in RTT and SAT. For RTT with the fast passage of cold air, the THB is located behind the strong cold front during the later stage, and these events rapidly ended owing to the northerly winds and wet scavenging of precipitation. For SAT with the slow cold air accumulation, the THB is mainly occupied by a weak high-pressure system with low wind speeds, strong descending airflow, and a low atmospheric boundary layer height, trapping the air pollutants accumulated in the THB and resulting in persistent heavy pollution incidents. This study provides new insights into the meteorological mechanism underlying the formation of severe air pollution episodes over a receptor region owing to the regional transport patterns of air pollutants.

Meteorology impact on PM<sub>2.5</sub> change over a receptor region in the regional transport of air pollutants: observational study of recent emission reductions in central China

Abstract. As an important issue in atmospheric environment, the contributions of anthropogenic emissions and meteorological conditions to air pollution have been little assessed over receptor regions in regional transport of air pollutants. In the present study of 5-year observations and modeling, we targeted the Twain-Hu Basin (THB), a large region of heavy PM2.5 pollution in central China, to assess the effects of meteorology on PM2.5 change over a receptor region in the regional transport of air pollutants. Based on observations of environment and meteorology over 2015–2019, the Kolmogorov–Zurbenko (KZ) filter was performed to decompose the PM2.5 variations into multiple timescale components over the THB, where the short-term, seasonal and long-term components accounted for, respectively, 47.5 %, 41.4 % and 3.7 % of daily PM2.5 changes. The short-term and seasonal components dominated the day-to-day PM2.5 variations with long-term component determining the change trend of PM2.5 concentrations over recent years. As the emission- and meteorology-related long-term PM2.5 components over the THB were identified, the meteorological contribution to the declining PM2.5 trend presented a distinct spatial pattern over the THB with northern positive rates up to 61.92 % and southern negative rates down to −24.93 %. The opposite effects of meteorology on PM2.5 pollution could accelerate and offset the effects of emission reductions in the northern and southern THB, which is attributed to the upwind diffusion and downward accumulation of air pollutants over receptor regions in regional PM2.5 transport. It is noteworthy that the increasing conversion efficiencies of SO2 and NO2 to sulfate and nitrate for secondary PM2.5 could have offset the effect of PM2.5 emission reduction on air pollution in the THB during recent years, revealing the enhancing contribution of gaseous precursor emissions to PM2.5 concentrations under control of anthropogenic emissions of PM2.5 and the gaseous precursors over receptor regions in the regional transport of air pollutants. Our results highlight the effects of emission mitigation and meteorological changes on the source–receptor relationship of the regional transport of air pollutants with the implication of long-range transport of air pollutants for regional and global environment changes.

Meteorological mechanism of regional PM2.5 transport building a receptor region for heavy air pollution over Central China

Regional transport of air pollutants is a key factor affecting air quality over the receptor region, where the meteorological mechanism of regional transport influence has not been fully understood. The Twain-Hu Basin (THB) in central China is located in the downwind area of major pollutant sources over central and eastern China (CEC) under the East Asian winter monsoonal winds. To understand the meteorological mechanism of regional PM2.5 transport building a receptor region for heavy air pollution, an ensemble of 8 typical heavy air pollution events with regional PM2.5 transport in January of 2015–2019 were selected objectively by using the MV-EOF (multivariable empirical orthogonal function) decomposition with multi-source observations, and the meteorological configurations driving the regional PM2.5 transport and building a receptor in the THB with heavy air pollution were investigated. The results showed that PM2.5 from the source area in northern China to the THB was actuated by cold air southward invasion with strong northerly winds in the lower troposphere, and the vertical structure of atmospheric circulation was characterized with the typical pattern of southward advance of cold front with the cold air confronting the warm air mass over the THB area. The warm air mass and the windward side of THB's basin terrain formed a “barrier” in regional transport of PM2.5 over central China, which were conducive to accumulating PM2.5 for heavy air pollution in the THB. Furthermore, an abnormal warm air layer in the middle troposphere acted as the upper “warm lid”, suppressing the vertical PM2.5 diffusion over the receptor region. With such the 3-D atmospheric structure, a key receptor region in the THB for heavy air pollution was built in regional PM2.5 transport over China. These findings could enrich the scientific understanding of the meteorological mechanism on air pollution with regional transport of source-receptor air pollutants in atmospheric environment change.

Variations and Sources of Organic Aerosol in Winter Beijing under Markedly Reduced Anthropogenic Activities During COVID-2019.

The COVID-19 outbreak provides a “controlled experiment” to investigate the response of aerosol pollution to the reduction of anthropogenic activities. Here we explore the chemical characteristics, variations, and emission sources of organic aerosol (OA) based on the observation of air pollutants and combination of aerosol mass spectrometer (AMS) and positive matrix factorization (PMF) analysis in Beijing in early 2020. By eliminating the impacts of atmospheric boundary layer and the Spring Festival, we found that the lockdown effectively reduced cooking-related OA (COA) but influenced fossil fuel combustion OA (FFOA) very little. In contrast, both secondary OA (SOA) and O3 formation was enhanced significantly after lockdown: less-oxidized oxygenated OA (LO-OOA, 37% in OA) was probably an aged product from fossil fuel and biomass burning emission with aqueous chemistry being an important formation pathway, while more-oxidized oxygenated OA (MO-OOA, 41% in OA) was affected by regional transport of air pollutants and related with both aqueous and photochemical processes. Combining FFOA and LO-OOA, more than 50% of OA pollution was attributed to combustion activities during the whole observation period. Our findings highlight that fossil fuel/biomass combustion are still the largest sources of OA pollution, and only controlling traffic and cooking emissions cannot efficiently eliminate the heavy air pollution in winter Beijing.

Regional transport patterns for heavy PM2.5 pollution driven by strong cold airflows in Twain-Hu Basin, Central China

Passages of strong cold airflow can quickly purge air pollutants over source region, e.g. North China Plain (NCP), while regional transport of air pollutants from source region would exacerbate air quality in the receptor region of Twain-Hu Basin (THB), due to the cold airflows modulated by atmospheric circulations over China. In this study, nine heavy pollution events (HPEs) in THB, Central China during the winters of 2015–2019 were identified for investigating the regional transport patterns of air pollutants from Central and Eastern China (CEC) to THB driven by strong cold airflows. Once regional transport of air pollutants from the CEC source regions was triggered by strong northerlies, PM2.5 pollution parcels then converged in the downwind THB receptor region through a dominant dynamic mechanism for most HPEs (66.7%) with downward airflow, which generally resulted in long durations of high PM2.5 concentrations and the impacted areas were modulated by synoptic patterns over the CEC. Non-dominant transport patterns were also observed with a cyclonic circulation of high wind speeds or a local stagnation of cold air over the THB, contributing to the highest and shortest PM2.5 peak or small impacted areas limited to the northern THB, respectively. It was estimated that the regional transport of PM2.5 from non-local regions accounted for 51.0%–85.7% of PM2.5 concentrations during the HPEs at Wuhan, revealing the dominance of regional transport of air pollutants from CEC to the THB with the meteorological drivers.

Influence of transboundary air pollution and meteorology on air quality in three major cities of Anhui Province, China

The variations of criteria air pollutants (CAPs), the influence of transboundary air pollution, and the meteorological effects on air quality in Hefei, Fuyang, and Suzhou were investigated during 2015–2019. In Hefei, continuous decreases in the concentrations of PM2.5 (5.5 μg/m3 yr−1) and PM10 (6.7 μg/m3 yr−1) were observed over time. PM2.5 and PM10 concentrations also exhibited substantial downward trends in Fuyang (PM2.5: 9.4 μg/m3 yr−1; PM10: 13.5 μg/m3 yr−1) and Suzhou (PM2.5: 10.9 μg/m3 yr−1; PM10: 9.1 μg/m3 yr−1) during 2017–2019. However, they increased during 2015–2017. NO2 decreased in Suzhou (9.3 μg/m3 yr−1) and Hefei (5.2 μg/m3 yr−1) during 2017–2019, and in Fuyang (5.2 μg/m3 yr−1) over 2016–2018. Alternatively, O3 increased by 8.3, 8.2, and 5.2 μg/m3 yr−1 in Fuyang, Suzhou, and Hefei, respectively. Despite the reductions, PM2.5 and PM10 still substantially exceeded the annual Grade I and Grade II CAAQS. The relationships between the pollutants and meteorological variables were relatively similar with the correlations varying by season. High primary pollutants concentrations were attributed to poorer dispersion, while elevated temperatures and greater solar radiation were related to higher O3 concentrations. The results of 120-h backward trajectories showed that the study areas were dominantly influenced by the northwesterly and northerly winds. Thus, elevated concentrations of most air pollutants observed in the study areas were associated with transboundary regional transport of air pollution from neighboring cities and provinces. This study shows that effective emissions control strategies that would simultaneously reduce PM2.5 and O3 across the cities in Anhui Province and the surrounding provinces are urgently needed.

Sulfur and Nitrogen Oxides in the Atmosphere of Lake Baikal: Sources, Automatic Monitoring, and Environmental Risks

This paper analyzes the results of the automatic (in situ) recording of the regional transport of pollutants from the large regional coal-fired thermal power plants in the atmospheric boundary layer above the southern basin of Lake Baikal. Due to high stacks (about 200 m), emissions from large thermal power plants rise to the altitudes of several hundreds of meters and spread over long distances from their source by tens and hundreds of kilometers. The continuous automatic monitoring of the atmosphere in the southern basin of Lake Baikal on top of the coastal hill (200 m above the lake) revealed the transport of a large number of sulfur oxides and nitrogen oxides in the form of high-altitude plumes from thermal power plants of the large cities located 70 to 100 km to the northwest of the lake (Irkutsk and Angarsk). The consequence of such transport is the increased acidity of precipitation in the southern basin of Lake Baikal and the additional influx of biogenic nitrogen compounds to the lake ecosystem. The spatial scale and possible risks of such regional transport of air pollution for the lake ecosystem require further closer study.

Characteristics of Atmospheric Particulate Matter Pollution and the Unique Wind and Underlying Surface Impact in the Twain-Hu Basin in Winter

To understand the recent characteristics of atmospheric environmental changes in the Twain-Hu(Hunan-Hubei) Basin, including the middle reaches of the Yangtze River, this paper uses near-surface PM2.5 and PM10 observational data for the Twain-Hu Basin in the winters of 2015 to 2019, combined with wind-speed, topography, the normalized difference vegetation index(NDVI), and other datasets. The results show that:① PM2.5 pollution occurred frequently in the winters of 2015-2019 in the Twain-Hu Basin, and Xiangyang and Jingmen in the western part of the basin, experience PM2.5 pollution on an average of 62 and 61 days in winter(PM2.5>75 μg·m-3). And the heavy pollution days in Xiangyang reached 19 more days(PM2.5>150 μg·m-3), indicating that the Twain-Hu Basin is an air pollution center in the middle reaches of the Yangtze River Basin; ② Spatially, pollution in the Twain-Hu Basin is heavier in the northwest than in the southeast, and in the urban agglomeration, which is mainly related to the regional transport of air pollutants by the winter monsoon and the high levels of emissions from urban areas; ③ A "U-shaped" nonlinear relationship was observed between near-surface wind speeds and PM2.5 and PM10 concentrations. The inflection points of PM2.5 and PM10 concentrations were 153 and 210 μg·m-3, respectively. This implies that the accumulation of local atmospheric particulate matter in the Twain-Hu Basin dominates light/medium pollution, while the regional transport of air pollutants dominates period of severe pollution; and ④ PM2.5 and PM10 in winter were significantly negatively correlated with terrain height and the NDVI, which reflects the atmospheric environmental effects of topography and urbanization.

Multilevel air quality evolution in Shenyang: Impact of elevated point emission reduction

Visibility observed at different altitudes is favorable to understand the causes of air pollution. We conducted 4-years of observations of visibility at 2.8 and 60 m and particulate matter (PM) concentrations from 2015 to 2018 in Shenyang, a provincial city in Northeast China. The results indicated that visibility increased with the increasing height in winter (especially at night), and decreased with height in summer (especially at the daytime). PM concentration exhibited opposite vertical variation to visibility, reflecting that visibility degrades with the increase of aerosol concentration in the air. The radiosonde meteorological data showed that weak turbulence in the planetary boundary layer (PBL) in winter favored aerosols’ accumulation near the surface. Whereas in summer, unstable atmospheric conditions, upper-level moister environment, and regional transport of air pollutants resulted in the deterioration of upper-level visibility. Inter-annual variation in the two-level visibility indicated that the upper-level visibility improved more significantly than low-level visibility, much likely due to the reduction in emission of elevated point sources in Shenyang. Our study suggested that strengthening the control of surface non-point emissions is a promising control strategy to improve Shenyang air quality.