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Aerosol spatial distribution obtained from satellite sensors is critical for understanding regional aerosol environments, anthropogenic aerosol emissions, and global climate change. The Directional Polarimetric Camera (DPC) is the first generation of multi-angle polarized sensor developed by China. It is on-board the GaoFen-5 satellite, running in 705 km sun-synchronous orbit with a 13:30 LT (local time) ascending node. The sensor has three polarized channels at 490, 670, and 865 nm and ∼ 9 viewing angles, mainly used for observing aerosols. The spatial resolution is ∼ 3.3 km at nadir, and global coverage is ∼ 2 d. In this study, the performance of aerosol optical depth (AOD) retrievals from the DPC/GaoFen-5 using the Generalized Retrieval of Atmosphere and Surface Properties (GRASP) algorithm were evaluated on a global basis for the first time. The results showed that the DPC GRASP/Model scheme, which used several forms of aerosol-type mixing, achieved good performance. By comparing with Aerosol Robotic Network (AERONET) observations, the correlation coefficient (R), root-mean-square error (RMSE), and expected error (EE%, ± (0.05+0.15×AOD)) were 0.9007 %, 0.0662 %, and 82.54 %, respectively. The scattering angle, number of averaged pixels, length of time steps, and radiative and polarized fitting residuals showed impacts on the results of AOD retrieval in the DPC GRASP/Model scheme. From the most AERONET sites, the R and EE% were larger than ∼ 0.9 % and ∼ 80%. Compared with Moderate resolution Imaging Spectroradiometer (MODIS) products, the spatial and temporal variations of aerosol could be caught by the DPC with the GRASP/Model scheme, showing a good performance. However, values of AOD were also underestimated by DPC, probably due to an overly strict cloud mask. The above findings validated the ability of the DPC sensor to monitor aerosols. It should contribute to the development of aerosol parameter retrieval from multi-angle polarized sensors in the future. [ABSTRACT FROM AUTHOR]

Satellite aerosol products such as the Dark Target (DT) produced from the MODerate resolution Imaging Spectroradiometer (MODIS) are useful for monitoring the progress of air pollution. Unfortunately, the DT often fails to retrieve during the heaviest aerosol events as well as the more moderate events in winter. Some of the literature attributes this lack of retrieval to the cloud mask. However, we found this lack of retrieval is mainly traced to thresholds used for masking of inland water and snow. Modifications to these two masks greatly increase 50 % of the retrievals of aerosol optical depth at 0.55 µ m (AOD) greater than 1.0. The "extra"-high-AOD retrievals tend to be biased when compared with a ground-based sun photometer (AErosol RObotic NETwork, AERONET). Reducing bias in new retrievals requires two additional steps. One is an update to the assumed aerosol optical properties (aerosol model); the haze in this region is both less absorbing and lower in altitude than what is assumed in the global algorithm. The second is accounting for the scale height of the aerosol, specifically that the heavy-aerosol events in the region are much closer to the surface than what is assumed by the global DT algorithm. The resulting combination of modified masking thresholds, new aerosol model, and lower aerosol layer scale height was applied to 3 months of MODIS observations (January–March 2013) over eastern China. After these two additional steps are implemented, the significant increase in new retrievals introduces no overall bias at a high-AOD regime but does degrade other overall validation statistics. We also find that the research algorithm is able to identify additional pollution events that AERONET instruments may not due to different spatial sampling. Mean AOD retrieved from the research algorithm increases from 0.11 to 0.18 compared to values calculated from the operational DT algorithm during January to March of 2013 over the study area. But near Beijing, where the severe pollution occurs, the new algorithm increases AOD by as much as 3.0 for each 0.5 ∘ grid box over the previous operational-algorithm values. [ABSTRACT FROM AUTHOR]

Satellite aerosol products such as the Dark Target (DT) produced from the MODerate resolution Imaging Spectroradiometer (MODIS), are useful for monitoring the progress of air pollution. Unfortunately, the DT often fails to retrieve during the heaviest aerosol events as well as the more moderate events in winter. Some literatures attribute this lack of retrieval to cloud mask. However, we found this lack of retrieval is mainly traced to thresholds used for masking of inland water and snow. Modifications to these two masks greatly increase the coverage of retrievals overall (50 %) and double the retrievals of aerosol optical depth at 0.55 µm (AOD) greater than 1.0. The extra high AOD retrievals tend to be biased when compared with ground-based sunphotometer (AERONET). Reducing bias in new retrievals requires two additional steps. One is an update to the assumed aerosol optical properties (aerosol model) - the haze in this region is both less absorbing and lower in altitude than what is assumed in the global algorithm. The second is accounting for the scale height of the aerosol, specifically that the heavy aerosol events in the region are much closer to the surface than what is assumed by the global DT algorithm. The resulting combination of modified masking thresholds, new aerosol model, and lower aerosol layer scale height was applied to three months of MODIS observations (Jan-March 2013) over eastern China. When compared with AERONET, 70 % of the research algorithm retrievals fall within ±(0.08 + 0.17 × AOD). We also find that the research algorithm is able to identify additional pollution events that a triad of AERONET instruments surrounding Beijing could not. Mean AOD retrieved from the research algorithm increases from 0.11 to 0.18 compared to values calculated from the operational DT algorithm during January to March of 2013 over the study area. But near Beijing where the severe pollution occurs, the new algorithm increases AOD by as much as 3.0 for each 0.5° grid box, over the previous operational algorithm values. [ABSTRACT FROM AUTHOR]

Multi-year observations of aerosol microphysical and optical properties, obtained through ground-based remote sensing at 50 China Aerosol Remote Sensing Network (CARSNET) sites, were used to characterize the aerosol climatology for representative remote, rural, and urban areas over China to assess effects on climate. The annual mean effective radii for total particles (ReffT) decreased from north to south and from rural to urban sites, and high total particle volumes were found at the urban sites. The aerosol optical depth at 440 nm (AOD 440nm) increased from remote and rural sites (0.12) to urban sites (0.79), and the extinction Ångström exponent (EAE 440–870 nm) increased from 0.71 at the arid and semi-arid sites to 1.15 at the urban sites, presumably due to anthropogenic emissions. Single-scattering albedo (SSA 440nm) ranged from 0.88 to 0.92, indicating slightly to strongly absorbing aerosols. Absorption AOD 440nm values were 0.01 at the remote sites versus 0.07 at the urban sites. The average direct aerosol radiative effect (DARE) at the bottom of atmosphere increased from the sites in the remote areas (-24.4 0 W m -2) to the urban areas (-103.28 W m -2), indicating increased cooling at the latter. The DARE for the top of the atmosphere increased from -4.79 W m -2 at the remote sites to -30.05 W m -2 at the urban sites, indicating overall cooling effects for the Earth–atmosphere system. A classification method based on SSA 440nm , fine-mode fraction (FMF), and EAE 440–870nm showed that coarse-mode particles (mainly dust) were dominant at the rural sites near the northwestern deserts, while light-absorbing, fine-mode particles were important at most urban sites. This study will be important for understanding aerosol climate effects and regional environmental pollution, and the results will provide useful information for satellite validation and the improvement of climate modelling. [ABSTRACT FROM AUTHOR]

Aerosol pollution in eastern China is an unfortunate consequence of the region's rapid economic and industrial growth. Here, sun photometer measurements from seven sites in the Yangtze River Delta (YRD) from 2011 to 2015 were used to characterize the climatology of aerosol microphysical and optical properties, calculate direct aerosol radiative forcing (DARF) and classify the aerosols based on size and absorption. Bimodal size distributions were found throughout the year, but larger volumes and effective radii of fine-mode particles occurred in June and September due to hygroscopic growth and/or cloud processing. Increases in the fine-mode particles in June and September caused AOD440 nm >1.00 at most sites, and annual mean AOD440 nm values of 0.71-0.76 were found at the urban sites and 0.68 at the rural site. Unlike northern China, the AOD440 nm was lower in July and August (~0.40-0.60) than in January and February (0.71-0.89) due to particle dispersion associated with subtropical anticyclones in summer. Low volumes and large bandwidths of both fine-mode and coarsemode aerosol size distributions occurred in July and August because of biomass burning. Single-scattering albedos at 440 nm (SSA440 nm/ from 0.91 to 0.94 indicated particles with relatively strong to moderate absorption. Strongly absorbing particles from biomass burning with a significant SSA wavelength dependence were found in July and August at most sites, while coarse particles in March to May were mineral dust. Absorbing aerosols were distributed more or less homogeneously throughout the region with absorption aerosol optical depths at 440 nm ~0.04-0.06, but inter-site differences in the absorption Angström exponent indicate a degree of spatial heterogeneity in particle composition. The annual mean DARF was -93±44 to -79±39Wm-2 at the Earth's surface and ~-40Wm-2 at the top of the atmosphere (for the solar zenith angle range of 50 to 80°) under cloud-free conditions. The fine mode composed a major contribution of the absorbing particles in the classification scheme based on SSA, fine-mode fraction and extinction Angström exponent. This study contributes to our understanding of aerosols and regional climate/air quality, and the results will be useful for validating satellite retrievals and for improving climate models and remote sensing algorithms. [ABSTRACT FROM AUTHOR]

Spatio-Temporal Variability of Aerosol Components, Their Optical and Microphysical Properties over North China during Winter Haze in 2012, as Derived from POLDER/PARASOL Satellite Observations. In the published article [[1]], there was an error regarding the affiliation for Yang Ou, Zhengqiang Li, and Ying Zhang. Footnotes 1 B Disclaimer/Publisher's Note: b The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). [Extracted from the article]

Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol products are used worldwide for their reliable accuracy. However, the aerosol optical depth (AOD) usually retrieved by the operational dark target (DT) algorithm of MODIS has been missing for most of the urban regions in Central China. This was due to a high surface reflectance and heavy aerosol loading, especially in winter, when a high cloud cover fraction and the frequent occurrence of haze events reduce the number of effective satellite observations. The retrieval of the AOD from limited satellite data is much needed and important for further aerosol investigations. In this paper, we propose an improved AOD retrieval method for 500 m MODIS data, which is based on an extended surface reflectance estimation scheme and dynamic aerosol models derived from ground-based sun-photometric observations. This improved method was applied to retrieve AOD during heavy aerosol loading and effectively complements the scarcity of AOD in correspondence with urban surface of a higher spatial resolution. The validation results showed that the retrieved AOD was consistent with MODIS DT AOD (R = ~0.87; RMSE = ~0.11) and ground measurements (R = ~0.89; RMSE = ~0.15) from both the Terra and the Aqua satellite. The method can be easily applied to different urban environments affected by air pollution and contributes to the research on aerosol. [ABSTRACT FROM AUTHOR]

Aerosols are important atmospheric constituents with significant impacts on both regional air quality and the global climate and environment. Aerosol effects on radiation, clouds and precipitation are strongly related to the optical properties of aerosols, which vary considerably in space and time. This study reviewed the current understanding of the optical and radiative properties of aerosols and aerosol effects in China, and presented future research prospects. The Chinese aerosol remote sensing network (CARSNET) has been developed and expanded to include >80 stations across China. An internationally recognized radiometric calibration and data processing method has been developed to provide high-quality observational data of key optical properties. Ground lidars are used to provide information on the vertical aerosol optical properties on the spatiotemporal scales. The spatio-temporal distribution and evolution of aerosols are also analyzed using multi-satellite observations. Particle type and component content information are derived from aerosol component retrieval algorithm. Given the advantages of aerosol assimilation reanalysis products in terms of temporal resolution and spatial coverage, such information can more accurately characterize the climatological impact of aerosol optical properties and aerosol–climate interactions. Measurements obtained using the above methods can quantitatively reveal the influence of aerosol direct radiation effects in different regions, and support the exploration of the potential mechanisms of aerosol impacts on weather and climate from the perspective of ground-based remote sensing, which will help to guide the direction of future research. [ABSTRACT FROM AUTHOR]

Aerosol optical properties were derived from ground-based sunphotometer observations between 2009-2013 at three urban sites of Shenyang, Anshan, Fushun in northeastern China. The annual means for extinction aerosol optical depths (EAOD) at 500 nm were 0.57±0.38, 0.52±0.35, and 0.41±0.31 at Shenyang, Anshan, Fushun, respectively. The corresponding annual means for the extinction Angstrom exponents (EAE) computed for the wavelengths of 440 and 870 nm were 0.86±0.32, 0.86±0.34 and 0.91±0.35, respectively, indicating that urban area of Northeast China were affected by both coarse and fine particles. Hygroscopic growth in summer and incursions of dust aerosols in spring were evidently revealed from the analysis of the relationship between EAE and δ EAE (the EAE difference, δ EAE=EAE(440,670)−EAE(670,870)). The annual mean absorption aerosol optical depths (AAOD 440 nm ) values at Shenyang, Anshan, Fushun were 0.15±0.11, 0.10±0.07, 0.08±0.04, respectively. The annual mean absorption Angstrom exponents (AAE 440–870 nm ) values were 0.86±0.24, 1.19±0.39, 1.33±0.36 at Shenyang, Anshan, Fushun, respectively. When the AAEs were close to unity at Anshan, the absorption aerosol particles evidently consisted of black carbon from coal combustion and motor vehicles. Larger AAEs at Fushun were indicative of absorbing aerosols mainly from biomass burning and mineral dust. The AAE at Shenyang was<1 which may be consistent with black carbon particles with absorbing or non-absorbing coatings. Analysis of the relationship between the AAEs and extinction Angstrom exponents showed that the aerosol populations at these three sites could be classified as “mixed-small particles” including anthropogenic particles and secondary organic aerosol with highly variable sphericity fractions. [ABSTRACT FROM AUTHOR]

The Directional Polarimetric Camera (DPC) is the first Chinese satellite-borne multi-angle polarimetric sensor onboard the GaoFen-5 (GF-5) satellite, which was launched on May 9th 2018. In this paper, we present the spatiotemporal variability of satellite-measured aerosol components (such as, black carbon, brown carbon, iron oxides contained in mineral dust etc.) over North China Plain (36–41°N, 113–123°E) for a heavily haze-polluted case, to demonstrate the first successful retrievals of aerosol component content and optical property directly from the DPC/GF-5 instrument measurements. We also reveal the relationship between absorbing/scattering component mass ratio and aerosol absorption based on the multi-angle polarimetric satellite measurements, which presents comparable agreement with previous laboratory studies and field studies for an indirect validation of aerosol component retrievals. In addition, the feasibility, stability and uncertainty of the aerosol optical property and component content retrievals from DPC multi-angle polarimetric observations were evaluated and assessed by sensitivity tests. The distributions of DPC/GF-5 aerosol optical depth (AOD) are in line with satellite-observed MODIS (Moderate-resolution Imaging Spectroradiometer) AOD and ground-based CARSNET (China Aerosol Remote Sensing Network) AOD products. Therefore, the multi-angle polarimetric satellite measurements show an encouraging potential for the comprehensive inversion of aerosol optical, microphysical and chemical properties. The results and further applications of this component algorithm in the future are helpful to reduce the uncertainty of climate change assessment by providing the measurements of aerosol component, in particular for black carbon, brown carbon, and iron oxides contained in dust, with large and extensive spatiotemporal scale. [Display omitted] • We propose a new aerosol retrieval application to multiangle polarimetric observation. • The first successful retrieval of aerosol component content from DPC is presented. • Satellite-measured BC, BrC and iron oxides are characterized in a haze-polluted case. • We reveal the link of satellite-measured absorbing/scattering component ratio to AAE. [ABSTRACT FROM AUTHOR]

PM 10 remote sensing is of great significance in the atmospheric environment studies. Contrary to intuitive perception, deriving PM 10 is more difficult than PM 2.5 from satellite measurements. This is because although the major satellite parameter Aerosol Optical Depth (AOD) contain contribution of all suspended particles, it is much more sensitive to fine particles than coarse particles. To address this challenge, a physically based remote sensing method for PM 10 is developed using two new semi-empirical physical models: the model of columnar volume-to-extinction ratio (VE 10) and the model of particle effective density. VE 10 is a key parameter bridging the non-linear relationship between aerosol extinction and volume concentration. A semi- empirical VE 10 model is developed based on the fine mode fraction (FMF), and the mean relative error of VE 10 modeling is 14.5%. Similarly, the particle effective density depends on the ratio of mass to volume. The particle effective density is effectively characterized by refractive index of matter, with the theoretical error of 13.8%. Both semi-empirical physical models are applied to the derivation of PM 10 , by using aerosol retrieval products of POLarization and Directionality of the Earth's Reflectances (POLDER) produced by the Generalized Retrieval of Atmosphere and Surface Properties (GRASP) algorithm. As a validation, PM 10 mass concentration is estimated over China from Jan. to Oct. in 2013. A fairly good correlation and consistency are achieved by inter-comparison with in-situ PM 10 measurements. • Volume-to-extinction ratio is alternative value to microphysical parameter. • Volume-to-extinction ratio is suitable for different aerosol types. • Volume-to-extinction ratio can be directly used for satellite data. • Particle effective density can be estimated from satellite refractive index data. • PM 10 results are well tested by using POLDER/GRASP products. [ABSTRACT FROM AUTHOR]

A new aerosol component approach that infers aerosol species fractions from satellite observations was developed and incorporated by Li et al. (2019) into Generalized Retrieval of Aerosol and Surface Properties (GRASP) algorithm. In the current work we employ this aerosol component approach for derivation and analysis of temporal and spatial characteristics of black and brown carbon (BC and BrC) concentrations in East and South Asia using POLDER/PARASOL satellite observations during 2005–2013. The inter-comparison of the satellite retrieved and in situ measured BC in China shows a reasonably good agreement for both monthly averaged and daily values. The distribution of light-absorbing carbon (BC and BrC) shows a significant temporal variation cycle with increases matching the months known for strong biomass burning or fossil fuels combustion emissions. The main concentrations of BC are found near the fire or anthropogenic emission regions, while concentrations of BrC are predominantly distanced from the emission sources. Seasonal means of BC/(BC + BrC) ratio in East and South Asia are also presented. High ratios (above 0.5) in Northeast India and North China during December, January, and February can be attributed to fossil fuels combustion and anthropogenic emissions. Medium ratios (around 0.2–0.3) over the Indo-China Peninsula and Northeast China during March, April, and May are associated with the biomass burning (e.g., forest fires, agriculture waste burning). The provided observationally-based retrievals of BC, BrC, and BC/(BC + BrC) are believed to be of high importance for constraining the aerosol modeling and reducing the uncertainties of the model estimations. • Satellite-retrieved and surface-measured BC show a reasonably good agreement. • Spatio-temporal characteristics of BC and brown carbon distributions are discussed. • Climatology of observationally-based BC/(BC + BrC) mass ratio over Asia is presented. [ABSTRACT FROM AUTHOR]

• A statistical comparison of GRASP total AOD, fine mode AOD and Fine Mode Fraction (FMF) with Sun–Sky Radiometer Observation Network (SONET) products over China was conducted. • Both GRASP «models» and GRASP «high-precision» products were evaluated at 8 SONET sites. • Both SONET-SDA and SONET-IP retrievals were compared with GRASP fine mode AOD. • The comparison between MODIS-FMF and GRASP-FMF was conducted. Monitoring fine mode aerosol pollutants is one of priorities in national environmental programs. Aerosol Optical Depth (AOD) and fine mode AOD are key parameters for estimation of particulate matter. POLDER/GRASP retrieval provides probably the most detailed aerosol products globally for 2005 – 2013. Current studies are focused on understanding the value of new satellite products for potential monitoring of air quality over China. As part of this objective, the quantitative evaluation of the products over China, a statistical comparison of total AOD, fine mode AOD and Fine Mode Fraction (FMF) with Sun–Sky Radiometer Observation Network (SONET) products is conducted. Two of available POLDER aerosol products: GRASP «models» and «high-precision» were evaluated at 8 SONET sites over China in this study. The results show the best agreement of the total AOD at 490 nm from «models» approach with AOD (500 nm) from SONET with correlation coefficient of ~0.96, slope of ~0.9, RMSE of 0.14 and bias (intercept) of 0.04, with about 76% of retrievals inside of Gfrac ranges. The validation results of the fine mode AOD at 490 nm with AOD the fine mode SONET-SDA product (500 nm) are quite comparable for both «high-precision» and «models» approaches, with slightly better results for «high-precision» data showing correlation coefficient of ~0.95, slope of ~0.78, RMSE of 0.14 and bias (intercept) of 0.07, with about 68% of retrievals inside of Gfrac ranges. The analysis is demonstrated that POLDER/GRASP results are one of most suitable aerosol products for characterization of FMF (Fine Mode Fraction) of aerosol. For example, compared with MODIS, the FMF from POLDER/GRASP is significantly closer to SONET-SDA FMF, with Gfrac increasing from 16.75% (MODIS) to 62.35% (POLDER/GRASP). Moreover, there is a clear similarity of spatial distribution of the fine mode AOD from GRASP with ground-level PM 2.5 observations. [ABSTRACT FROM AUTHOR]