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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.

Currently, mitigating severe particle pollution in autumn and winter is the key to further improving the air quality of China. The source contributions and transboundary transport of fine particles (PM2.5) in pollution episodes are closely related to large-scale or synoptic-scale atmospheric circulation. How to effectively reduce emissions to control haze pollution under different synoptic conditions is rarely reported. In this study, we classify the synoptic conditions over central China from 2013 to 2018 by using Lamb–Jenkinson method and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Final (FNL) operational global analysis data. The effectiveness of emission control to reduce PM2.5 pollution during winter haze episodes under potential synoptic controls is simulated by GEOS-Chem model. Among the 10 identified synoptic patterns, four types account for 87 % of the total pollution days. Two typical synoptic modes are characterized by low surface wind speed and stable weather conditions or high relative humidity (A or C type) over central China due to a high-pressure system or a southwest trough and low-pressure system, blocking pollutants dispersion. Sensitivity simulations show that these two heavy pollution processes are mainly contributed by local emission sources with ∼82 % for A type and ∼85 % for C type, respectively. The other two patterns lead to pollution of transport characteristics affected by northerly or southerly winds (NW or SW type), carrying air pollution from northern or southern China to central China. The contribution of pollution transmission from northern and southern China is 36.9 % and 7.6 %, respectively of PM2.5, and local emission sources contribute 41 % and 69 %, respectively. We also estimate the effectiveness of emission reduction in these four typical severe pollution synoptic processes. By only reducing SO2 and NOx emission and not controlling NH3, the enhanced nitrate counteracts the effect of sulfate reduction on PM2.5 mitigation, with a less than 4 % decrease in PM2.5. In addition, to effectively mitigate haze pollution of NW- and SW-type synoptic-controlled episodes, local emission control actions should be in coordination with regional collaborative actions.

The regional transport of air pollutants, controlled by emission sources and meteorological factors, results in a complex source–receptor relationship of air pollution change. Wuhan, a metropolis in the Yangtze River middle basin (YRMB) of central China, experienced heavy air pollution characterized by hourly PM2.5 concentrations reaching 471.1 µg m−3 in January 2016. To investigate the regional transport of PM2.5 over central eastern China (CEC) and the meteorological impact on wintertime air pollution in the YRMB area, observed meteorological and other relevant environmental data from January 2016 were analyzed. Our analysis presented noteworthy cases of heavy PM2.5 pollution in the YRMB area with unique “non-stagnant” meteorological conditions of strong northerly winds, no temperature inversion, and additional unstable structures in the atmospheric boundary layer. This unique set of conditions differed from the stagnant meteorological conditions characterized by near-surface weak winds, air temperature inversion, and stable structure in the boundary layer that are typically observed in heavy air pollution over most regions in China. The regional transport of PM2.5 over CEC aggravated PM2.5 levels, thus creating heavy air pollution in the YRMB area. This demonstrates a source–receptor relationship between the originating air pollution regions in CEC and the receiving YRMB region. Furthermore, a backward trajectory simulation using a Flexible Particle dispersion (FLEXPART) Weather Research and Forecasting (WRF) model to integrate the air pollutant emission inventory over China was used to explore the patterns of regional transport of PM2.5 governed by the strong northerly winds in the cold air activity of the East Asian winter monsoon season. It was estimated that the regional transport of PM2.5 from non-local air pollutant emissions contributes more than 65 % of the PM2.5 concentrations to the heavy air pollution in the YRMB region during the study period, revealing the importance of the regional transport of air pollutants over China as a causative factor of heavy air pollution over the YRMB area.

As an important issue in atmospheric environment, the contributions of anthropogenic emissions and meteorological conditions to air pollution have been few assessed over the receptor region in regional transport of air pollutants. In this study on observations of environment and meteorology over 2015–2019, the Kolmogorov–Zurbenko (KZ) filter was performed to decompose the PM2.5 variations into multi-time scale components over the Twain-Hu Basin (THB), a receptor region in regional transport of air pollutants in central China, where the short-term, seasonal and long-term components accounted for respectively 47.5 %, 41.4 % and 3.7 % to 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. The emission- and meteorology-related long-term PM2.5 components over the THB were identified. The meteorological contribution to PM2.5 declining trend presented the 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 the receptor region 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 offset the effects 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 with controlling anthropogenic emissions of PM2.5 and the gaseous precursors over the receptor region in regional transport of air pollutants.

Anthropogenic emission, meteorological conditions, and regional transport are the three major factors influencing heavy air pollution in China. The Hunan and Hubei provinces in the middle Yangtze River region border China's main air pollution areas, serving as the hub of regional transport of air pollutants. The meteorological formation mechanism of regional transport of air pollutants on heavy air pollution in the Hunan and Hubei provinces still remain urgent to be addressed in depth. In this study, multivariate empirical orthogonal function (MV-EOF) analysis was performed to objectively select eight typical heavy pollution events in the two provinces that occured in January 2015–2019. Based on the regional surface environment, meteorological network data, atmospheric sounding data, ERA-interim reanalysis data, and a numerical simulation experiment, this study investigated the pattern of regional transport of air pollutants in the two provinces and its mechanism of regional meteorological conditions. The results showed that transporting air pollutants mainly passed through two transport pathways, namely the Nanxiang Basin-Yunmeng Plain pathway and the Dabie Mountain's Hilly Area-Yunmeng Plain pathway, existing anomalous near-surface northerly winds in the two provinces and their upstream area accompanied by southward penetration of a shallow cold layer, all of which jointly provide a dynamic condition for regional air pollutant transport. The weak cold-air mass degenerated as it passed through the Hunan–Hubei Plain, causing warm air to accumulate in the near-surface layer of the downstream area, where winds slowed down and converged, buffering the air pollutant transport and resulting in pollutants accumulation; the near-surface atmosphere of the Hunan and Hubei provinces was a non-stagnant condition (dry intrusion of cold air, anomalous northerly winds, and positive anomalies of boundary-layer height), which is conducive to the horizontal transport of air pollutants. However, the mid-high layers, characterized by temperature inversion and the presence of a warm lid, had a stable stratification, inhibiting the diffusion of air pollutants to the upper layers; there is an obvious longitudinal vertical circulation above the Hunan–Hubei Plain, which results in the sinking and accumulation of air pollutants, thereby promoting rapid accumulation of air pollutants in the Hunan and Hubei provinces. In addition, extended empirical orthogonal function (EEOF) analysis was performed, revealing a quasi-4-d periodic oscillation pattern of air pollutants transport in the Hunan and Hubei provinces, which provides a reference for early prediction of its regional transport. The findings are of practical value in broadening the scientific understanding of the differences in the formation mechanism of heavy atmospheric pollution between the various regions of China and promoting environmental and ecological protection of the middle Yangtze Basin.

Currently solving the severe particle pollution in autumn and winter is the key to further improve the air quality of China. The source contributions and transboundary transport of fine particles (PM2.5) in pollution episodes are closely related to large-scale or synoptic-scale atmospheric circulation. Under different synoptic conditions, how to effectively reduce emissions to control haze pollution is rarely reported. In this study, we classify the synoptic conditions over Central China from 2013 to 2018 by using Lamb-Jenkension method and the NCEP/NCAR FNL operational global analysis data. The effectiveness of emission control to reduce PM2.5 pollution during winter haze episodes under potential synoptic controls is simulated by GEOS-Chem model. Among the ten identified synoptic patterns, four types account for 87 % of the total pollution days. Two typical synoptic modes of them are characterized by small surface wind speed and stable weather conditions/high relative humidity (A/C-type) over Central China due to a high-pressure system/a southwest trough low-pressure system, blocking pollutants dispersion. Sensitivity simulations show that these two heavy pollution processes are mainly contributed by local emission sources with ~82 % for A-type and ~85 % for C-type, respectively. The other two patterns lead to pollution of transportation characteristics affected by northerly/southerly winds (NW/SW-type), carrying air pollution from northern/southern China to Central China. The contribution of pollution transportation from North/South China is 36.9 %/7.6 % of PM2.5 and local emission sources contribute 41 %/69 %. We also estimate the effectiveness of emission reduction in these four typical severe pollution synoptic processes. By only reducing SO2 and NOx emission and not controlling NH3, the enhanced nitrate counteracts the effect of sulfate reduction on PM2.5 mitigations with less than 4% decrease in PM2.5. In addition, to effectively mitigate haze pollution in NW/SW-type synoptic controlled episodes, local emission control actions should be in coordination with regional collaborative actions.

Regional transport of air pollutants controlled by both emission sources and meteorological factors results in a complex source-receptor relationship of air pollution change. Wuhan, a metropolis in the Yangtze River Middle Basin (YRMB) of central China experienced heavy air pollution characterized by excessive PM2.5 concentrations reaching 471.1 μg m−3 in January 2016. In order to investigate the regional transport of PM2.5 over China and the meteorological impact on wintertime air pollution in the YRMB area, observational meteorological and other relevant environmental data from January 2016 were analyzed. Our analysis presented the noteworthy cases of heavy PM2.5 pollution in the YRMB area with the unique “non-stagnant” meteorological conditions of strong northerly winds, no temperature inversion and additional unstable structures in the atmospheric boundary layer. This unique set of conditions differed from the stagnant meteorological conditions characterized by near-surface weak winds, air temperature inversion, and stable structure in the boundary layer observed in heavy air pollution over most regions in China. The regional transport of PM2.5 over central-eastern China aggravated PM2.5 levels present in the YRMB area, thus demonstrating the source-receptor relationship between the originating air pollution regions in central-eastern China and the receiving YRMB regions. Furthermore, a backward trajectory simulation using FLEXPART-WRF to integrate the air pollutant emission inventory over China was used to explore the patterns of regional transport of PM2.5 governed by the strong northerly winds in the cold air activity of the East Asian winter monsoon over central-eastern China, which contributes markedly to the heavy PM2.5 pollution in the YRMB area. It was estimated that the regional transport of PM2.5 of non-local air pollutant emissions could contribute more than 65 % of the PM2.5 concentrations to the heavy air pollution in the YRMB region during the study period, revealing the importance of the regional transport of air pollutants over central-eastern China in the formation of heavy air pollution over the YRMB region.

Black carbon (BC), from the incomplete combustion sources (mainly fossil fuel, biofuel and open biomass burning), is chemically inertness and optical absorbance in the atmosphere. It has significant impacts on global climate, regional air quality, and human health. During the transportation, its physical-chemical characteristics, optical properties and sources would change dramatically. To investigate the BC properties (i.e., mass concentration, sources and optical properties) during the intra-regional transport between the south edge of North China Plain (SE-NCP) and Central China (CC), simultaneous observations of BC at a megacity (Wuhan, WH) in CC, three borderline cities (Xiangyang, XY, Suixian, SX and Hong'an, HA, distributing from the west to east) between SE-NCP and CC and a city (Luohe, LH) in SE-NCP were conducted during the typical winter haze episodes. Using Aethalometer, the highest equivalent BC (eBC) mass concentrations and aerosol absorption coefficients (σabs) were found in the city (LH) at SE-NCP, followed by the borderline cities (XY, SX and HA) and megacity (WH). The levels, sources, optical properties (i.e., σabs and absorption Ångström exponent, AAE) and geographic origins of eBC were different between clean and pollution episodes. Compared to clean days, the higher eBC levels (increased by 26.4–163 %) and σabs (increased by 18.2–236 %) were found during pollution episodes due to more combustion of fossil fuel (contributing for 51.1–277 %), supported by the decreased AAE (by 7.40–12.7 %). Non-parametric wind regression (NWR) and concentration-weighted trajectory (CWT) results showed that the geographic origins of biomass burning (BCbb) and fossil fuel (BCff) combustion derived BC were different. Based on cluster analysis of trajectories, air parcels from south direction dominated for border sites during clean days, with contributions of 46.0–58.2 %, while trajectories from the northeast had higher contributions (37.5–51.2 %) during pollution episodes. At the SE-NCP site (LH), transboundary influences from south direction (CC) exhibited more frequent impact (with the air parcels from this direction contributed 47.8 % of all the parcels) on the ambient eBC levels during pollution episodes. At WH, eBC was mainly from the northeast transport route during the whole observation period. Two transportation cases showed that from upwind to downwind direction, the mass concentrations of eBC, BCbb and BCff all increased, while AAE decreased. This study highlighted that intra-regional prevention and control for dominated sources of specific sites should be considered to improve the regional air quality.