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The Sichuan Basin (SCB) with complex mountain-basin topography, located immediately to the east of the Tibetan Plateau (TP) in southwest China, is identified as a region with severe PM2.5 pollution over China. To comprehensively understand the SCB terrain effect on the atmospheric environment change, we investigated the effect degree and meteorological mechanism of mountain-valley winds on wintertime PM2.5 in the western SCB, thermally driven mountain-valley winds between the mountains of eastern TP and the western edge of SCB based on near-surface observations of PM2.5 and the ERA5 reanalysis data of meteorology. The results show that the western SCB exhibits a significant diurnal change of mountain-valley winds, shifting between daytime easterly flows and nighttime westerly flows over the western edge of SCB. The frequency of the mountain-valley winds was accounted for 39% of the study’s duration, with the valley and mountain wind-controlling periods being from 2:00 pm to 5:00 pm and 01:00 am to 05:00 am in local time, respectively, and the remaining time being the transition period. Notably, a reduction of 4.5~20.7μg m-3 in near-surface PM2.5 concentrations averaged over the western edge of SCB, decreasing 6~74% in the PM2.5 pollution during the days of mountain-valley winds, which indicates that the mountain-valley winds could alleviate wintertime near-surface PM2.5 in improving air quality over the western SCB. The easterly wind speed was uplifted by 18% during the valley wind-controlling period, corresponding to a 22% reduction in the near-surface PM2.5 levels driven by the mountain-valley winds. The westerly wind speed increased by 50% at the mountain wind-controlling period with a 20% reduction in PM2.5, reflecting the attenuating effect of the favorable atmospheric diffusion conditions on PM2.5 pollution over the western SCB. The daytime noticeably enhanced easterly wind and the strong updraft flows over the eastern slope of TP promoted the dilution and diffusion of air pollutants over the western edge of SCB, and the nocturnal downhill flows along the slope in the westerly winds on the western edge of SCB brought clean TP air to the polluted SCB region in southwest China.

Regional PM

In recent years, ozone pollution becomes a serious environmental issue in China. A good understanding of source-receptor relationships of ozone transport from aboard and inside China is beneficial to mitigating ozone pollution there. To date, these issues have not been comprehensively assessed, especially for highly polluted regions in the central and eastern China (CEC), including the North China Plain (NCP), Twain-Hu region (THR), Yangtze River Delta (YRD), Pearl River Delta (PRD), and Sichuan Basin (SCB). Here, based on simulations over 2013-2020 from a well-validated chemical transport model, GEOS-Chem, we show that foreign ozone accounts for a large portion of surface ozone over CEC, ranging from 25.0% in THR to 39.4% in NCP. Focusing on transport of domestic ozone between the five regions in CEC, we find that atmospheric transport can largely modulate regional interactions of ozone pollution in China. At the surface, THR receives the largest amount of ozone from the other four regions (54.2% of domestic ozone in the receptor region, the same in below), followed by PRD (32.3%), SCB (26.7%), YRD (21.1%), and NCP (18.0%). Meanwhile, YRD exports largest amount of ozone to the other regions, ranging from 8.9% in SCB to 28.4% in THR. Although SCB is relatively isolated and thus impacts NCP, YRD, and PRD weakly (2.2%), export of SCB ozone to THR reaches 9.3%. The regional ozone transport over CEC, occurring mostly in the lower troposphere, is mainly modulated by the East Asian monsoon circulations, proximity between source and receptor regions, seasonal changes of ozone production, and topography.

The terrain effect on atmospheric environment is poorly understood in particular for the polluted region with underlying complex topography. Therefore, this study targeted the Sichuan Basin (SCB), a deep basin with severe PM

Radar data are essential to convection nowcasting and nudging-based radar data assimilation through diabatic initialization is one of the most effective approaches for forecasting convective systems with numerical weather prediction (NWP) models, used at several advanced global weather centers. It is desired to assess the uncertainty and physical consistency of this assimilation process. This paper investigated impacts of relaxation coefficient, radar data update intervals and continuous assimilation time duration and addressed the key issues and possible solutions of the radar data assimilation based on the WRF hydrometeor and latent heat nudging (HLHN) developed at the National Center for Atmospheric Research (NCAR). It is revealed that excessively large relaxation coefficient forced the model to observations with a tendency greater than the physical terms of the convection, causing the dynamic imbalances and serious convection “ramp-down” right after the free forecast starts. Assimilating high update frequency radar data can make the tendency terms moderate and sustained thereby maintaining the assimilation effect and reducing fortuitous convection. HLHN requires a minimum continuous assimilation duration to contain the initial forced disturbance of the model. For a summer Meiyu precipitation case studied, the minimum duration is ~1 h. Appropriate selection of the HLHN parameters is able to effectively improve the temperature, humidity, and dynamic fields of the model. In addition, several issues still remain to be solved to further enhance HLHN.

The Sichuan Basin (SCB), to the east of the Tibetan Plateau (TP), experiences severe ozone (O

Surface radiation is crucial to atmospheric boundary layer development and air pollution formation. Several studies have revealed that surface radiation plays a vital role in developing the daytime convective boundary layer that controls the explosive growth of PM

Deep basins create a uniquely favorable condition for the formation of air pollution, and the Sichuan Basin (SCB) in Southwest China is such a basin featuring frequent heavy pollution. A wintertime heavy haze pollution event in SCB was studied with conventional and intensive observation data and the WRF-chem model to explore the three-dimensional distribution of PM2.5 for understanding the impact of regional pollutant emissions, basin circulations associated with plateaus, and downwind transport to the adjacent areas. It was found that the vertical structure of PM2.5 over SCB was characterized with a remarkable hollow sandwiched by high PM2.5 layers at heights of 1.5–3 km and the highly polluted near-surface layer. The southwesterlies passed over the Tibetan Plateau (TP) and Yunan-Guizhou Plateau (YGP) resulted in a lee vortex over SCB, which helped form and maintain heavy PM2.5 pollution. The basin PM2.5 was lifted into the free troposphere and transported outside of SCB. At the bottom of SCB, high PM2.5 concentrations were mostly located in the northwest and southern regions. Due to the blocking effect of the plateau terrain on the northeasterly winds, PM2.5 gradually increased from northeast to southwest in the basin. In the lower free troposphere, the high PM2.5 centers were distributed over the northwestern and southwestern SCB areas, as well as the central SCB region. For this event, the regional emissions from SCB contributed 75.4–94.6 % to the surface PM2.5 concentrations in SCB. The SCB emission export was the major source of the PM2.5 over the eastern regions of TP and the northern regions of YGP, with contribution rates of 72.7 % and 70.5 %, respectively, during the dissipation stage of heavy air pollution over SCB, which was regarded as the major pollutant sources affecting atmospheric environment changes in Southwest China.

Long-term variations in aerosol optical properties, types, and radiative forcing over the Sichuan Basin (SCB) and surrounding regions in Southwest China were investigated based on two-decade data (2001−2020) from the Moderate Resolution Imaging Spectroradiometer, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation, and the Santa Barbara DISORT Atmospheric Radiative Transfer model. The results showed that the aerosol optical depth (AOD550nm) in the SCB, a major polluted region in Southwest China, experienced an increasing tendency at a rate of +0.052 yr−1 during 2001–2006; thereafter, it decreased speedy up from −0.020 to −0.058 yr−1 over recent years, whereas the interannual variation in Angstrom exponent (AE470–660nm) presented a persistently increasing trend during 2001–2020, with a rate of +0.014 yr−1. An improved atmospheric environment but an enhanced fine particle contribution to regional aerosols in the SCB was observed. Over the polluted SCB region, the dominant aerosol types were biomass burning/urban industrial and mixed-type aerosols with the proportions of 80.7%–87.5% in regional aerosols, with a higher frequency of clean aerosols in recent years, reflecting an effect of controlling anthropogenic emission in the SCB owing to governmental regulation. By contrast, few changes were observed in the aerosol types and amounts in the eastern Tibetan Plateau (ETP), where clean continental aerosols dominate with high proportion of 93.7% in the clean atmospheric environment. A significant decline in polluted anthropogenic aerosols was observed below 3 km over the SCB, resulting in the regional aerosol extinction coefficients at 532 nm (EC532nm) were declined by −0.22 km−1 from 2013 to 2020. Notably, the decreases in aerosol radiative forcing within the atmosphere were found in the SCB and the adjacent northern Yunnan-Guizhou Plateau (NYGP) and ETP, with −41.6%, −33.7%, and −13.6%, respectively during 2013–2020. This indicates that such an attenuated aerosol heating rate in the atmosphere, caused by aerosol variation, could alter the atmospheric thermal structure over the SCB and surrounding areas for regional changes of environment and climate in recent years.

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.

Using ground-based data, meteorological observations, and atmospheric environmental monitoring data, a comparative analysis of the microphysical and optical properties, and radiative forcing of aerosols was conducted between three stations in different developed environments during a severe air pollution episode during the Spring Festival over Beijing. During the most polluted period, the daily peak values of the aerosol optical depth were ~1.62, ~1.73, and ~0.74, which were about 2.6, 2.9, and 2.1 times higher than the background levels at the CAMS, Xianghe, and Shangdianzi sites, respectively. The daily peak values of the single scattering albedo were ~0.95, ~0.96, and ~0.87. The volume of fine-mode particles varied from 0.04 to 0.21 µm3 µm–2, 0.06 to 0.17 µm3 µm–2, and 0.01 to 0.10 µm3 µm–2, which were about 0.3 to 5.8, 1.1 to 4.7, and 1.2 to 8.9 times greater than the background values, respectively. The daily absorption aerosol optical depth was ~0.01 to ~0.13 at CAMS, ~0.03 to ~0.14 at Xianghe, and ~0.01 to ~0.09 at Shangdianzi, and the absorption Angstrom exponents reflected a significant increase in organic aerosols over CAMS and Xianghe and in black carbon over Shangdianzi. Aerosol radiative forcing at the bottom of the atmosphere varied from –20 to –130, –40 to –150, and –10 to –110 W m–2 for the whole holiday period, indicating the cooling effect. The potential source contribution function and concentration-weighted trajectory analysis showed that Beijing, the southern parts of Hebei and Shanxi, and the central northern part of Shandong contributed greatly to the pollution.

Surface radiation is crucial to atmospheric boundary layer development and air pollution formation. Several studies have revealed that surface radiation plays a vital role in developing the daytime convective boundary layer that controls the explosive growth of PM2.5 concentration; however, less attention has been paid to the effects of changing nighttime surface radiation on the near-surface temperature inversion layer and PM2.5 accumulation. In this study, we used long-term observations of meteorological and environmental data and atmospheric boundary layer measurements during a severe PM2.5 pollution event to investigate the effect of changes in nocturnal surface radiation on the increase in PM2.5 concentrations. The results showed that surface radiation cooling was enhanced (weakened) by decreased (increased) cloud cover fraction by changing longwave radiation at night; this strengthened (weakened) near-surface temperature inversion intensity and promoted (prevented) the accumulated increase in PM2.5. This observational study using 5-year data further confirmed the cloud radiative effect on the nighttime accumulation of PM2.5 with a significant negative correlation between nighttime averages of surface PM2.5 concentrations and cloud cover fractions. This reveals an important mechanism for the impact of surface radiation cooling modulated by cloud cover change on the nighttime accumulated increase in PM2.5. This finding extends our understanding of air pollutant accumulation at night with potential implications for atmospheric environment change.

Water vapour content greatly influences the aerosol extinction coefficient in the atmosphere. Based on 27 months of the Cloud-aerosol lidar and Infrared Pathfinder Satellite Observation data, the e...

Since the 1980s, an industrial development has bloomed in China including the Yunnan–Guizhou Plateau (YGP), a remote region in Southwest China. To analyze the regional variations in air quality over YGP during the industrial development, we adopt the equivalent visibility by excluding the influence of natural factors on the observed visibility based on the meteorological data observed at 203 sites over YGP from 1980 to 2010 in this study. The YGP air quality exhibits the tremendous spatial differences in a general distribution from western good to eastern poor air quality. A similar pattern shifts seasonally in the spatial distribution with a typical seasonality of air quality over YGP between summertime low air pollution and wintertime high air pollution. The increasing and decreasing trends in visibility are mostly concentrated, respectively, in the YGP regions with high and low visibility, displaying the phenomenon of polarization in air quality change over YGP during 1980–2010. The regional mean visibility of the YGP presents a significant declining trend with change rate of −1.5 km decade−1 for air quality deterioration. The seasonal differences in visibility between summer (33.6 km) and winter (25.2 km) became obscure with the interannual change trends of visibility with stronger declines (−2.29 km decade−1) in summer and weaker decreases (−0.89 km decade−1) in winter over 1980–2010, which lead to a indistinct seasonality of air quality change over YGP. The remote YGP had experienced more frequent haze pollution, especially in the eastern plateau over 31 years. In accompany of increasing energy consumption in the industrial development, population growth is an important factor influencing the interannual change of YGP air quality. The distinct spatial distribution in the YGP terrain exerts an impact on poor air quality in lower flatlands harbored by mountainous topography with good air quality. Declined monsoon winds could meteorologically drive the air quality change with less regional transport of air pollutant over YGP during 1980–2010. Implications of the climate change for atmospheric environment will be having potential utility for sustainable development in China.

The chemical composition dataset of Aerosol Reanalysis of NASA’s Modern-Era Retrospective Analysis for Research and Application, version 2 (MERRAero) has not been thoroughly evaluated with observation data in mainland China due to the lack of long-term chemical components data. Using the 5-year data of PM10 mass concentrations and chemical compositions obtained from the routine sampling measurements at the World Meteorological Organization the Global Atmosphere Watch Programme regional background stations, Jing Sha (JS) and Lin’An (LA), in central and eastern China, we comprehensively evaluate the surface PM10 concentrations and chemical compositions such as sulfate (SO42−), organic carbon (OC) and black carbon (BC) derived from MERRAero. Overall, the concentrations of PM10, SO42−, OC and BC from the MERRAero agreed well with the measurements, despite a slight and consistent overestimation of BC concentrations and a moderate and persistent underestimation of PM10 concentrations throughout the study period. The MERRAero reanalysis of aerosol compositions performs better during the summertime than wintertime. By considering the nitrate particles in PM10 reconstruction, MERRAero performance can be significantly improved. The unreasonable seasonal variations of PM10 chemical compositions at station LA by MERRAero could be causative factors for the larger MERRAero discrepancies during 2016–2017 than the period of 2011–2013.

Compared with the 21-year climatological mean over the same period during 2000–2020, the aerosol optical depth (AOD) and Angstrom exponent (AE) during the COVID-19 lockdown (January 24–February 29, 2020) decreased and increased, respectively, in most regions of Central-Eastern China (CEC). The AOD (AE) values decreased (increased) by 39.2% (29.4%) and 31.0% (45.3%) in Hubei and Wuhan, respectively, because of the rigorous restrictions. These inverse changes reflected the reduction of total aerosols in the air and the contribution of the increase in fine-mode particles during the lockdown. The surface PM2.5 had a distinct spatial distribution over CEC during the lockdown, with high concentrations in North China and East China. In particular, relatively high PM2.5 concentrations were notable in the lower flatlands of Hubei Province in Central China, where six PM2.5 pollution events were identified during the lockdown. Using the observation data and model simulations, we found that 50% of the pollution episodes were associated with the long-range transport of air pollutants from upstream CEC source regions, which then converged in the downstream Hubei receptor region. However, local pollution was dominant for the remaining episodes because of stagnant meteorological conditions. The long-range transport of air pollutants substantially contributed to PM2.5 pollution in Hubei, reflecting the exceptional importance of meteorology in regional air quality in China., Graphical abstract Unlabelled Image, Highlights • Inverse changes in AOD and AE during lockdown were found over CEC. • Pollution events were frequently observed during the strictest lockdown over Hubei. • Air pollution was caused by regional transport of upstream air pollutants to Hubei. • Local pollution was accumulated despite of large reductions in air pollutant emissions.

Recently, the frequent occurrence of haze with aerosol pollution in China has attracted worldwide attention. Air pollutant emissions in conjunction with changing meteorological conditions create environment pollution in China. Aerosol pollution is spatially centralized in four regions of China, including the North China Plain, Yangtze River Delta, Pearl River Delta, and Sichuan Basin. In this observational study, a new center of aerosol pollution was identified in the Twain-Hu Basin (THB), covering the Hubei and Hunan provinces in central China. Based on the analysis of 19 years of satellite remote sensing data from the Moderate Resolution Imaging Spectroradiometer (MODIS), the THB experiences high aerosol optical depth (AOD) values exceeding 0.9. The fine mode fraction (FMF) values below 0.3 were also detected over the aerosol polluted THB region, where aerosol pollution was dominated by the mixed aerosol type. This reflects the role of intense human activities and the unique aerosol processes involved in the regional aerosol pollution over central China. The interannual AOD variations for THB present an increasing trend (mostly >0.02 yr−1) between 2000 and 2011 and a significant descending trend (mostly