The Twain-Hu basin (THB), covering the lower plain of Hubei and Hunan provinces in Central China, has experienced severe air pollution in recent years. However, the terrain effects of such sub-basin on air quality over the THB have been incomprehensibly understood. A heavy PM_2.5 pollution event occurred over the THB during 4–10 January 2019. By using the observations and WRF-Chem simulations, we investigated the underlying mechanisms of sub-basin effects on the air pollution with several sensitivity experiments. Observationally, air pollution in the western THB urban area with an average PM_2.5 concentration of 189.8 μg m⁻³, which was more serious than the eastern urban area with the average PM_2.5 concentration of 106.3 μg m⁻³, reflecting a different influence of topography on air pollution over the THB. Simulation results revealed that the terrain effect can contribute 12.0% to increasing the PM_2.5 concentrations in the western THB, but slightly mitigate the pollution extent in the eastern THB with the contribution of −4.6% to PM_2.5 during the heavy pollution episode. In particular, the sub-basin terrain was conducive to the accumulation of PM_2.5 by regional transport with the contribution of 39.1 %, and contrarily lowered its local pollution by −57.0% via the enhanced atmospheric boundary layer height and ventilation coefficients. Given a heavy air pollution episode occurring over the THB, such inverse contribution of terrain effects reflected a unique importance of sub-basin topography in regional transport of air pollutants for air pollution in central China.

The extent of PM_2.5 pollution has reduced in traditional polluted regions such as the North China Plain (NCP), Yangtze River Delta (YRD), Sichuan Basin (SB), and Pearl River Delta (PRD) over China in recent years. Despite this, the Twain-Hu Basin (THB), which covers the lower flatlands in Hubei and Hunan provinces in central China, was found to be a high PM_2.5 pollution region, with annual mean PM_2.5 concentrations of 41–63 μg·m⁻³, which is larger than the values in YRD, SB, and PRD during 2014–2019, and high aerosol optical depth values (>0.8) averaged over 2000–2019 from the MODIS products. Heavy pollution events (HPEs) are frequently observed in the THB, with HPE-averaged concentrations of PM_2.5 reaching up to 183–191 μg·m⁻³, which exceeds their counterparts in YRD, SB, and PRD for 2014–2019, highlighting the THB as a center of heavy PM_2.5 pollution in central China. During 2014–2019, approximately 65.2% of the total regional HPEs over the THB were triggered by the regional transport of PM_2.5 over Central and Eastern China (CEC). This occurred in view of the co-existing HPEs in the NCP and the THB, with a lag of almost two days in the THB-PM_2.5 peak, which is governed by the strong northerlies of the East Asian monsoon (EAM) over CEC. Such PM_2.5 transport from upstream source regions in CEC contributes 60.3% of the surface PM_2.5 pollution over the THB receptor region. Hence, a key PM_2.5 receptor of the THB in regional pollutant transport alters the distribution patterns of PM_2.5 pollution over China, which is attributable to the climate change of EAMs. This study indicates a complex relationship between sources and receptors of atmospheric aerosols for air quality applications.

Particle size distribution, water soluble ions, and black carbon (BC) concentration in a long-term haze-fog episode were measured using a wide-range particle spectrometer (WPS), a monitor for aerosols and gases (MARGA), and an aethalometer (AE33) in Nanjing from 16 to 27 November, 2018. The observation included five processes of clean, mist, mix, haze, and fog. Combined with meteorological elements, the HYSPLIT model, and the IMPROVE model, we analyzed the particle size distribution, chemical composition, and optical properties of aerosols in different processes. The particle number size distribution (PNSD) in five processes differed: It was bimodal in mist and fog and unimodal in clean, mix, and haze. The particle surface area size distribution (PSSD) in different processes showed a bimodal distribution, and the second peak of the mix and fog processes shifted to a larger particle size at 480 nm. The dominant air masses in five processes differed and primarily originated in the northeast direction in the clean process and the southeast direction in the haze process. In the mist, mix, and fog processes local air masses dominated. NO₃⁻ was the primary component of water soluble ions, with the lowest proportion of 45.6% in the clean process and the highest proportion of 53.0% in the mix process. The ratio of NH₄⁺ in the different processes was stable at approximately 23%. The ratio of SO₄²⁻ in the clean process was 26.2%, and the ratio of other processes was approximately 20%. The average concentration of BC in the fog processes was 10,119 ng·m⁻³, which was 3.55, 1.80, 1.60, and 1.46 times that in the processes of clean, mist, mix, and haze, respectively. In the different processes, BC was primarily based on liquid fuel combustion. NO₃⁻, SO₄²⁻, and BC were the main contributors to the atmospheric extinction coefficient and contributed more than 90% in different processes. NO₃⁻ contributed 398.43 Mm⁻¹ in the mix process, and SO₄²⁻ and BC contributed 167.90 Mm⁻¹ and 101.19 Mm⁻¹, respectively, during the fog process.

To clarify the aerosol optical properties under different pollution levels and their impacting factors, hourly organic carbon (OC), elemental carbon (EC), and water-soluble ion (WSI) concentrations in PM_2.5 were observed by using monitoring for aerosols and gases (MARGA) and a semicontinuous OC/EC analyzer (Model RT-4) in Wuhan from 9 to 26 January 2018. The aerosol extinction coefficient (b_ext) was reconstructed using the original Interagency Monitoring of Protected Visual Environment (IMPROVE) formula with a modification to include sea salt aerosols. A good correlation was obtained between the reconstructed b_ext and measured b_ext converted from visibility. b_ext presented a unimodal distribution on polluted days (PM_2.5 mass concentrations > 75 μg⋅m⁻³), peaking at 19:00. b_ext on clean days (PM_2.5 mass concentrations < 75 μg⋅m⁻³) did not change much during the day, while on polluted days, it increased rapidly starting at 12:00 due to the decrease of wind speed and increase of relative humidity (RH). PM_2.5 mass concentrations, the aerosol scattering coefficient (b_scat), and the aerosol extinction coefficient increased with pollution levels. The value of b_ext was 854.72 Mm⁻¹ on bad days, which was 4.86, 3.1, 2.29, and 1.28 times of that obtained on excellent, good, acceptable, and poor days, respectively. When RH < 95%, b_ext exhibited an increasing trend with RH under all pollution levels, and the higher the pollution level, the bigger the growth rate was. However, when RH > 95%, b_ext on acceptable, poor and bad days decreased, while b_ext on excellent and good days still increased. The overall b_ext in Wuhan in January was mainly contributed by NH₄NO₃ (25.2%) and organic matter (20.1%). The contributions of NH₄NO₃ and (NH₄)₂SO₄ to b_ext increased significantly with pollution levels. On bad days, NH₄NO₃ and (NH₄)₂SO₄ contributed the most to b_ext, accounting for 38.2% and 27.0%, respectively.

As the new core region of the haze pollution, the terrain effect of sub-basin and water networks over the Twin-Hu Basin (THB) in the Yangtze River Middle-Reach (YRMR) had great impacts on the variations and distributions of air pollutants. In this study, trace gases (NH₃, HNO₃, and HCl), water-soluble ions (WSIs), organic carbon (OC), and elemental carbon (EC) were measured in PM_2.5 from 9 January to 27 January 2018, in Wuhan using monitoring for aerosols and gases (MARGA) and a semi-continuous OC/EC analyzer (Model RT-4). The characteristics of air pollutants during a haze episode were discussed, and the PM_2.5 sources were quantitatively analyzed on haze and non-haze days using the principal component analysis/absolute principal component scores (PCA/APCS) model. The average PM_2.5 concentration was 122.61 μg·m⁻³ on haze days, which was 2.20 times greater than it was on non-haze days. The concentrations of secondary water soluble ions (WSIs) including NO₃⁻, SO₄²⁻, and NH₄⁺ increased sharply on haze days, which accounted for 91.61% of the total WSIs and were 2.43 times larger than the values on non-haze days. The heterogeneous oxidation reactions of NO₂ and SO₂ during haze episodes were proven to be the major sources of sulfate and nitrate in PM_2.5. On haze days, the concentrations of EC, primary organic carbon (POC), and secondary organic carbon (SOC) were 1.68, 1.69, and 1.34 times larger than those on non-haze days, the CO, HNO₃, and NH₃ concentrations enhanced and relatively low SO₂, O₃, and HNO₂ levels were observed on haze days. The diurnal variations of different pollutants distinctly varied on haze days. The PM_2.5 in Wuhan primarily originated from the secondary formation, combustion, dust, industry, and vehicle exhaust sources. The source contributions of the secondary formation + combustion sources to PM_2.5 on haze days were 2.79 times larger than the level on non-haze days. The contribution of the vehicle exhaust + combustion source on haze days were 0.59 times the value on non-haze days. This description is supported by a summary of how pollutant concentrations and patterns vary in the THB compared to the variations in other pollution regions in China, which have been more completely described.

A long- and large-range heavy dust episode occurred from 3 to 8 May 2017 in China. To explore the impacts of this long-range dust transport episode on the chemical compositions and size distributions of urban aerosols, such instruments as an online analyzer for monitoring aerosols and gases (MARGA) and a wide-range particle spectrometer (WPS) were mainly used to monitor chemical components, such as PM_2.5 and aerosol size distributions in the range of 10 nm to 10 μm, in Nanjing in this study. During the dust episode, the average concentrations of PM_2.5 and PM₁₀ and ions of Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻, NO₃⁻, and NH₄⁺ were 66.2, 233.9, and 1.1, 1.5, 1.1, 11.4, 7.8 and 4.4 μg·m⁻³, which were 4.4, 5.8, 3.7, 15, 1.38, 1.84, 1.66 and 1.83 times higher than the values observed before the episode and 2.2, 3.3, 5.5, 5.0, 1.57, 1.97, 1.39 and 1.69 times the levels after the episode. The dusts were demonstrated to have differential impacts on the water-soluble gases in the air. During the dust episode, the concentrations of HCl, SO₂ and NH₃ were comparably low, while the HNO₂ and HNO₃ concentrations were high. The diurnal variations in pollutants, including SO₂, HNO₃, Cl⁻, Ca²⁺, Mg²⁺, PM_2.5 and PM₁₀, were strongly impacted by the dust episode. However, those variations in NH_3, NO₃⁻, SO₄²⁻ and NH₄⁺ were only slightly influenced. Pollutants were distinctively featured in the various dust stages. The concentration of HNO₂ was relatively high in the earliest stage but was substituted by those of SO₂, PM₁₀, PM_2.5, Ca²⁺, Mg²⁺ HNO₃ and Cl⁻ in the explosion stage. The aerosol number concentrations exhibited unimodal distributions in the earliest and explosion stages but showed bimodal distributions in the duration and dissipation stages. Additionally, the aerosol size distributions were observed to shift to larger particle segments in different dust stages. The surface area concentrations exhibited four peaks in different dust stages and exhibited trimodal distributions in the non-dust episode. The surface area concentration of fine particles first increased during the earliest stage, while that of coarse particles first decreased during the dissipation stage.