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The optical and microphysical properties of aerosols remain one of the greatest uncertainties associated with evaluating the climate forcing attributed to aerosols. Although the trends in aerosol optical depth (AOD) at global and regional scales have been widely examined, little attention has been paid to the trends in type-dependent AODs related to aerosol particle properties. Here, using the aerosol optical component dataset from the Multi-angle Imaging SpectroRadiometer (MISR) instrument, we investigate decadal-scale trends in total aerosol loading as well as AODs for five aerosol components by particle size and morphology during 2003–2018 over land. Relationships between the total AOD (TAOD) trends and type-dependent AOD changes were examined, and the relative contribution of each type-dependent AOD to the overall TAOD trends was quantified. By dividing the TAOD values into four different aerosol pollution levels (APLs) with splits at 0.15, 0.40, and 0.80, we further explored the relationships between TAOD changes and interannual variations in the frequency-of-occurrences (FoOs) of these APLs. Long-term trends in FoOs in the different APLs show that there was a significant improvement in air quality between 2003 and 2018 in most land areas, except South Asia, corresponding to a shift from lightly polluted to clean conditions. However, the effects of different APLs on TAOD changes are regionally dependent and their extent of correlation varied spatially. Moreover, we observed that the annual mean TAOD has decreased by 0.47% a−1 over land since 2003 (P

East Asian dust aerosols play a vital role in the local and regional climate through its direct, indirect, and semidirect effects, but the dominant factors affecting the interannual variation of dust aerosols over East Asia and their regional differences remain unclear. This study verified the accuracy of MEERA-2 dust data in East Asia, analyzed the interannual trends of dust in East Asia from 2000 to 2019 using the MERRA-2 dust column mass density (DCMD) and identified the dominant factors affecting the interannual variation during the dusty season (March–July) by developing the regional multiple linear regression models, combined with correlation and partial correlation analysis. The comparison with the dust index (DI) calculated from ground-based observations of dust events frequency indicated that MERRA-2 DCMD exhibited high spatial agreement (R > 0.8) with ground-based observations in most regions (especially in the dust source region of North China). The trend analysis revealed that DCMD in East Asia decreased significantly after 2000, particularly in the dusty season (March–July). These significant decreases were generally highly correlated with increases in normalized differential vegetation index (NDVI), volumetric soil moisture (VSM), and precipitation (PPT) and with decreases in wind speed (WS). Furthermore, WS dominated the interannual variation in the dust concentration over the East Asian dust source regions and their downstream. By contrast, PPT, through its wet deposition effect, dominated the variation in the rest of the regions away from the dust source regions. The study findings may help clarify the associations between local meteorological and surface factors and long-term variations in dust aerosols over East Asia.

Although a remarkable reduction in the frequency of sand and dust storms (SDSs) in the past several decades has been reported over northern China (NC), two unexpected mega SDSs occurred on March 15–20, 2021 and March 27–29, 2021 (abbreviated as the “3.15” and “3.27” SDS events), which has reawakened widespread concern. This study characterizes the origins, transport processes, magnitudes of impact, and meteorological causes of these two SDS events using a long-term (2000–2021) dust optical depth (DOD) dataset retrieved from MODIS measurements and a comprehensive set of multiple satellite and ground-based observations combined with atmospheric reanalysis data. During the 3.15/3.27 event, the invasion of dust plumes greatly degraded the air quality over large areas of NC, reaching extremely hazardous levels, with the maximum daily mean PM10 concentration of 7058 µg m−3 (2670 µg m−3) recorded on March 15 (28). CALIOP observations show that during the 3.15 event the dust plume was lifted to an altitude of 4–8 km, and its range of impact extended from the dust source to the eastern coast of China. In contrast, the lifting height of the dust plume during the 3.27 event was lower than that during 3.15 event, which was also confirmed by ground-based Lidar observations. The MODIS-retrieved DOD data registered these two massive SDS events as the most intense episode in the same period in history over the past two decades. These two extreme SDS events were associated with both atmospheric circulation extremes and local meteorological anomalies that favored enhanced dust emissions in the Gobi Desert (GD) across southern Mongolia and NC. Meteorological analysis revealed that both SDS events were triggered by an exceptionally strong Mongolian cyclone generated at nearly the same location (along the central and eastern plateau of Inner Mongolia) in conjunction with a surface-level cold high-pressure system at the rear, albeit with differences in magnitude and spatial extent of impact. In the GD, the early melting of spring snow caused by near-surface temperature anomalies over dust source regions, together with negative soil moisture anomalies induced by decreased precipitation, formed drier and barer soil surfaces, which allowed for increased emissions of dust into the atmosphere by strongly enhanced surface winds generated by the Mongolian cyclone.

Serious pollution events caused by residential heating activities occur frequently in northeastern China, and these increased particulate matter levels and gaseous pollutants could contribute to both local emissions and regional climate change. In this study, meteorological data, air trajectories and aerosol vertical distributions were used to study the pollutant sources and movement paths during the heating season (winter–spring transition) in the Harbin-Changchun megalopolis, northeastern China. Higher concentration of fine-mode particles (PM2.5/PM10 ∼ 0.70–0.80) and the trace gas SO2 (35–45 μg m−3) could be seem as an indicator of the air quality in the Harbin-Changchun megalopolis during the residential heating seasons, caused by local emissions. The apparent changes of meteorological elements in the high latitude and cold regions over Northeastern China could have important effects on the pollutant formation, secondary transformation, aerosol accumulation and vertical transport over the heating season. The PM2.5 concentrations were highly associated with the SO2 and NO2 from both fossil fuel combustion and biomass burning. The obvious diurnal pollutants variation shown thermodynamic conditions could be considered one of the important environmental elements responsible for residential heating processes. The mixture effect of local emissions and aerosol convective activities were found to contribute to the aerosol extinction and air pollution in the Harbin-Changchun megalopolis. These findings could help to improve our understanding of the association between aerosol pollutants influenced by the anthropogenic activities and the aerosol regional climate effects during the heating season in northeastern China.

Fine particulate matter (PM2.5) pollution in Eastern China (EC) has raised concerns due to its adverse effects on air quality, climate, and human health. This study investigated the long-term variation trend in satellite-derived PM2.5 concentrations and how it was related to pollutant emissions and meteorological parameters over EC and seven regions of interest (ROIs) during 1998–2016. Over EC, the annual mean PM2.5 increased before 2006 due to the enhanced emissions of primary PM2.5, NOx and SO2, but decreased with the reduced SO2 emissions after 2006 evidently in response to China's clean air policies. In addition, results from statistical analyses indicated that in the North China Plain (NCP), Northeast China (NEC), Sichuan Basin (SCB) and Central China (CC) planetary boundary layer height (PBLH) was the dominant meteorological driver for the PM2.5 decadal changes, and in the Pearl River Delta (PRD) wind speed is the leading factor. Overall, the variation in meteorological parameters accounted for 48% of the variances in PM2.5 concentrations over EC. The population-weighted PM2.5 over EC increased from 36.4 μg/m3 in 1998–2004 (P1) to 49.4 μg/m3 in 2005–2010 (P2) then decreased to 46.5 μg/m3 in 2011–2016 (P3). In the NCP and NEC, the percentages of the population living above the World Health Organization (WHO) Interim Target-1 (IT-1, 35 μg/m3) have risen steadily over the past 20 yr, reaching maxima of 97.3% and 78.8% in P3, respectively, but decreases of ∼30% from P2 to P3 were found for the SCB and PRD.

The Sichuan Basin (SB), covering 260,000 km2 immediately to the east of the Tibetan Plateau (TP) with a large drop exceeding 3000 m in elevation over a short horizontal distance, is a region of high haze pollution in China. The terrain effects of this unique deep basin on haze pollution have not been well investigated. During 14–19 January 2014, heavy haze pollution engulfed the SB with high PM2.5 concentrations. By using the WRF-Chem model, the topographic effects on haze pollution in the SB and the underlying mechanisms were investigated by two sets of simulation with and without the basin topography. The simulation results show that the SB topography lead to an increase of 48 μg m−3 in the basin-averaged surface PM2.5 contributing about 44% to the PM2.5 concentrations during heavy haze pollution, indicating an important role of the basin topography in worsening air pollution. The surface PM2.5 level enhancements varied spatially from 0 to 30 μg m−3 in the eastern basin to 60–120 μg m−3 in the western basin, respectively corresponding to 0–20% and 50–70% relative to the concentrations simulated without the SB topographic condition, reflecting the stronger topographic effect of the TP in the western SB. The topographic effects intensify the haze pollution via reducing wind speed, raising air temperature and humidity in the lower troposphere, as well as dropping boundary layer height in the basin. Resulted from the impact of TP on mid-latitude westerly winds, a typical leeside vortex of immediately upstream TP over the basin was developed with accentuating PM2.5 accumulations in the basin, implying a strong influence of TP on air quality in the SB.

As one of the largest metropolitan areas in the world, Beijing has a high aerosol loading. Although the government has taken various measures to reduce emissions, the large-scale and intermittent occurrence of pollution is still a major issue. To understand the mechanisms and characteristics of pollution, Raman-Mie lidar and CE-318 sun photometer were used to study the aerosol vertical distribution and optical properties during dust, haze and clean periods in Beijing that occurred in September to October 2017. Combined with meteorological data, the sources and transmission paths of the pollution events were analyzed. The results are as follows. For the dust event, irregular particles with a large linear particle depolarization ratio played a leading role. The aerosol boundary layer height reached 3.5 km. The volume concentration of coarse particles was obviously larger in the volume size distribution. Strong northwest winds at high altitudes cause high-altitude long-distance transmission of pollutants. And this process can exacerbate air quality of Beijing. For the haze event, high-spherical water-soluble particles were dominant. The extinction coefficient exceeded 1 km−1 while the linear particle depolarization ratio was relatively small. The aerosol boundary layer height was

Since haze and other air pollution are frequently seen in the North China Plain (NCP), detail information on aerosol optical and radiative properties and its type classification is demanded for the study of regional environmental pollution. Here, a multiyear ground-based synchronous sun photometer observation at seven sites on North China Plain megalopolis from 2013 to 2018 was conducted. First, the annual and seasonal variation of these characteristics as well as the intercomparsion were analyzed. Then the potential relationships between these properties with meteorological factors and the aerosol type classification were discussed. The results show: Particle volume exhibited a decreasing trend from the urban downtown to suburban and the rural region. The annual average aerosol optical depth at 440 nm (AOD440) varied from ∼0.43 to 0.86 over the NCP. Annual average single-scattering albedo at 440 nm (SSA440) varied from ∼0.89 to 0.93, indicating a moderate to slight absorption capacity. Average absorption aerosol optical depth at 440 nm (AAOD440) varied from ∼0.07 to 0.10. The absorption Angstrom exponent (AAE) (∼0.89–1.40) indicated the multi-types of absorptive matters originated form nature and anthropogenic emission. The discussion of aerosol composition showed a smaller particle size of aerosol from biomass burning and/or fossil foil consumption with enhanced aerosol scattering and enlarged light extinction. Aerosol classification indicated a large percentage of mixed absorbing aerosol (∼20%–49%), which showed increasing trend between relative humidity (RH) with aerosol scattering and dust was an important environmental pollutant compared to southern China.

The Yangtze River Delta (YRD) is one of the most densely populated regions in China with severe air quality issues that have not been fully understood. Thus, in this study, based on 1-year (2013) continuous measurement at a National Reference Climatological Station (NRCS, 30.22° N, 120.17° E; 41.7 m a.s.l.) in the center of Hangzhou in the YRD, we investigated the seasonal characteristics, interspecies relationships, and the local emissions and the regional potential source contributions of trace gases (including O3, NOx, NOy, SO2, and CO) and particulate matter (PM2.5 and PM10). Results revealed that severe two-tier air pollution (photochemical and haze pollution) occurred in this region, with frequent exceedances in O3 (38 days) and PM2.5 (62 days). O3 and PM2.5 both exhibited distinct seasonal variations with reversed patterns: O3 reaching a maximum in warm seasons (May and July) but PM2.5 reaching a maximum in cold seasons (November to January). The overall results from interspecies correlation indicated a strong local photochemistry favoring the O3 production under a volatile organic compound (VOC)-limited regime, whereas it moved towards an optimum O3 production zone during warm seasons, accompanied by the formation of secondary fine particulates under high O3. The emission maps of PM2.5, CO, NOx, and SO2 demonstrated that local emissions were significant for these species on a seasonal scale. The contributions from the regional transport among inland cities (Zhejiang, Jiangsu, Anhui, and Jiangxi Province) on a seasonal scale were further confirmed to be crucial to air pollution at the NRCS site by using backward trajectory simulations. Air masses transported from the offshore areas of the Yellow Sea, East Sea, and South Sea were also found to be highly relevant to the elevated O3 at the NRCS site through the analysis of potential source contribution function (PSCF). Case studies of photochemical pollution (O3) and haze (PM2.5) episodes both suggested the combined importance of local atmospheric photochemistry and synoptic conditions during the accumulation (related with anticyclones) and dilution process (related with cyclones). Apart from supplementing a general picture of the air pollution state in the city of Hangzhou in the YRD region, this study specifically elucidates the role of local emission and regional transport, and it interprets the physical and photochemical processes during haze and photochemical pollution episodes. Moreover, this work suggests that cross-regional control measures are crucial to improve air quality in the YRD region, and it further emphasizes the importance of local thermally induced circulation for air quality.

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.

The industrial city of Shenyang in northeastern China has undergone a period of rapid development; long-term aerosol vertical properties could be relevant to more clearly understanding local emissions and their regional transportation. Aerosol optical depth (AOD), planetary boundary layer (PBL) height, and the vertical profiles of extinction coefficient, were measured and analyzed with ground-based Lidar during 2016 in Shenyang. Ground-level particulate matter mass concentrations, meteorological parameters, backward trajectories, and Moderate Resolution Imaging Spectroradiometer products were used to study the pollutant sources in four cases using the potential source contribution function and concentration-weighted trajectory methods. The results indicate that the AOD was 0.10 ± 0.10 to 0.23 ± 0.34 from January to May, and approximately 0.49 ± 0.39 in July. The PBL height was highest in March (1318.7 ± 696.5 m) and lowest in winter (877.1 ± 508.1 m to 950.7 ± 762.3 m). The mass concentrations of PM10, PM2.5, and PM1.0 were highest in January at 148.2 ± 77.8 µg m–3, 106.0 ± 58.8 µg m–3, and 33.8 ± 20.5 µg m–3; and lowest in June at 56.2 ± 27.8 µg m–3, 33.7 ± 18.3 µg m–3, and 9.3 ± 6.0 µg m–3, respectively. The concentrations of SO2 and CO were higher in winter and lower in summer, whereas O3 concentrations were higher in summer and lower in winter. The monthly extinction coefficient was affected by dust events in spring and new particle generation in summer, as well as by biomass-burning and coal-burning emissions in autumn and winter. Four pollution sources—from northwestern, eastern, northern, and northeastern China—were selected to analyze the different paths and sources of pollutants affecting Shenyang. The results of this paper will be helpful in the study of continuous year-round aerosol vertical properties and the regional pollution features of northeast China.

The optical and radiative properties of aerosols during a severe haze episode from 15 to 22 December 2016 over Beijing, Shijiazhuang, and Jiaozuo in the North China Plain were analyzed based on the ground-based and satellite data, meteorological observations, and atmospheric environmental monitoring data. The aerosol optical depth at 500 nm was 1.4 as the haze pollution developed. The Angstrom exponent was > 0.80 for most of the study period. The daily single-scattering albedo was > 0.85 over all of the North China Plain on the most polluted days and was > 0.97 on some particular days. The volumes of fine and coarse mode particles during the haze event were approximately 0.05–0.21 and 0.01–0.43 μm3, respectively—that is, larger than those in the time without haze. The daily absorption aerosol optical depth was about 0.01–0.11 in Beijing, 0.01–0.13 in Shijiazhuang, and 0.01–0.04 in Jiaozuo, and the average absorption Angstrom exponent varied between 0.6 and 2.0. The aerosol radiative forcing at the bottom of the atmosphere varied from –23 to –227,–34 to –199, and –29 to –191 W m–2 for the whole haze period, while the aerosol radiative forcing at the top of the atmosphere varied from –4 to –98, –10 to –51, and –21 to –143 W m–2 in Beijing, Shijiazhuang, and Jiaozuo, respectively. Satellite observations showed that smoke, polluted dust, and polluted continental components of aerosols may aggravate air pollution during haze episodes. The analysis of the potential source contribution function and concentration-weighted trajectory showed that the contribution from local emissions and pollutants transport from upstream areas were 190–450 and 100–410 μg m–3, respectively.

This study compares the aerosol optical properties measured by CE318 sunphotometers at the Institute of Atmospheric Physics and the Chinese Academy of Meteorological Sciences in Beijing between January 2013 and July 2015 to provide the framework to quantify the spatial and temporal variability of aerosol properties. Aerosol optical parameters included extinction (scattering plus absorption) aerosol optical depth (EAOD), extinction Angstrom exponent (EAE), columnar water vapor (CWV), absorption aerosol optical depth (AAOD), absorption Angstrom exponent (AAE), extinction aerosol optical depth of fine particles (EAODf), extinction aerosol optical depth of coarse particles (EAODc), real parts of the refractive index (REFR), imaginary parts of the refractive index (REFI), single scattering albedo (SSA), asymmetry factor (ASYM), size distribution, and sphericity fraction. Comparison of aerosol optical properties using the simultaneous observations between two sites showed that correlation coefficients were larger than or equal to 0.98 for EAOD, EAE, and CWV, but smaller than or equal to 0.90 for AAOD, REFR, REFI, and SSA; the percentage differences for most of the parameters were less than 2%, but for EAODf were relatively large, ranging from 4.35 to 6.45%; the mean size distributions simultaneously showed bi-modal patterns, with two peak volumes at the radii of 0.15 and 2.94 μm; two kinds of tri-peak model were apparent during the study period; a case of EAODs at 440 nm differing by more than 0.2 between the two sites reflected the effect of local aerosol pollution. Comparison of aerosol characterization inferred by absorption properties using all the inversion data showed that classification using EAE, AAE, and sphericity fraction indicated the main aerosol type was “U/IB the volume size distribution retrievals binned by AAE exhibited coarse mode particles became dominant as AAE increased to 2.0; the SSA retrievals binned by AAE demonstrated SSA transitioned from spectra representing dust to U/IB averaged SSA for all the retrievals and SSA data partitioned by the EAE and η 675 nm suggested there were more absorbing aerosols at IAP. The results of the study will be beneficial in validating satellite observations and model simulation results, providing more accurate input parameters for model simulations.

Water vapor (WV) has a vital effect on global climate change. Using satellite data observed by AURA/MLS and ERA-Interim reanalysis datasets, the spatial distributions and temporal variations of WV were analyzed. It was found that high WV content in the UTLS over the southern Tibetan Plateau is more apparent in summer, due to monsoon-induced strong upward motions. The WV content showed the opposite distribution at 100 hPa, though, during spring and winter. And a different distribution at 121 hPa indicated that the difference in WV content between the northern and southern plateau occurs between 121 and 100 hPa in spring and between 147 and 121 hPa in winter. In the UTLS, it diminishes rapidly with increase in altitude in these two seasons, and it shows a “V” structure in winter. There has been a weak increasing trend in WV at 100 hPa, but a downtrend at 147 and 215 hPa, during the past 12 years. At the latter two heights, the WV content in summer has been much higher than in other seasons. Furthermore, WV variation showed a rough wave structure in spring and autumn at 215 hPa. The variation of WV over the Tibetan Plateau is helpful in understanding the stratosphere-troposphere exchange (STE) and climate change.

This study assesses the performance of SKYNET in comparison to AERONET (Aerosol Robotic Network) for retrieving aerosol optical properties (AOPs) in Beijing, China. The results obtained from simultaneous measurements show high correlation coefficients (> 0.994) for aerosol optical depth (AOD) at each wavelength. The highest correlation coefficient for Ångström exponent is 0.825, at 500–870 nm. The single scattering albedo (SSA) of SKYNET is systematically larger than that of AERONET at each wavelength, and adjusting the SVA (solid view angle) and SA (surface albedo) input values can easily affect the value of SKYNET SSA. The volume size distribution patterns derived from the two networks’ instruments are both bimodal, which is typical, while the coarse-mode volume of SKYNET is larger than that of AERONET on average. According to the frequency distribution of aerosol particles, coarser aerosol particles often present in autumn and finer particles usually exist in winter, and there are more absorbent aerosol particles in winter. SKYNET data, combined with meteorological data, CALIPSO (Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations) data, backward trajectories, and WPSCF (weighted potential source contribution function) and WCWT (weighted concentrated weighted trajectory) analyses are used to analyze a serious pollution event in winter over Beijing. The results suggest that it was not only affected by local emissions but also by regional transport. The AOPs under three weather conditions (clean, dusty, haze) in Beijing are discussed. The values of AOD on haze days are about 10.3, 10.0, 8.7, 6.3 and 6.2 times larger than those on clean days at 400, 500, 670, 870 and 1020 nm, respectively; and under haze conditions, the PM2.5 (fine particulate matter) is about 7.6 times larger than that under clean conditions. The values of AOD on dusty days are about 7.1, 7.4, 7.0, 5.3 and 5.2 times larger than those on clean days at 400, 500, 670, 870 and 1020 nm, respectively; and under haze conditions, the PM2.5 is about 5.2 times larger than that under clean conditions.

The aerosol microphysical, optical and radiative properties of the whole column and upper planetary boundary layer (PBL) were investigated during 2013 to 2018 based on long-term sun-photometer observations at a surface site (~106 m a.s.l.) and a mountainous site (~1225 m a.s.l.) in Beijing. Raman-Mie lidar data combined with radiosonde data were used to explore the aerosol radiative effects to PBL during dust and haze episodes. The results showed size distribution exhibited mostly bimodal pattern for the whole column and the upper PBL throughout the year, except in July for the upper PBL, when a trimodal distribution occurred due to the coagulation and hygroscopic growth of fine particles. The seasonal mean values of aerosol optical depth at 440 nm for the upper PBL were 0.31 ± 0.34, 0.30 ± 0.37, 0.17 ± 0.30 and 0.14 ± 0.09 in spring, summer, autumn and winter, respectively. The single-scattering albedo at 440 nm of the upper PBL varied oppositely to that of the whole column, with the monthly mean value between 0.91 and 0.96, indicating weakly to slightly strong absorptive ability at visible spectrum. The monthly mean direct aerosol radiative forcing at the Earth's surface and the top of the atmosphere varied from −40 ± 7 to −105 ± 25 and from −18 ± 4 to −49 ± 17 W m−2, respectively, and the maximum atmospheric heating was found in summer (~66 ± 12 W m−2). From a radiative point of view, during dust episode, the presence of mineral dust heated the lower atmosphere, thus promoting vertical turbulence, causing more air pollutants being transported to the upper air by the increasing PBLH. In contrast, during haze episode, a large quantity of absorbing aerosols (such as black carbon) had a cooling effect on the surface and a heating effect on the upper atmosphere, which favored the stabilization of PBL and occurrence of inversion layer, contributing to the depression of the PBLH.

Carbonaceous aerosols and gaseous pollutants emitted from wildfires play a crucial role in both the global climate system and regional air quality. Here, using multisource satellite and ground-based observations combined with reanalysis data, we investigate the three-dimensional evolution of biomass-burning emissions from a forest wildfire event in Liangshan, Southwest China, which occurred from 29 March to 1 April 2020. The meteorological field analysis showed that the negative anomaly of relative humidity and precipitation, as well as the positive anomaly of near-surface wind speed, created favourable conditions for the occurrence and spread of this wildfire event. During the fire, satellite observations suggested a maximum fire radiation power of over 100 MW. In addition, there were significant short-term effects of fire activity on regional air quality, with downwind surface PM2.5 concentrations at the Xichang site reaching a maximum of approximately 470 μg·m −3 on March 31. Driven by a southwesterly airflow, large amounts of smoke aerosols were transported rapidly to downstream areas, significantly deteriorating air quality, with the maximum value of the aerosol optical depth (AOD) exceeding 2. Moreover, the quantitative evaluation based on Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2) reanalysis showed that the instantaneous maximum values of the column mass concentration of black carbon (BC) and organic carbon (OC) reached 9.8 g·m−2 and 1.8 g·m−2 during the fire respectively. Further analysis suggested that the interaction between the lower and upper atmosphere constrained the smoke aerosols to altitudes below approximately 5 km, which was also supported by the vertical distribution of elevated smoke aerosols observed by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP).

Submicron particle matter (PM1) that rapidly reaches exceedingly high levels in several or more hours in the North China Plain (NCP) has been threating~400 million individuals' health for decades. The precise cause of the rapid rise in PM1 remains uncertain. Based on sophisticated measurements in PM1 characterizations and corresponding boundary-layer (BL) meteorology in the NCP, it demonstrates that this rising is mainly driven by BL meteorological variability. Large increases in near-ground inversions and decreases in vertical heat/momentum fluxes during the day-night transition result in a significant reduction in mixing space. The PM1 that is vertically distributed before accumulates at the near-ground and then experiences a rapid rise. Besides meteorological variability, a part of the rise in organics is ascribed to an increase of coal combustion at midnight. The daily-based accumulation of PM1 is attributed to day-to-day vertical meteorological variability, particularly diminishing mixing layer height exacerbated by aerosol-radiation feedback. Resolved by a multiple linear regression model, BL meteorological variability can explain 71% variances of PM1. In contrast, secondary chemical reactions facilitate the daily-based accumulation of PM1 rather than the rapid rise. Our results show that BL meteorological variability plays a dominant role in PM1 rising and day-to-day accumulation, which is crucial for understanding the mechanism of heavy pollution formation.

Different types of aerosols play a crucial role in the climate system according to their vertical distributions and optical-radiative properties. Southwest China (SWC), consisting of the Sichuan Basin (SCB), the Yunnan-Guizhou Plateau (YGP) and the Tibetan Plateau (TP), is one of the most topographically complex regions in the world and is also facing varying degrees of air pollution problems caused by local, high-intensity anthropogenic emissions and trans-regional transport of aerosols from surrounding areas. However, a comprehensive understanding of the regional three-dimensional (3D) characteristics of aerosol distribution and evolution over SWC has not to be achieved yet. Here, the seasonal distributions and vertical structures of four types of aerosols, including dust, polluted continental/smoke, polluted dust and elevated smoke, were detected by using the quality controlled CALIPSO Level 2 nighttime aerosol profile product from 2007 to 2016 in this study. The loading of aerosols over the SCB was higher than that over the other regions, and of the sources, polluted continental/smoke and polluted dust, related to anthropogenic sources, contributed the most to the aerosols. However, we also found that polluted continental/smoke and polluted dust have declined since 2013, which was due to the strict regulatory measures implemented for industry emissions and biomass burning in agriculture in recent years. The elevated smoke aerosols over the YGP at an altitude of 3 km in spring were significantly higher than those in other seasons, while the polluted continental/smoke aerosols at an altitude of 1–2 km were higher in autumn and winter than in the other seasons and was considered to be from local biomass burning. The aerosol optical depth (AOD) over the TP was nearly one magnitude lower than that over the other two regions. However, either the dust or polluted continental/smoke from the Qaidam Basin or elevated smoke elevated from the Indo-Gangetic Basin could have been transported to the layer as high as 6 km over the TP. Additionally, the heights of the highest frequencies of AOD for all types of aerosols for the SCB, YGP and TP were 0–2 km, 2–4 km and 4–6 km, respectively. In summer, elevated smoke can rise to heights of 8–10 km on the TP. The results of this study provide an observational basis for further understanding the trans-regional transport of aerosols and their climate effects in SWC.

Northeast China (NEC) has unique climate characteristics and emission sources; continued urbanisation has aggravated regional pollution. The in situ observation data concerning PM2.5, visibility, surface meteorological elements and synchronous aerosol vertical extinction profiles obtained from ground-based Lidar were investigated to better understand local and regional particulate pollution in NEC. The WRF (3.7.1)/CAMx (6.40) model was employed to quantitative investigate the contribution of regional transport to PM2.5 in Shenyang. The results suggested that PM2.5 increased significantly from 9 to 14 January over NEC and the Northern China (NC), with monthly PM2.5 highest in Shijiazhuang and Baoding of NC about 145.2 ± 88.9 and 136.8 ± 83.1 μg m−3, respectively. The distribution of SO2 and NO2 for PM2.5 implied SO2 was more influence on PM2.5 in NEC, while NO2 has larger impact on PM2.5 in NC. The significant increasing of relative humidity (RH) and temperatures exhibited in the pollution indicate water vapor and warm air flow during the transport. The development of the southwest airflow was conducive to pollutant transport across the Beijing–Tianjin–Hebei (or Jing-Jin-Ji) megalopolis to NEC, and together with the local emissions in NEC to affect air quality. The modelling results pointed out that contribution of regional transport to PM2.5 in Shenyang was about 80.12% at 00:00 LT in 10 January, of which the contribution of BTH was about 61.52%; the total regional contribution to PM2.5 in Shenyang reaching 60.70% at 02:00 LT on 13 January including 34.56% contributed by BTH region. Aerosol vertical extinction indicated the particle layer appeared in the near-surface and in the upper atmospheric layer from 0.5 to 1.0 km following the development of transport event. The findings of this study can facilitate a comprehensive understanding of the local and regional air pollution in NEC and helpful for national environment pollution controls and improvement.

Northeast China has undergone rapid urbanisation with increased anthropogenic emissions, and the types and absorption properties of aerosols may affect regional climate change. MERRA-2 (Modern-Era Retrospective analysis for Research and Applications, version 2) distributions of aerosol optical depth (AOD), Angstrom exponent (AE), and absorption aerosol optical depth (AAOD) from 1980 to 2017 was studied to estimate the climatology of aerosol optical properties over Northeast China. The highest AOD and AAOD occurred in Liaoning Province range from 0.3 to 0.4 and 0.02–0.03, respectively. The spacial distribution of black carbon (BC) AOD was similar to AAOD with maximum value in Liaoning province about 0.04 related to the emission sources and human activities. The seasonal interdecadal distribution indicated larger dust (DU) AOD in Liaoning (0.12) and organic carbon (OC) AOD in Heilongjiang (0.18). The contribution of SO4 to total AOD was significant in autumn and winter, and BC particles contributed 70% to total AAOD in all seasons. The decadal change in AOD was positive for 2000–2009 (0.2/decadal) due to the increased dust events happening in spring. The positive correlation between AOD and relative humidity (RH) at surface was about 0.4–0.6; the negative correlation between AOD and surface wind speed (WS) (−0.6), planetary boundary layer height (PBLH) (−0.2 to −0.6), sea level pressure (SLP) (−0.2) was found over the study period. This study’s findings enable more comprehensive understanding of the distribution of aerosols optical properties and regional climatology in Northeast China.

Are there still persistent heavy aerosol pollution episodes (HPEs) in Beijing one year after the implementation of the "Action Plan for the Prevention and Control of Air Pollution ("Ten Statements of Atmosphere" in China: 2013-2017)"? Will the cumulative aerosol pollution still induce significant two-way feedback between PM2.5 and the planetary boundary layer structure? Answers to these matters are particular concerns of the government and the public. The analysis of the vertical structure of the aerosol and meteorological factors in planetary boundary layer shows that the two-way feedback between unfavorable meteorological conditions and PM2.5 pollution cumulating is still the primary mechanism for the maintenance of HPEs, accounting especially ~66% to 88% for explosive rise in PM2.5, in autumn and winter in Beijing area a year after the "Ten Statements of Atmosphere". This effect also shows that the concentration of PM2.5 in Beijing had not fallen low enough to decouple the influence of unfavorable meteorological factors. The increased level of PM2.5 mass during the explosive rise stage was similar to those of the precursor gases of NO2, SO2 and CO, as well as to the declining ratio of the boundary layer height (BLH), which also suggest that the interaction between PM2.5 cumulating and the boundary layer structure is playing a leading role for the maintenance of HPEs and the PM2.5 explosive rise in Beijing. The depolarization ratio signal of the Lidar also shows that the transit of mineral aerosols from the northwest over Beijing often appears in the upper layer of the planetary boundary layer or higher atmosphere during the late or subside stage of HPEs.

As the central part of eastern China, the Yangtze River Delta (YRD) region, with its rapid economic growth and industrial expansion, has experienced severe air quality issues. In this study, the monthly variation and interaction between aerosol direct radiative forcing (ADRF) and aerosol vertical structure during 2013–2015 over the YRD were investigated using ground-based observations from a Micro Pulse Lidar (MPL) and a CE-318 sun-photometer. Combining satellite products from MODIS and CALIPSO, and reanalysis wind fields, an integrated discussion of a biomass burning episode in Hangzhou during August 2015 was conducted by applying analysis of optical properties, planetary boundary layer (PBL), spatial-temporal and vertical distributions, backward trajectories, Potential Source Contribution Function (PSCF), and Concentration Weighted Trajectory (CWT). The results reveal that a shallower PBL coincides with higher scattering extinction at low altitude, resulting in less heating to the atmosphere and radiative forcing to the surface, which in turn further depresses the PBL. In months with a deeper PBL, the extinction coefficient decreases rapidly with altitude, showing stronger atmospheric heating effects and ADRF to the surface, facilitating the turbulence and vertical diffusion of aerosol particles, which further reduces the extinction and raises the PBL. Because of the hygroscopic growth facilitated by high relative humidity, June stands out for its high scattering extinction coefficient and relatively low PBL, and the reduced ADRF at the surface and the enhanced cooling effect on near-surface layer in turn depresses the PBL. Absorptive aerosols transported from biomass burning events located in Zhejiang, Jiangxi, and Taiwan provinces at 1.5 km, result in high ADRF efficiency for atmospheric heating. And the enhanced heating effect on near-surface layer caused by absorptive particles facilitates PBL development in August over the YRD.

The Sichuan and Chongqing regions suffer from severe haze weather in winter due to the unfavourable atmospheric diffusion conditions. Reanalysis and precipitation datasets were applied in this study to calculate and distinguish air stagnation events using a developed criterion, and the impacts of the occurrence of air stagnation events on air quality were analysed in combination with the PM2.5 concentration data for the winters of 2013-2016. The highest occurrence frequency of air stagnation events was observed in 2013, and the lowest, 2015. The meteorological conditions during winter in the Sichuan Basin were inclined to form unfavourable atmospheric diffusion conditions, and the occurrence frequency of air stagnation days was up to 76.6% on average during the four winters. The effects of air stagnation events on air quality were most obvious in the western and southern Sichuan Basin. The mean concentrations of PM2.5 during air stagnation days were higher by 41.9% than those during non-air stagnation days. The PM2.5 concentrations were adjusted using the favourable atmospheric diffusion conditions in 2015 as a baseline to quantify the PM2.5 contribution to the improvement of air quality in the other years, which revealed that the level of PM2.5 in the Sichuan and Chongqing regions was declining at a rate of approximately 10.7% overall during the winters of 2013-2016, implying that the air pollutant reduction measures have been highly effective. Furthermore, the occurrence frequency of air stagnation days and events were increased in recent ten years of 2007-2016, with linear slopes of 0.61yr-1 and 0.26yr-1, respectively. The study revealed that the government might face a greater challenge in improving the air quality over winter and should pay more attention to reduction of pollutant emission in areas of Chengdu, Chongqing and cities in the south of the Sichuan Basin.

We analyzed the frequency and intensity of sand and dust storms (SDSs) in East Asia from 2007 to 2016 using observational data from ground stations, numerical modeling, and vegetation indices obtained from both satellite and reanalysis data. The relationships of SDSs with surface conditions and the synoptic circulation pattern were also analyzed. The statistical analyses demonstrated that the number and intensity of SDS events recorded in spring during 2007 to 2016 showed a decreasing trend. The total number of spring SDSs decreased from at least ten events per year before 2011 to less than ten events per year after 2011. The overall average annual variation of the surface dust concentration in the main dust source regions decreased 33.24μg/m3 (-1.75%) annually. The variation in the temperatures near and below the ground surface and the amount of precipitation and soil moisture all favored an improvement in vegetation coverage, which reduced the intensity and frequency of SDSs. The strong winds accompanying the influx of cold air from high latitudes showed a decreasing trend, leading to a decrease in the number of SDSs and playing a key role in the decadal decrease of SDSs. The decrease in the intensity of the polar vortex during study period was closely related to the decrease in the intensity and frequency of SDSs.

Variations in the optical properties of aerosols and their radiative forcing were investigated based on long-term synchronous observations made at three-minute intervals from 2011 to 2015 over seven adjacent CARSNET (China Aerosol Remote Sensing NETwork) urban (Hangzhou), suburban (Xiaoshan, Fuyang, LinAn, Tonglu, Jiande) and rural (ChunAn) stations in the Yangtze River Delta region, eastern China. The aerosol optical depth (AOD) varied from 0.68 to 0.76, with two peaks in June and September, and decreased from the eastern coast to western inland areas. The ratio of the AOD of fine-mode particles to the total AOD was > 0.90 and the extinction Angström exponent was > 1.20 throughout the year at all seven sites. The Moderate Resolution Imaging Spectroradiometer (MODIS) C6 retrieval AOD was validated by comparison with ground-based observations. The correlation coefficients (R2) between the MODIS C6 AOD data and the values measured on the ground were ~ 0.73–0.89. The single-scattering albedo varied from 0.91 to 0.94, indicating that scattering aerosol particles are dominant in this region. The real parts of the refractive index were ~ 1.41–1.43, with no significant difference among the seven urban, suburban and rural sites. Large imaginary parts of the refractive index were seen in August at all urban, suburban and rural sites. The fine-mode radii in the Yangtze River Delta region were ~ 0.2–0.3 μm with a volume of 0.10–0.12 μm3 and the coarse-mode radii were ~ 2.0 μm with a volume close to 0.07 μm3. The fine-mode aerosols were obviously larger in June and September than in other months at almost the sites. The absorption AOD was low in the winter. The absorption Angström exponent and the extinction Angström exponent were used to classify the different types of aerosol and the components of mixtures. The aerosols caused negative radiative forcing both at the Earth's surface and at the top of the atmosphere all year round in the Yangtze River Delta region of eastern China.

This research systematically studied heavy rain that occurred on 5 August 2014 over the southeastern Tibetan Plateau (31°N–35°N, 96°E–103°E) using orbital data from the Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR), the TRMM Multi-satellite Precipitation Analysis (TMPA) products, and the European Centre for Medium-range Weather Forecast (ECMWF) Re-Analysis Interim reanalysis data (ERA-Interim). The data studied included spatial and temporal distributions of the precipitation; horizontal distributions and vertical structures of the precipitation system; convective storm top altitudes and types of rain; mean rainfall profiles; the influence of water vapor content before and after the rainfall; and the atmospheric circulation background. The results suggest that most precipitation on the Tibetan Plateau occurs in the southeast, and that the maximum near-surface precipitation rate for this event was more than 100 mm·h−1. The convection was so powerful that the convective storm top altitude surpassed 16 km. Furthermore, the water vapor content caused obvious changes in the upper troposphere and lower stratosphere (UTLS) area. The mean rainfall profile can be roughly divided into four layers and showed that the maximum rainfall rate appeared at about 5.5 km. Deep weak precipitation provided the largest contribution to the total precipitation, while the highest average precipitation rate was from deep strong convective precipitation. The atmospheric circulation situation is conducive to the formation of strong convective weather, and the terrain is also an external factor affecting precipitation for this event.

This study analyzed the long-term variations and trends of haze pollution and its relationships with emission and meteorological factors using the haze days (HDs) data derived from surface observation stations in Sichuan-Chongqing (SCC) region during 1980&ndash, 2016. The results showed that the multi-year mean number of HDs were 68.7 and 4.9 days for the Sichuan-Basin (SCB) and the rest of SCC region, respectively. The seasonally averaged HDs over SCB reached its maximum in winter (34.7 days), followed by autumn (17.0 days) and spring (11.6 days), and with the minimum observed in summer (5.5 days). The inter-annual variations of HDs in 18 main cities revealed that Zigong, Neijiang, and Yibin, which are located in the southern of SCB, have been the most polluted areas over the SCC region in the past decades. A notable increasing trend in annual HDs over the majority of SCC region was found during 1980&ndash, 1995, then the trend sharply reversed during 1996&ndash, 2005, while it increased, fluctuating at some cities after 2006. Seasonally, the increased trend in spring and autumn seems to be the strongest during 1980&ndash, 1995, whereas the decreased trend in spring and winter was stronger than other seasons during 1996&ndash, 2005. In addition, a remarkable increasing trend was found in winter since 2006. Using correlation analysis between HDs and emission and meteorological factors during different periods, we found that the variability of local precipitation days (PDs), planetary boundary layer height (PBLH), near-surface wind speed (WS), and relatively humidity (RH) play different roles in influencing the haze pollution change during different historical periods. The joint effect of sharp increase of anthropogenic emissions, reduced PDs and WS intensified the haze pollution in SCB during 1980&ndash, 1995. In contrast, decreased HDs during 1996&ndash, 2005 are mainly attributable to the reduction of PM2.5 emission and the increase of PDs (especially in winter). In addition, the decrease of PDs is likely to be responsible for the unexpected increase in winter HDs over SCB in the last decade.

An extreme haze event occurred in the Beijing area from 17 to 23 December 2015. Ground-based measurements and satellite observations during this event were used to further our understanding of the formation process of haze pollution and aerosol optical properties. The results suggest that high relative humidity, poor diffusion conditions (low wind speed and stable stratification) and favorable secondary transformation conditions under the hygroscopic growth of aerosol and high emissions led to this serious haze episode. During the haze period, the daily average value was 1.15 and 0.42 for aerosol optical depth (AOD500nm) and columnar water-vapor (CWV, in cm), respectively. On 19 December, the correlation coefficient between CWV and AOD500nm was 0.91, indicating the effect of hygroscopic growth of fine-mode articles. The daily average values for Ångström exponent, fine-mode fraction, aerosol absorption optical depth, and Ångström absorption exponent were 1.19, 0.81, 0.11 and 1.47, respectively, which suggests that fine aerosol particles were dominant in the atmosphere and fine-mode particles were the dominant contributor to atmospheric extinction during the haze period. Moreover, it also reflects that there were more absorbing aerosol particles during the haze period. Compared with other polluted periods with a bimodal distribution, there was an obvious trimodal distribution on 19 December. There were three peaks at radii of about 0.1 μm, 0.5–0.8 μm and 4 μm. Satellite observations show that there was an obvious aerosol layer in the Beijing area during the haze period, concentrated at ground level to within 2 km in the upper layers. The types of aerosol were mainly composed of mixed pollution aerosols.